aboutsummaryrefslogtreecommitdiff
path: root/manual/signal.texi
blob: fc05d222c8048fd4547011b33a741f137cbd455b (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
2903
2904
2905
2906
2907
2908
2909
2910
2911
2912
2913
2914
2915
2916
2917
2918
2919
2920
2921
2922
2923
2924
2925
2926
2927
2928
2929
2930
2931
2932
2933
2934
2935
2936
2937
2938
2939
2940
2941
2942
2943
2944
2945
2946
2947
2948
2949
2950
2951
2952
2953
2954
2955
2956
2957
2958
2959
2960
2961
2962
2963
2964
2965
2966
2967
2968
2969
2970
2971
2972
2973
2974
2975
2976
2977
2978
2979
2980
2981
2982
2983
2984
2985
2986
2987
2988
2989
2990
2991
2992
2993
2994
2995
2996
2997
2998
2999
3000
3001
3002
3003
3004
3005
3006
3007
3008
3009
3010
3011
3012
3013
3014
3015
3016
3017
3018
3019
3020
3021
3022
3023
3024
3025
3026
3027
3028
3029
3030
3031
3032
3033
3034
3035
3036
3037
3038
3039
3040
3041
3042
3043
3044
3045
3046
3047
3048
3049
3050
3051
3052
3053
3054
3055
3056
3057
3058
3059
3060
3061
3062
3063
3064
3065
3066
3067
3068
3069
3070
3071
3072
3073
3074
3075
3076
3077
3078
3079
3080
3081
3082
3083
3084
3085
3086
3087
3088
3089
3090
3091
3092
3093
3094
3095
3096
3097
3098
3099
3100
3101
3102
3103
3104
3105
3106
3107
3108
3109
3110
3111
3112
3113
3114
3115
3116
3117
3118
3119
3120
3121
3122
3123
3124
3125
3126
3127
3128
3129
3130
3131
3132
3133
3134
3135
3136
3137
3138
3139
3140
3141
3142
3143
3144
3145
3146
3147
3148
3149
3150
3151
3152
3153
3154
3155
3156
3157
3158
3159
3160
3161
3162
3163
3164
3165
3166
3167
3168
3169
3170
3171
3172
3173
3174
3175
3176
3177
3178
3179
3180
3181
3182
3183
3184
3185
3186
3187
3188
3189
3190
3191
3192
3193
3194
3195
3196
3197
3198
3199
3200
3201
3202
3203
3204
3205
3206
3207
3208
3209
3210
3211
3212
3213
3214
3215
3216
3217
3218
3219
3220
3221
3222
3223
3224
3225
3226
3227
3228
3229
3230
3231
3232
3233
3234
3235
3236
3237
3238
3239
3240
3241
3242
3243
3244
3245
3246
3247
3248
3249
3250
3251
3252
3253
3254
3255
3256
3257
3258
3259
3260
3261
3262
3263
3264
3265
3266
3267
3268
3269
3270
3271
3272
3273
3274
3275
3276
3277
3278
3279
3280
3281
3282
3283
3284
3285
3286
3287
3288
3289
3290
3291
3292
3293
3294
3295
3296
3297
3298
3299
3300
3301
3302
3303
3304
3305
3306
3307
3308
3309
3310
3311
3312
3313
3314
3315
3316
3317
3318
3319
3320
3321
3322
3323
3324
3325
3326
3327
3328
3329
3330
3331
3332
3333
3334
3335
3336
3337
3338
3339
3340
3341
3342
3343
3344
3345
3346
3347
@node Signal Handling, Process Startup, Non-Local Exits, Top
@chapter Signal Handling

@cindex signal
A @dfn{signal} is a software interrupt delivered to a process.  The
operating system uses signals to report exceptional situations to an
executing program.  Some signals report errors such as references to
invalid memory addresses; others report asynchronous events, such as
disconnection of a phone line.

The GNU C library defines a variety of signal types, each for a
particular kind of event.  Some kinds of events make it inadvisable or
impossible for the program to proceed as usual, and the corresponding
signals normally abort the program.  Other kinds of signals that report
harmless events are ignored by default.

If you anticipate an event that causes signals, you can define a handler
function and tell the operating system to run it when that particular
type of signal arrives.

Finally, one process can send a signal to another process; this allows a
parent process to abort a child, or two related processes to communicate
and synchronize.

@menu
* Concepts of Signals::         Introduction to the signal facilities.
* Standard Signals::            Particular kinds of signals with
                                 standard names and meanings.
* Signal Actions::              Specifying what happens when a
                                 particular signal is delivered.
* Defining Handlers::           How to write a signal handler function.
* Interrupted Primitives::	Signal handlers affect use of @code{open},
				 @code{read}, @code{write} and other functions.
* Generating Signals::          How to send a signal to a process.
* Blocking Signals::            Making the system hold signals temporarily.
* Waiting for a Signal::        Suspending your program until a signal
                                 arrives.
* Signal Stack::                Using a Separate Signal Stack.
* BSD Signal Handling::         Additional functions for backward
			         compatibility with BSD.
@end menu

@node Concepts of Signals
@section Basic Concepts of Signals

This section explains basic concepts of how signals are generated, what
happens after a signal is delivered, and how programs can handle
signals.

@menu
* Kinds of Signals::            Some examples of what can cause a signal.
* Signal Generation::           Concepts of why and how signals occur.
* Delivery of Signal::          Concepts of what a signal does to the
                                 process.
@end menu

@node Kinds of Signals
@subsection Some Kinds of Signals

A signal reports the occurrence of an exceptional event.  These are some
of the events that can cause (or @dfn{generate}, or @dfn{raise}) a
signal:

@itemize @bullet
@item
A program error such as dividing by zero or issuing an address outside
the valid range.

@item
A user request to interrupt or terminate the program.  Most environments
are set up to let a user suspend the program by typing @kbd{C-z}, or
terminate it with @kbd{C-c}.  Whatever key sequence is used, the
operating system sends the proper signal to interrupt the process.

@item
The termination of a child process.

@item
Expiration of a timer or alarm.

@item
A call to @code{kill} or @code{raise} by the same process.

@item
A call to @code{kill} from another process.  Signals are a limited but
useful form of interprocess communication.

@item
An attempt to perform an I/O operation that cannot be done.  Examples
are reading from a pipe that has no writer (@pxref{Pipes and FIFOs}),
and reading or writing to a terminal in certain situations (@pxref{Job
Control}).
@end itemize

Each of these kinds of events (excepting explicit calls to @code{kill}
and @code{raise}) generates its own particular kind of signal.  The
various kinds of signals are listed and described in detail in
@ref{Standard Signals}.

@node Signal Generation
@subsection Concepts of Signal Generation
@cindex generation of signals

In general, the events that generate signals fall into three major
categories: errors, external events, and explicit requests.

An error means that a program has done something invalid and cannot
continue execution.  But not all kinds of errors generate signals---in
fact, most do not.  For example, opening a nonexistent file is an error,
but it does not raise a signal; instead, @code{open} returns @code{-1}.
In general, errors that are necessarily associated with certain library
functions are reported by returning a value that indicates an error.
The errors which raise signals are those which can happen anywhere in
the program, not just in library calls.  These include division by zero
and invalid memory addresses.

An external event generally has to do with I/O or other processes.
These include the arrival of input, the expiration of a timer, and the
termination of a child process.

An explicit request means the use of a library function such as
@code{kill} whose purpose is specifically to generate a signal.

Signals may be generated @dfn{synchronously} or @dfn{asynchronously}.  A
synchronous signal pertains to a specific action in the program, and is
delivered (unless blocked) during that action.  Most errors generate
signals synchronously, and so do explicit requests by a process to
generate a signal for that same process.  On some machines, certain
kinds of hardware errors (usually floating-point exceptions) are not
reported completely synchronously, but may arrive a few instructions
later.

Asynchronous signals are generated by events outside the control of the
process that receives them.  These signals arrive at unpredictable times
during execution.  External events generate signals asynchronously, and
so do explicit requests that apply to some other process.

A given type of signal is either typically synchronous or typically
asynchronous.  For example, signals for errors are typically synchronous
because errors generate signals synchronously.  But any type of signal
can be generated synchronously or asynchronously with an explicit
request.

@node Delivery of Signal
@subsection How Signals Are Delivered
@cindex delivery of signals
@cindex pending signals
@cindex blocked signals

When a signal is generated, it becomes @dfn{pending}.  Normally it
remains pending for just a short period of time and then is
@dfn{delivered} to the process that was signaled.  However, if that kind
of signal is currently @dfn{blocked}, it may remain pending
indefinitely---until signals of that kind are @dfn{unblocked}.  Once
unblocked, it will be delivered immediately.  @xref{Blocking Signals}.

@cindex specified action (for a signal)
@cindex default action (for a signal)
@cindex signal action
@cindex catching signals
When the signal is delivered, whether right away or after a long delay,
the @dfn{specified action} for that signal is taken.  For certain
signals, such as @code{SIGKILL} and @code{SIGSTOP}, the action is fixed,
but for most signals, the program has a choice: ignore the signal,
specify a @dfn{handler function}, or accept the @dfn{default action} for
that kind of signal.  The program specifies its choice using functions
such as @code{signal} or @code{sigaction} (@pxref{Signal Actions}).  We
sometimes say that a handler @dfn{catches} the signal.  While the
handler is running, that particular signal is normally blocked.

If the specified action for a kind of signal is to ignore it, then any
such signal which is generated is discarded immediately.  This happens
even if the signal is also blocked at the time.  A signal discarded in
this way will never be delivered, not even if the program subsequently
specifies a different action for that kind of signal and then unblocks
it.

If a signal arrives which the program has neither handled nor ignored,
its @dfn{default action} takes place.  Each kind of signal has its own
default action, documented below (@pxref{Standard Signals}).  For most kinds
of signals, the default action is to terminate the process.  For certain
kinds of signals that represent ``harmless'' events, the default action
is to do nothing.

When a signal terminates a process, its parent process can determine the
cause of termination by examining the termination status code reported
by the @code{wait} or @code{waitpid} functions.  (This is discussed in
more detail in @ref{Process Completion}.)  The information it can get
includes the fact that termination was due to a signal, and the kind of
signal involved.  If a program you run from a shell is terminated by a
signal, the shell typically prints some kind of error message.

The signals that normally represent program errors have a special
property: when one of these signals terminates the process, it also
writes a @dfn{core dump file} which records the state of the process at
the time of termination.  You can examine the core dump with a debugger
to investigate what caused the error.

If you raise a ``program error'' signal by explicit request, and this
terminates the process, it makes a core dump file just as if the signal
had been due directly to an error.

@node Standard Signals
@section Standard Signals
@cindex signal names
@cindex names of signals

@pindex signal.h
@cindex signal number
This section lists the names for various standard kinds of signals and
describes what kind of event they mean.  Each signal name is a macro
which stands for a positive integer---the @dfn{signal number} for that
kind of signal.  Your programs should never make assumptions about the
numeric code for a particular kind of signal, but rather refer to them
always by the names defined here.  This is because the number for a
given kind of signal can vary from system to system, but the meanings of
the names are standardized and fairly uniform.

The signal names are defined in the header file @file{signal.h}.

@comment signal.h
@comment BSD
@deftypevr Macro int NSIG
The value of this symbolic constant is the total number of signals
defined.  Since the signal numbers are allocated consecutively,
@code{NSIG} is also one greater than the largest defined signal number.
@end deftypevr

@menu
* Program Error Signals::       Used to report serious program errors.
* Termination Signals::         Used to interrupt and/or terminate the
                                 program.
* Alarm Signals::               Used to indicate expiration of timers.
* Asynchronous I/O Signals::    Used to indicate input is available.
* Job Control Signals::         Signals used to support job control.
* Operation Error Signals::     Used to report operational system errors.
* Miscellaneous Signals::       Miscellaneous Signals.
* Signal Messages::             Printing a message describing a signal.
@end menu

@node Program Error Signals
@subsection Program Error Signals
@cindex program error signals

The following signals are generated when a serious program error is
detected by the operating system or the computer itself.  In general,
all of these signals are indications that your program is seriously
broken in some way, and there's usually no way to continue the
computation which encountered the error.

Some programs handle program error signals in order to tidy up before
terminating; for example, programs that turn off echoing of terminal
input should handle program error signals in order to turn echoing back
on.  The handler should end by specifying the default action for the
signal that happened and then reraising it; this will cause the program
to terminate with that signal, as if it had not had a handler.
(@xref{Termination in Handler}.)

Termination is the sensible ultimate outcome from a program error in
most programs.  However, programming systems such as Lisp that can load
compiled user programs might need to keep executing even if a user
program incurs an error.  These programs have handlers which use
@code{longjmp} to return control to the command level.

The default action for all of these signals is to cause the process to
terminate.  If you block or ignore these signals or establish handlers
for them that return normally, your program will probably break horribly
when such signals happen, unless they are generated by @code{raise} or
@code{kill} instead of a real error.

@vindex COREFILE
When one of these program error signals terminates a process, it also
writes a @dfn{core dump file} which records the state of the process at
the time of termination.  The core dump file is named @file{core} and is
written in whichever directory is current in the process at the time.
(On the GNU system, you can specify the file name for core dumps with
the environment variable @code{COREFILE}.)  The purpose of core dump
files is so that you can examine them with a debugger to investigate
what caused the error.

@comment signal.h
@comment ISO
@deftypevr Macro int SIGFPE
The @code{SIGFPE} signal reports a fatal arithmetic error.  Although the
name is derived from ``floating-point exception'', this signal actually
covers all arithmetic errors, including division by zero and overflow.
If a program stores integer data in a location which is then used in a
floating-point operation, this often causes an ``invalid operation''
exception, because the processor cannot recognize the data as a
floating-point number.
@cindex exception
@cindex floating-point exception

Actual floating-point exceptions are a complicated subject because there
are many types of exceptions with subtly different meanings, and the
@code{SIGFPE} signal doesn't distinguish between them.  The @cite{IEEE
Standard for Binary Floating-Point Arithmetic (ANSI/IEEE Std 754-1985
and ANSI/IEEE Std 854-1987)}
defines various floating-point exceptions and requires conforming
computer systems to report their occurrences.  However, this standard
does not specify how the exceptions are reported, or what kinds of
handling and control the operating system can offer to the programmer.
@end deftypevr

BSD systems provide the @code{SIGFPE} handler with an extra argument
that distinguishes various causes of the exception.  In order to access
this argument, you must define the handler to accept two arguments,
which means you must cast it to a one-argument function type in order to
establish the handler.  The GNU library does provide this extra
argument, but the value is meaningful only on operating systems that
provide the information (BSD systems and GNU systems).

@table @code
@comment signal.h
@comment BSD
@item FPE_INTOVF_TRAP
@vindex FPE_INTOVF_TRAP
Integer overflow (impossible in a C program unless you enable overflow
trapping in a hardware-specific fashion).
@comment signal.h
@comment BSD
@item FPE_INTDIV_TRAP
@vindex FPE_INTDIV_TRAP
Integer division by zero.
@comment signal.h
@comment BSD
@item FPE_SUBRNG_TRAP
@vindex FPE_SUBRNG_TRAP
Subscript-range (something that C programs never check for).
@comment signal.h
@comment BSD
@item FPE_FLTOVF_TRAP
@vindex FPE_FLTOVF_TRAP
Floating overflow trap.
@comment signal.h
@comment BSD
@item FPE_FLTDIV_TRAP
@vindex FPE_FLTDIV_TRAP
Floating/decimal division by zero.
@comment signal.h
@comment BSD
@item FPE_FLTUND_TRAP
@vindex FPE_FLTUND_TRAP
Floating underflow trap.  (Trapping on floating underflow is not
normally enabled.)
@comment signal.h
@comment BSD
@item FPE_DECOVF_TRAP
@vindex FPE_DECOVF_TRAP
Decimal overflow trap.  (Only a few machines have decimal arithmetic and
C never uses it.)
@ignore @c These seem redundant
@comment signal.h
@comment BSD
@item FPE_FLTOVF_FAULT
@vindex FPE_FLTOVF_FAULT
Floating overflow fault.
@comment signal.h
@comment BSD
@item FPE_FLTDIV_FAULT
@vindex FPE_FLTDIV_FAULT
Floating divide by zero fault.
@comment signal.h
@comment BSD
@item FPE_FLTUND_FAULT
@vindex FPE_FLTUND_FAULT
Floating underflow fault.
@end ignore
@end table

@comment signal.h
@comment ISO
@deftypevr Macro int SIGILL
The name of this signal is derived from ``illegal instruction''; it
usually means your program is trying to execute garbage or a privileged
instruction.  Since the C compiler generates only valid instructions,
@code{SIGILL} typically indicates that the executable file is corrupted,
or that you are trying to execute data.  Some common ways of getting
into the latter situation are by passing an invalid object where a
pointer to a function was expected, or by writing past the end of an
automatic array (or similar problems with pointers to automatic
variables) and corrupting other data on the stack such as the return
address of a stack frame.

@code{SIGILL} can also be generated when the stack overflows, or when
the system has trouble running the handler for a signal.
@end deftypevr
@cindex illegal instruction

@comment signal.h
@comment ISO
@deftypevr Macro int SIGSEGV
@cindex segmentation violation
This signal is generated when a program tries to read or write outside
the memory that is allocated for it, or to write memory that can only be
read.  (Actually, the signals only occur when the program goes far
enough outside to be detected by the system's memory protection
mechanism.)  The name is an abbreviation for ``segmentation violation''.

Common ways of getting a @code{SIGSEGV} condition include dereferencing
a null or uninitialized pointer, or when you use a pointer to step
through an array, but fail to check for the end of the array.  It varies
among systems whether dereferencing a null pointer generates
@code{SIGSEGV} or @code{SIGBUS}.
@end deftypevr

@comment signal.h
@comment BSD
@deftypevr Macro int SIGBUS
This signal is generated when an invalid pointer is dereferenced.  Like
@code{SIGSEGV}, this signal is typically the result of dereferencing an
uninitialized pointer.  The difference between the two is that
@code{SIGSEGV} indicates an invalid access to valid memory, while
@code{SIGBUS} indicates an access to an invalid address.  In particular,
@code{SIGBUS} signals often result from dereferencing a misaligned
pointer, such as referring to a four-word integer at an address not
divisible by four.  (Each kind of computer has its own requirements for
address alignment.)

The name of this signal is an abbreviation for ``bus error''.
@end deftypevr
@cindex bus error

@comment signal.h
@comment ISO
@deftypevr Macro int SIGABRT
@cindex abort signal
This signal indicates an error detected by the program itself and
reported by calling @code{abort}.  @xref{Aborting a Program}.
@end deftypevr

@comment signal.h
@comment Unix
@deftypevr Macro int SIGIOT
Generated by the PDP-11 ``iot'' instruction.  On most machines, this is
just another name for @code{SIGABRT}.
@end deftypevr

@comment signal.h
@comment BSD
@deftypevr Macro int SIGTRAP
Generated by the machine's breakpoint instruction, and possibly other
trap instructions.  This signal is used by debuggers.  Your program will
probably only see @code{SIGTRAP} if it is somehow executing bad
instructions.
@end deftypevr

@comment signal.h
@comment BSD
@deftypevr Macro int  SIGEMT
Emulator trap; this results from certain unimplemented instructions
which might be emulated in software, or the operating system's
failure to properly emulate them.
@end deftypevr

@comment signal.h
@comment Unix
@deftypevr Macro int  SIGSYS
Bad system call; that is to say, the instruction to trap to the
operating system was executed, but the code number for the system call
to perform was invalid.
@end deftypevr

@node Termination Signals
@subsection Termination Signals
@cindex program termination signals

These signals are all used to tell a process to terminate, in one way
or another.  They have different names because they're used for slightly
different purposes, and programs might want to handle them differently.

The reason for handling these signals is usually so your program can
tidy up as appropriate before actually terminating.  For example, you
might want to save state information, delete temporary files, or restore
the previous terminal modes.  Such a handler should end by specifying
the default action for the signal that happened and then reraising it;
this will cause the program to terminate with that signal, as if it had
not had a handler.  (@xref{Termination in Handler}.)

The (obvious) default action for all of these signals is to cause the
process to terminate.

@comment signal.h
@comment ISO
@deftypevr Macro int SIGTERM
@cindex termination signal
The @code{SIGTERM} signal is a generic signal used to cause program
termination.  Unlike @code{SIGKILL}, this signal can be blocked,
handled, and ignored.  It is the normal way to politely ask a program to
terminate.

The shell command @code{kill} generates @code{SIGTERM} by default.
@pindex kill
@end deftypevr

@comment signal.h
@comment ISO
@deftypevr Macro int SIGINT
@cindex interrupt signal
The @code{SIGINT} (``program interrupt'') signal is sent when the user
types the INTR character (normally @kbd{C-c}).  @xref{Special
Characters}, for information about terminal driver support for
@kbd{C-c}.
@end deftypevr

@comment signal.h
@comment POSIX.1
@deftypevr Macro int SIGQUIT
@cindex quit signal
@cindex quit signal
The @code{SIGQUIT} signal is similar to @code{SIGINT}, except that it's
controlled by a different key---the QUIT character, usually
@kbd{C-\}---and produces a core dump when it terminates the process,
just like a program error signal.  You can think of this as a
program error condition ``detected'' by the user.

@xref{Program Error Signals}, for information about core dumps.
@xref{Special Characters}, for information about terminal driver
support.

Certain kinds of cleanups are best omitted in handling @code{SIGQUIT}.
For example, if the program creates temporary files, it should handle
the other termination requests by deleting the temporary files.  But it
is better for @code{SIGQUIT} not to delete them, so that the user can
examine them in conjunction with the core dump.
@end deftypevr

@comment signal.h
@comment POSIX.1
@deftypevr Macro int SIGKILL
The @code{SIGKILL} signal is used to cause immediate program termination.
It cannot be handled or ignored, and is therefore always fatal.  It is
also not possible to block this signal.

This signal is usually generated only by explicit request.  Since it
cannot be handled, you should generate it only as a last resort, after
first trying a less drastic method such as @kbd{C-c} or @code{SIGTERM}.
If a process does not respond to any other termination signals, sending
it a @code{SIGKILL} signal will almost always cause it to go away.

In fact, if @code{SIGKILL} fails to terminate a process, that by itself
constitutes an operating system bug which you should report.

The system will generate @code{SIGKILL} for a process itself under some
unusual conditions where the program cannot possible continue to run
(even to run a signal handler).
@end deftypevr
@cindex kill signal

@comment signal.h
@comment POSIX.1
@deftypevr Macro int SIGHUP
@cindex hangup signal
The @code{SIGHUP} (``hang-up'') signal is used to report that the user's
terminal is disconnected, perhaps because a network or telephone
connection was broken.  For more information about this, see @ref{Control
Modes}.

This signal is also used to report the termination of the controlling
process on a terminal to jobs associated with that session; this
termination effectively disconnects all processes in the session from
the controlling terminal.  For more information, see @ref{Termination
Internals}.
@end deftypevr

@node Alarm Signals
@subsection Alarm Signals

These signals are used to indicate the expiration of timers.
@xref{Setting an Alarm}, for information about functions that cause
these signals to be sent.

The default behavior for these signals is to cause program termination.
This default is rarely useful, but no other default would be useful;
most of the ways of using these signals would require handler functions
in any case.

@comment signal.h
@comment POSIX.1
@deftypevr Macro int SIGALRM
This signal typically indicates expiration of a timer that measures real
or clock time.  It is used by the @code{alarm} function, for example.
@end deftypevr
@cindex alarm signal

@comment signal.h
@comment BSD
@deftypevr Macro int SIGVTALRM
This signal typically indicates expiration of a timer that measures CPU
time used by the current process.  The name is an abbreviation for
``virtual time alarm''.
@end deftypevr
@cindex virtual time alarm signal

@comment signal.h
@comment BSD
@deftypevr Macro int SIGPROF
This signal is typically indicates expiration of a timer that measures
both CPU time used by the current process, and CPU time expended on
behalf of the process by the system.  Such a timer is used to implement
code profiling facilities, hence the name of this signal.
@end deftypevr
@cindex profiling alarm signal


@node Asynchronous I/O Signals
@subsection Asynchronous I/O Signals

The signals listed in this section are used in conjunction with
asynchronous I/O facilities.  You have to take explicit action by
calling @code{fcntl} to enable a particular file descriptor to generate
these signals (@pxref{Interrupt Input}).  The default action for these
signals is to ignore them.

@comment signal.h
@comment BSD
@deftypevr Macro int SIGIO
@cindex input available signal
@cindex output possible signal
This signal is sent when a file descriptor is ready to perform input
or output.

On most operating systems, terminals and sockets are the only kinds of
files that can generate @code{SIGIO}; other kinds, including ordinary
files, never generate @code{SIGIO} even if you ask them to.

In the GNU system @code{SIGIO} will always be generated properly
if you successfully set asynchronous mode with @code{fcntl}.
@end deftypevr

@comment signal.h
@comment BSD
@deftypevr Macro int SIGURG
@cindex urgent data signal
This signal is sent when ``urgent'' or out-of-band data arrives on a
socket.  @xref{Out-of-Band Data}.
@end deftypevr

@comment signal.h
@comment SVID
@deftypevr Macro int SIGPOLL
This is a System V signal name, more or less similar to @code{SIGIO}.
It is defined only for compatibility.
@end deftypevr

@node Job Control Signals
@subsection Job Control Signals
@cindex job control signals

These signals are used to support job control.  If your system
doesn't support job control, then these macros are defined but the
signals themselves can't be raised or handled.

You should generally leave these signals alone unless you really
understand how job control works.  @xref{Job Control}.

@comment signal.h
@comment POSIX.1
@deftypevr Macro int SIGCHLD
@cindex child process signal
This signal is sent to a parent process whenever one of its child
processes terminates or stops.

The default action for this signal is to ignore it.  If you establish a
handler for this signal while there are child processes that have
terminated but not reported their status via @code{wait} or
@code{waitpid} (@pxref{Process Completion}), whether your new handler
applies to those processes or not depends on the particular operating
system.
@end deftypevr

@comment signal.h
@comment SVID
@deftypevr Macro int SIGCLD
This is an obsolete name for @code{SIGCHLD}.
@end deftypevr

@comment signal.h
@comment POSIX.1
@deftypevr Macro int SIGCONT
@cindex continue signal
You can send a @code{SIGCONT} signal to a process to make it continue.
This signal is special---it always makes the process continue if it is
stopped, before the signal is delivered.  The default behavior is to do
nothing else.  You cannot block this signal.  You can set a handler, but
@code{SIGCONT} always makes the process continue regardless.

Most programs have no reason to handle @code{SIGCONT}; they simply
resume execution without realizing they were ever stopped.  You can use
a handler for @code{SIGCONT} to make a program do something special when
it is stopped and continued---for example, to reprint a prompt when it
is suspended while waiting for input.
@end deftypevr

@comment signal.h
@comment POSIX.1
@deftypevr Macro int SIGSTOP
The @code{SIGSTOP} signal stops the process.  It cannot be handled,
ignored, or blocked.
@end deftypevr
@cindex stop signal

@comment signal.h
@comment POSIX.1
@deftypevr Macro int SIGTSTP
The @code{SIGTSTP} signal is an interactive stop signal.  Unlike
@code{SIGSTOP}, this signal can be handled and ignored.

Your program should handle this signal if you have a special need to
leave files or system tables in a secure state when a process is
stopped.  For example, programs that turn off echoing should handle
@code{SIGTSTP} so they can turn echoing back on before stopping.

This signal is generated when the user types the SUSP character
(normally @kbd{C-z}).  For more information about terminal driver
support, see @ref{Special Characters}.
@end deftypevr
@cindex interactive stop signal

@comment signal.h
@comment POSIX.1
@deftypevr Macro int SIGTTIN
A process cannot read from the user's terminal while it is running
as a background job.  When any process in a background job tries to
read from the terminal, all of the processes in the job are sent a
@code{SIGTTIN} signal.  The default action for this signal is to
stop the process.  For more information about how this interacts with
the terminal driver, see @ref{Access to the Terminal}.
@end deftypevr
@cindex terminal input signal

@comment signal.h
@comment POSIX.1
@deftypevr Macro int SIGTTOU
This is similar to @code{SIGTTIN}, but is generated when a process in a
background job attempts to write to the terminal or set its modes.
Again, the default action is to stop the process.  @code{SIGTTOU} is
only generated for an attempt to write to the terminal if the
@code{TOSTOP} output mode is set; @pxref{Output Modes}.
@end deftypevr
@cindex terminal output signal

While a process is stopped, no more signals can be delivered to it until
it is continued, except @code{SIGKILL} signals and (obviously)
@code{SIGCONT} signals.  The signals are marked as pending, but not
delivered until the process is continued.  The @code{SIGKILL} signal
always causes termination of the process and can't be blocked, handled
or ignored.  You can ignore @code{SIGCONT}, but it always causes the
process to be continued anyway if it is stopped.  Sending a
@code{SIGCONT} signal to a process causes any pending stop signals for
that process to be discarded.  Likewise, any pending @code{SIGCONT}
signals for a process are discarded when it receives a stop signal.

When a process in an orphaned process group (@pxref{Orphaned Process
Groups}) receives a @code{SIGTSTP}, @code{SIGTTIN}, or @code{SIGTTOU}
signal and does not handle it, the process does not stop.  Stopping the
process would probably not be very useful, since there is no shell
program that will notice it stop and allow the user to continue it.
What happens instead depends on the operating system you are using.
Some systems may do nothing; others may deliver another signal instead,
such as @code{SIGKILL} or @code{SIGHUP}.  In the GNU system, the process
dies with @code{SIGKILL}; this avoids the problem of many stopped,
orphaned processes lying around the system.

@ignore
On the GNU system, it is possible to reattach to the orphaned process
group and continue it, so stop signals do stop the process as usual on
a GNU system unless you have requested POSIX compatibility ``till it
hurts.''
@end ignore

@node Operation Error Signals
@subsection Operation Error Signals

These signals are used to report various errors generated by an
operation done by the program.  They do not necessarily indicate a
programming error in the program, but an error that prevents an
operating system call from completing.  The default action for all of
them is to cause the process to terminate.

@comment signal.h
@comment POSIX.1
@deftypevr Macro int SIGPIPE
@cindex pipe signal
@cindex broken pipe signal
Broken pipe.  If you use pipes or FIFOs, you have to design your
application so that one process opens the pipe for reading before
another starts writing.  If the reading process never starts, or
terminates unexpectedly, writing to the pipe or FIFO raises a
@code{SIGPIPE} signal.  If @code{SIGPIPE} is blocked, handled or
ignored, the offending call fails with @code{EPIPE} instead.

Pipes and FIFO special files are discussed in more detail in @ref{Pipes
and FIFOs}.

Another cause of @code{SIGPIPE} is when you try to output to a socket
that isn't connected.  @xref{Sending Data}.
@end deftypevr

@comment signal.h
@comment GNU
@deftypevr Macro int SIGLOST
@cindex lost resource signal
Resource lost.  This signal is generated when you have an advisory lock
on an NFS file, and the NFS server reboots and forgets about your lock.

In the GNU system, @code{SIGLOST} is generated when any server program
dies unexpectedly.  It is usually fine to ignore the signal; whatever
call was made to the server that died just returns an error.
@end deftypevr

@comment signal.h
@comment BSD
@deftypevr Macro int SIGXCPU
CPU time limit exceeded.  This signal is generated when the process
exceeds its soft resource limit on CPU time.  @xref{Limits on Resources}.
@end deftypevr

@comment signal.h
@comment BSD
@deftypevr Macro int SIGXFSZ
File size limit exceeded.  This signal is generated when the process
attempts to extend a file so it exceeds the process's soft resource
limit on file size.  @xref{Limits on Resources}.
@end deftypevr

@node Miscellaneous Signals
@subsection Miscellaneous Signals

These signals are used for various other purposes.  In general, they
will not affect your program unless it explicitly uses them for something.

@comment signal.h
@comment POSIX.1
@deftypevr Macro int SIGUSR1
@comment signal.h
@comment POSIX.1
@deftypevrx Macro int SIGUSR2
@cindex user signals
The @code{SIGUSR1} and @code{SIGUSR2} signals are set aside for you to
use any way you want.  They're useful for simple interprocess
communication, if you write a signal handler for them in the program
that receives the signal.

There is an example showing the use of @code{SIGUSR1} and @code{SIGUSR2}
in @ref{Signaling Another Process}.

The default action is to terminate the process.
@end deftypevr

@comment signal.h
@comment BSD
@deftypevr Macro int SIGWINCH
Window size change.  This is generated on some systems (including GNU)
when the terminal driver's record of the number of rows and columns on
the screen is changed.  The default action is to ignore it.

If a program does full-screen display, it should handle @code{SIGWINCH}.
When the signal arrives, it should fetch the new screen size and
reformat its display accordingly.
@end deftypevr

@comment signal.h
@comment BSD
@deftypevr Macro int SIGINFO
Information request.  In 4.4 BSD and the GNU system, this signal is sent
to all the processes in the foreground process group of the controlling
terminal when the user types the STATUS character in canonical mode;
@pxref{Signal Characters}.

If the process is the leader of the process group, the default action is
to print some status information about the system and what the process
is doing.  Otherwise the default is to do nothing.
@end deftypevr

@node Signal Messages
@subsection Signal Messages
@cindex signal messages

We mentioned above that the shell prints a message describing the signal
that terminated a child process.  The clean way to print a message
describing a signal is to use the functions @code{strsignal} and
@code{psignal}.  These functions use a signal number to specify which
kind of signal to describe.  The signal number may come from the
termination status of a child process (@pxref{Process Completion}) or it
may come from a signal handler in the same process.

@comment string.h
@comment GNU
@deftypefun {char *} strsignal (int @var{signum})
This function returns a pointer to a statically-allocated string
containing a message describing the signal @var{signum}.  You
should not modify the contents of this string; and, since it can be
rewritten on subsequent calls, you should save a copy of it if you need
to reference it later.

@pindex string.h
This function is a GNU extension, declared in the header file
@file{string.h}.
@end deftypefun

@comment signal.h
@comment BSD
@deftypefun void psignal (int @var{signum}, const char *@var{message})
This function prints a message describing the signal @var{signum} to the
standard error output stream @code{stderr}; see @ref{Standard Streams}.

If you call @code{psignal} with a @var{message} that is either a null
pointer or an empty string, @code{psignal} just prints the message
corresponding to @var{signum}, adding a trailing newline.

If you supply a non-null @var{message} argument, then @code{psignal}
prefixes its output with this string.  It adds a colon and a space
character to separate the @var{message} from the string corresponding
to @var{signum}.

@pindex stdio.h
This function is a BSD feature, declared in the header file @file{signal.h}.
@end deftypefun

@vindex sys_siglist
There is also an array @code{sys_siglist} which contains the messages
for the various signal codes.  This array exists on BSD systems, unlike
@code{strsignal}.

@node Signal Actions
@section Specifying Signal Actions
@cindex signal actions
@cindex establishing a handler

The simplest way to change the action for a signal is to use the
@code{signal} function.  You can specify a built-in action (such as to
ignore the signal), or you can @dfn{establish a handler}.

The GNU library also implements the more versatile @code{sigaction}
facility.  This section describes both facilities and gives suggestions
on which to use when.

@menu
* Basic Signal Handling::       The simple @code{signal} function.
* Advanced Signal Handling::    The more powerful @code{sigaction} function.
* Signal and Sigaction::        How those two functions interact.
* Sigaction Function Example::  An example of using the sigaction function.
* Flags for Sigaction::         Specifying options for signal handling.
* Initial Signal Actions::      How programs inherit signal actions.
@end menu

@node Basic Signal Handling
@subsection Basic Signal Handling
@cindex @code{signal} function

The @code{signal} function provides a simple interface for establishing
an action for a particular signal.  The function and associated macros
are declared in the header file @file{signal.h}.
@pindex signal.h

@comment signal.h
@comment GNU
@deftp {Data Type} sighandler_t
This is the type of signal handler functions.  Signal handlers take one
integer argument specifying the signal number, and have return type
@code{void}.  So, you should define handler functions like this:

@smallexample
void @var{handler} (int @code{signum}) @{ @dots{} @}
@end smallexample

The name @code{sighandler_t} for this data type is a GNU extension.
@end deftp

@comment signal.h
@comment ISO
@deftypefun sighandler_t signal (int @var{signum}, sighandler_t @var{action})
The @code{signal} function establishes @var{action} as the action for
the signal @var{signum}.

The first argument, @var{signum}, identifies the signal whose behavior
you want to control, and should be a signal number.  The proper way to
specify a signal number is with one of the symbolic signal names
described in @ref{Standard Signals}---don't use an explicit number, because
the numerical code for a given kind of signal may vary from operating
system to operating system.

The second argument, @var{action}, specifies the action to use for the
signal @var{signum}.  This can be one of the following:

@table @code
@item SIG_DFL
@vindex SIG_DFL
@cindex default action for a signal
@code{SIG_DFL} specifies the default action for the particular signal.
The default actions for various kinds of signals are stated in
@ref{Standard Signals}.

@item SIG_IGN
@vindex SIG_IGN
@cindex ignore action for a signal
@code{SIG_IGN} specifies that the signal should be ignored.

Your program generally should not ignore signals that represent serious
events or that are normally used to request termination.  You cannot
ignore the @code{SIGKILL} or @code{SIGSTOP} signals at all.  You can
ignore program error signals like @code{SIGSEGV}, but ignoring the error
won't enable the program to continue executing meaningfully.  Ignoring
user requests such as @code{SIGINT}, @code{SIGQUIT}, and @code{SIGTSTP}
is unfriendly.

When you do not wish signals to be delivered during a certain part of
the program, the thing to do is to block them, not ignore them.
@xref{Blocking Signals}.

@item @var{handler}
Supply the address of a handler function in your program, to specify
running this handler as the way to deliver the signal.

For more information about defining signal handler functions,
see @ref{Defining Handlers}.
@end table

If you set the action for a signal to @code{SIG_IGN}, or if you set it
to @code{SIG_DFL} and the default action is to ignore that signal, then
any pending signals of that type are discarded (even if they are
blocked).  Discarding the pending signals means that they will never be
delivered, not even if you subsequently specify another action and
unblock this kind of signal.

The @code{signal} function returns the action that was previously in
effect for the specified @var{signum}.  You can save this value and
restore it later by calling @code{signal} again.

If @code{signal} can't honor the request, it returns @code{SIG_ERR}
instead.  The following @code{errno} error conditions are defined for
this function:

@table @code
@item EINVAL
You specified an invalid @var{signum}; or you tried to ignore or provide
a handler for @code{SIGKILL} or @code{SIGSTOP}.
@end table
@end deftypefun

@strong{Compatibility Note:} A problem when working with the
@code{signal} function is that it has a different semantic on BSD and
SVID system.  The difference is that on SVID systems the signal handler
is deinstalled after an signal was delivered.  On BSD systems the
handler must be explicitly deinstalled.  In the GNU C Library we use the
BSD version by default.  To use the SVID version you can either use the
function @code{sysv_signal} (see below) or use the @code{_XOPEN_SOURCE}
feature select macro (@pxref{Feature Test Macros}).  Generally it should
be avoided to use this functions due to the compatibility problems.  It
is better to use @code{sigaction} if it is available since the results
are much more reliable.

Here is a simple example of setting up a handler to delete temporary
files when certain fatal signals happen:

@smallexample
#include <signal.h>

void
termination_handler (int signum)
@{
  struct temp_file *p;

  for (p = temp_file_list; p; p = p->next)
    unlink (p->name);
@}

int
main (void)
@{
  @dots{}
  if (signal (SIGINT, termination_handler) == SIG_IGN)
    signal (SIGINT, SIG_IGN);
  if (signal (SIGHUP, termination_handler) == SIG_IGN)
    signal (SIGHUP, SIG_IGN);
  if (signal (SIGTERM, termination_handler) == SIG_IGN)
    signal (SIGTERM, SIG_IGN);
  @dots{}
@}
@end smallexample

@noindent
Note how if a given signal was previously set to be ignored, this code
avoids altering that setting.  This is because non-job-control shells
often ignore certain signals when starting children, and it is important
for the children to respect this.

We do not handle @code{SIGQUIT} or the program error signals in this
example because these are designed to provide information for debugging
(a core dump), and the temporary files may give useful information.

@comment signal.h
@comment GNU
@deftypefun sighandler_t sysv_signal (int @var{signum}, sighandler_t @var{action})
The @code{sysv_signal} implements the behaviour of the standard
@code{signal} function as found on SVID systems.  The difference to BSD
systems is that the handler is deinstalled after a delivery of a signal.

@strong{Compatibility Note:} As said above for @code{signal}, this
function should be avoided when possible.  @code{sigaction} is the
preferred method.
@end deftypefun

@comment signal.h
@comment SVID
@deftypefun sighandler_t ssignal (int @var{signum}, sighandler_t @var{action})
The @code{ssignal} function does the same thing as @code{signal}; it is
provided only for compatibility with SVID.
@end deftypefun

@comment signal.h
@comment ISO
@deftypevr Macro sighandler_t SIG_ERR
The value of this macro is used as the return value from @code{signal}
to indicate an error.
@end deftypevr

@ignore
@comment RMS says that ``we don't do this''.
Implementations might define additional macros for built-in signal
actions that are suitable as a @var{action} argument to @code{signal},
besides @code{SIG_IGN} and @code{SIG_DFL}.  Identifiers whose names
begin with @samp{SIG_} followed by an uppercase letter are reserved for
this purpose.
@end ignore


@node Advanced Signal Handling
@subsection Advanced Signal Handling
@cindex @code{sigaction} function

The @code{sigaction} function has the same basic effect as
@code{signal}: to specify how a signal should be handled by the process.
However, @code{sigaction} offers more control, at the expense of more
complexity.  In particular, @code{sigaction} allows you to specify
additional flags to control when the signal is generated and how the
handler is invoked.

The @code{sigaction} function is declared in @file{signal.h}.
@pindex signal.h

@comment signal.h
@comment POSIX.1
@deftp {Data Type} {struct sigaction}
Structures of type @code{struct sigaction} are used in the
@code{sigaction} function to specify all the information about how to
handle a particular signal.  This structure contains at least the
following members:

@table @code
@item sighandler_t sa_handler
This is used in the same way as the @var{action} argument to the
@code{signal} function.  The value can be @code{SIG_DFL},
@code{SIG_IGN}, or a function pointer.  @xref{Basic Signal Handling}.

@item sigset_t sa_mask
This specifies a set of signals to be blocked while the handler runs.
Blocking is explained in @ref{Blocking for Handler}.  Note that the
signal that was delivered is automatically blocked by default before its
handler is started; this is true regardless of the value in
@code{sa_mask}.  If you want that signal not to be blocked within its
handler, you must write code in the handler to unblock it.

@item int sa_flags
This specifies various flags which can affect the behavior of
the signal.  These are described in more detail in @ref{Flags for Sigaction}.
@end table
@end deftp

@comment signal.h
@comment POSIX.1
@deftypefun int sigaction (int @var{signum}, const struct sigaction *@var{action}, struct sigaction *@var{old-action})
The @var{action} argument is used to set up a new action for the signal
@var{signum}, while the @var{old-action} argument is used to return
information about the action previously associated with this symbol.
(In other words, @var{old-action} has the same purpose as the
@code{signal} function's return value---you can check to see what the
old action in effect for the signal was, and restore it later if you
want.)

Either @var{action} or @var{old-action} can be a null pointer.  If
@var{old-action} is a null pointer, this simply suppresses the return
of information about the old action.  If @var{action} is a null pointer,
the action associated with the signal @var{signum} is unchanged; this
allows you to inquire about how a signal is being handled without changing
that handling.

The return value from @code{sigaction} is zero if it succeeds, and
@code{-1} on failure.  The following @code{errno} error conditions are
defined for this function:

@table @code
@item EINVAL
The @var{signum} argument is not valid, or you are trying to
trap or ignore @code{SIGKILL} or @code{SIGSTOP}.
@end table
@end deftypefun

@node Signal and Sigaction
@subsection Interaction of @code{signal} and @code{sigaction}

It's possible to use both the @code{signal} and @code{sigaction}
functions within a single program, but you have to be careful because
they can interact in slightly strange ways.

The @code{sigaction} function specifies more information than the
@code{signal} function, so the return value from @code{signal} cannot
express the full range of @code{sigaction} possibilities.  Therefore, if
you use @code{signal} to save and later reestablish an action, it may
not be able to reestablish properly a handler that was established with
@code{sigaction}.

To avoid having problems as a result, always use @code{sigaction} to
save and restore a handler if your program uses @code{sigaction} at all.
Since @code{sigaction} is more general, it can properly save and
reestablish any action, regardless of whether it was established
originally with @code{signal} or @code{sigaction}.

On some systems if you establish an action with @code{signal} and then
examine it with @code{sigaction}, the handler address that you get may
not be the same as what you specified with @code{signal}.  It may not
even be suitable for use as an action argument with @code{signal}.  But
you can rely on using it as an argument to @code{sigaction}.  This
problem never happens on the GNU system.

So, you're better off using one or the other of the mechanisms
consistently within a single program.

@strong{Portability Note:} The basic @code{signal} function is a feature
of @w{ISO C}, while @code{sigaction} is part of the POSIX.1 standard.  If
you are concerned about portability to non-POSIX systems, then you
should use the @code{signal} function instead.

@node Sigaction Function Example
@subsection @code{sigaction} Function Example

In @ref{Basic Signal Handling}, we gave an example of establishing a
simple handler for termination signals using @code{signal}.  Here is an
equivalent example using @code{sigaction}:

@smallexample
#include <signal.h>

void
termination_handler (int signum)
@{
  struct temp_file *p;

  for (p = temp_file_list; p; p = p->next)
    unlink (p->name);
@}

int
main (void)
@{
  @dots{}
  struct sigaction new_action, old_action;

  /* @r{Set up the structure to specify the new action.} */
  new_action.sa_handler = termination_handler;
  sigemptyset (&new_action.sa_mask);
  new_action.sa_flags = 0;

  sigaction (SIGINT, NULL, &old_action);
  if (old_action.sa_handler != SIG_IGN)
    sigaction (SIGINT, &new_action, NULL);
  sigaction (SIGHUP, NULL, &old_action);
  if (old_action.sa_handler != SIG_IGN)
    sigaction (SIGHUP, &new_action, NULL);
  sigaction (SIGTERM, NULL, &old_action);
  if (old_action.sa_handler != SIG_IGN)
    sigaction (SIGTERM, &new_action, NULL);
  @dots{}
@}
@end smallexample

The program just loads the @code{new_action} structure with the desired
parameters and passes it in the @code{sigaction} call.  The usage of
@code{sigemptyset} is described later; see @ref{Blocking Signals}.

As in the example using @code{signal}, we avoid handling signals
previously set to be ignored.  Here we can avoid altering the signal
handler even momentarily, by using the feature of @code{sigaction} that
lets us examine the current action without specifying a new one.

Here is another example.  It retrieves information about the current
action for @code{SIGINT} without changing that action.

@smallexample
struct sigaction query_action;

if (sigaction (SIGINT, NULL, &query_action) < 0)
  /* @r{@code{sigaction} returns -1 in case of error.} */
else if (query_action.sa_handler == SIG_DFL)
  /* @r{@code{SIGINT} is handled in the default, fatal manner.} */
else if (query_action.sa_handler == SIG_IGN)
  /* @r{@code{SIGINT} is ignored.} */
else
  /* @r{A programmer-defined signal handler is in effect.} */
@end smallexample

@node Flags for Sigaction
@subsection Flags for @code{sigaction}
@cindex signal flags
@cindex flags for @code{sigaction}
@cindex @code{sigaction} flags

The @code{sa_flags} member of the @code{sigaction} structure is a
catch-all for special features.  Most of the time, @code{SA_RESTART} is
a good value to use for this field.

The value of @code{sa_flags} is interpreted as a bit mask.  Thus, you
should choose the flags you want to set, @sc{or} those flags together,
and store the result in the @code{sa_flags} member of your
@code{sigaction} structure.

Each signal number has its own set of flags.  Each call to
@code{sigaction} affects one particular signal number, and the flags
that you specify apply only to that particular signal.

In the GNU C library, establishing a handler with @code{signal} sets all
the flags to zero except for @code{SA_RESTART}, whose value depends on
the settings you have made with @code{siginterrupt}.  @xref{Interrupted
Primitives}, to see what this is about.

@pindex signal.h
These macros are defined in the header file @file{signal.h}.

@comment signal.h
@comment POSIX.1
@deftypevr Macro int SA_NOCLDSTOP
This flag is meaningful only for the @code{SIGCHLD} signal.  When the
flag is set, the system delivers the signal for a terminated child
process but not for one that is stopped.  By default, @code{SIGCHLD} is
delivered for both terminated children and stopped children.

Setting this flag for a signal other than @code{SIGCHLD} has no effect.
@end deftypevr

@comment signal.h
@comment BSD
@deftypevr Macro int SA_ONSTACK
If this flag is set for a particular signal number, the system uses the
signal stack when delivering that kind of signal.  @xref{Signal Stack}.
If a signal with this flag arrives and you have not set a signal stack,
the system terminates the program with @code{SIGILL}.
@end deftypevr

@comment signal.h
@comment BSD
@deftypevr Macro int SA_RESTART
This flag controls what happens when a signal is delivered during
certain primitives (such as @code{open}, @code{read} or @code{write}),
and the signal handler returns normally.  There are two alternatives:
the library function can resume, or it can return failure with error
code @code{EINTR}.

The choice is controlled by the @code{SA_RESTART} flag for the
particular kind of signal that was delivered.  If the flag is set,
returning from a handler resumes the library function.  If the flag is
clear, returning from a handler makes the function fail.
@xref{Interrupted Primitives}.
@end deftypevr

@node Initial Signal Actions
@subsection Initial Signal Actions
@cindex initial signal actions

When a new process is created (@pxref{Creating a Process}), it inherits
handling of signals from its parent process.  However, when you load a
new process image using the @code{exec} function (@pxref{Executing a
File}), any signals that you've defined your own handlers for revert to
their @code{SIG_DFL} handling.  (If you think about it a little, this
makes sense; the handler functions from the old program are specific to
that program, and aren't even present in the address space of the new
program image.)  Of course, the new program can establish its own
handlers.

When a program is run by a shell, the shell normally sets the initial
actions for the child process to @code{SIG_DFL} or @code{SIG_IGN}, as
appropriate.  It's a good idea to check to make sure that the shell has
not set up an initial action of @code{SIG_IGN} before you establish your
own signal handlers.

Here is an example of how to establish a handler for @code{SIGHUP}, but
not if @code{SIGHUP} is currently ignored:

@smallexample
@group
@dots{}
struct sigaction temp;

sigaction (SIGHUP, NULL, &temp);

if (temp.sa_handler != SIG_IGN)
  @{
    temp.sa_handler = handle_sighup;
    sigemptyset (&temp.sa_mask);
    sigaction (SIGHUP, &temp, NULL);
  @}
@end group
@end smallexample

@node Defining Handlers
@section Defining Signal Handlers
@cindex signal handler function

This section describes how to write a signal handler function that can
be established with the @code{signal} or @code{sigaction} functions.

A signal handler is just a function that you compile together with the
rest of the program.  Instead of directly invoking the function, you use
@code{signal} or @code{sigaction} to tell the operating system to call
it when a signal arrives.  This is known as @dfn{establishing} the
handler.  @xref{Signal Actions}.

There are two basic strategies you can use in signal handler functions:

@itemize @bullet
@item
You can have the handler function note that the signal arrived by
tweaking some global data structures, and then return normally.

@item
You can have the handler function terminate the program or transfer
control to a point where it can recover from the situation that caused
the signal.
@end itemize

You need to take special care in writing handler functions because they
can be called asynchronously.  That is, a handler might be called at any
point in the program, unpredictably.  If two signals arrive during a
very short interval, one handler can run within another.  This section
describes what your handler should do, and what you should avoid.

@menu
* Handler Returns::             Handlers that return normally, and what
                                 this means.
* Termination in Handler::      How handler functions terminate a program.
* Longjmp in Handler::          Nonlocal transfer of control out of a
                                 signal handler.
* Signals in Handler::          What happens when signals arrive while
                                 the handler is already occupied.
* Merged Signals::		When a second signal arrives before the
				 first is handled.
* Nonreentrancy::               Do not call any functions unless you know they
                                 are reentrant with respect to signals.
* Atomic Data Access::          A single handler can run in the middle of
                                 reading or writing a single object.
@end menu

@node Handler Returns
@subsection Signal Handlers that Return

Handlers which return normally are usually used for signals such as
@code{SIGALRM} and the I/O and interprocess communication signals.  But
a handler for @code{SIGINT} might also return normally after setting a
flag that tells the program to exit at a convenient time.

It is not safe to return normally from the handler for a program error
signal, because the behavior of the program when the handler function
returns is not defined after a program error.  @xref{Program Error
Signals}.

Handlers that return normally must modify some global variable in order
to have any effect.  Typically, the variable is one that is examined
periodically by the program during normal operation.  Its data type
should be @code{sig_atomic_t} for reasons described in @ref{Atomic
Data Access}.

Here is a simple example of such a program.  It executes the body of
the loop until it has noticed that a @code{SIGALRM} signal has arrived.
This technique is useful because it allows the iteration in progress
when the signal arrives to complete before the loop exits.

@smallexample
@include sigh1.c.texi
@end smallexample

@node Termination in Handler
@subsection Handlers That Terminate the Process

Handler functions that terminate the program are typically used to cause
orderly cleanup or recovery from program error signals and interactive
interrupts.

The cleanest way for a handler to terminate the process is to raise the
same signal that ran the handler in the first place.  Here is how to do
this:

@smallexample
volatile sig_atomic_t fatal_error_in_progress = 0;

void
fatal_error_signal (int sig)
@{
@group
  /* @r{Since this handler is established for more than one kind of signal, }
     @r{it might still get invoked recursively by delivery of some other kind}
     @r{of signal.  Use a static variable to keep track of that.} */
  if (fatal_error_in_progress)
    raise (sig);
  fatal_error_in_progress = 1;
@end group

@group
  /* @r{Now do the clean up actions:}
     @r{- reset terminal modes}
     @r{- kill child processes}
     @r{- remove lock files} */
  @dots{}
@end group

@group
  /* @r{Now reraise the signal.  Since the signal is blocked,}
     @r{it will receive its default handling, which is}
     @r{to terminate the process.  We could just call}
     @r{@code{exit} or @code{abort}, but reraising the signal}
     @r{sets the return status from the process correctly.} */
  raise (sig);
@}
@end group
@end smallexample

@node Longjmp in Handler
@subsection Nonlocal Control Transfer in Handlers
@cindex non-local exit, from signal handler

You can do a nonlocal transfer of control out of a signal handler using
the @code{setjmp} and @code{longjmp} facilities (@pxref{Non-Local
Exits}).

When the handler does a nonlocal control transfer, the part of the
program that was running will not continue.  If this part of the program
was in the middle of updating an important data structure, the data
structure will remain inconsistent.  Since the program does not
terminate, the inconsistency is likely to be noticed later on.

There are two ways to avoid this problem.  One is to block the signal
for the parts of the program that update important data structures.
Blocking the signal delays its delivery until it is unblocked, once the
critical updating is finished.  @xref{Blocking Signals}.

The other way to re-initialize the crucial data structures in the signal
handler, or make their values consistent.

Here is a rather schematic example showing the reinitialization of one
global variable.

@smallexample
@group
#include <signal.h>
#include <setjmp.h>

jmp_buf return_to_top_level;

volatile sig_atomic_t waiting_for_input;

void
handle_sigint (int signum)
@{
  /* @r{We may have been waiting for input when the signal arrived,}
     @r{but we are no longer waiting once we transfer control.} */
  waiting_for_input = 0;
  longjmp (return_to_top_level, 1);
@}
@end group

@group
int
main (void)
@{
  @dots{}
  signal (SIGINT, sigint_handler);
  @dots{}
  while (1) @{
    prepare_for_command ();
    if (setjmp (return_to_top_level) == 0)
      read_and_execute_command ();
  @}
@}
@end group

@group
/* @r{Imagine this is a subroutine used by various commands.} */
char *
read_data ()
@{
  if (input_from_terminal) @{
    waiting_for_input = 1;
    @dots{}
    waiting_for_input = 0;
  @} else @{
    @dots{}
  @}
@}
@end group
@end smallexample


@node Signals in Handler
@subsection Signals Arriving While a Handler Runs
@cindex race conditions, relating to signals

What happens if another signal arrives while your signal handler
function is running?

When the handler for a particular signal is invoked, that signal is
automatically blocked until the handler returns.  That means that if two
signals of the same kind arrive close together, the second one will be
held until the first has been handled.  (The handler can explicitly
unblock the signal using @code{sigprocmask}, if you want to allow more
signals of this type to arrive; see @ref{Process Signal Mask}.)

However, your handler can still be interrupted by delivery of another
kind of signal.  To avoid this, you can use the @code{sa_mask} member of
the action structure passed to @code{sigaction} to explicitly specify
which signals should be blocked while the signal handler runs.  These
signals are in addition to the signal for which the handler was invoked,
and any other signals that are normally blocked by the process.
@xref{Blocking for Handler}.

When the handler returns, the set of blocked signals is restored to the
value it had before the handler ran.  So using @code{sigprocmask} inside
the handler only affects what signals can arrive during the execution of
the handler itself, not what signals can arrive once the handler returns.

@strong{Portability Note:} Always use @code{sigaction} to establish a
handler for a signal that you expect to receive asynchronously, if you
want your program to work properly on System V Unix.  On this system,
the handling of a signal whose handler was established with
@code{signal} automatically sets the signal's action back to
@code{SIG_DFL}, and the handler must re-establish itself each time it
runs.  This practice, while inconvenient, does work when signals cannot
arrive in succession.  However, if another signal can arrive right away,
it may arrive before the handler can re-establish itself.  Then the
second signal would receive the default handling, which could terminate
the process.

@node Merged Signals
@subsection Signals Close Together Merge into One
@cindex handling multiple signals
@cindex successive signals
@cindex merging of signals

If multiple signals of the same type are delivered to your process
before your signal handler has a chance to be invoked at all, the
handler may only be invoked once, as if only a single signal had
arrived.  In effect, the signals merge into one.  This situation can
arise when the signal is blocked, or in a multiprocessing environment
where the system is busy running some other processes while the signals
are delivered.  This means, for example, that you cannot reliably use a
signal handler to count signals.  The only distinction you can reliably
make is whether at least one signal has arrived since a given time in
the past.

Here is an example of a handler for @code{SIGCHLD} that compensates for
the fact that the number of signals received may not equal the number of
child processes generate them.  It assumes that the program keeps track
of all the child processes with a chain of structures as follows:

@smallexample
struct process
@{
  struct process *next;
  /* @r{The process ID of this child.}  */
  int pid;
  /* @r{The descriptor of the pipe or pseudo terminal}
     @r{on which output comes from this child.}  */
  int input_descriptor;
  /* @r{Nonzero if this process has stopped or terminated.}  */
  sig_atomic_t have_status;
  /* @r{The status of this child; 0 if running,}
     @r{otherwise a status value from @code{waitpid}.}  */
  int status;
@};

struct process *process_list;
@end smallexample

This example also uses a flag to indicate whether signals have arrived
since some time in the past---whenever the program last cleared it to
zero.

@smallexample
/* @r{Nonzero means some child's status has changed}
   @r{so look at @code{process_list} for the details.}  */
int process_status_change;
@end smallexample

Here is the handler itself:

@smallexample
void
sigchld_handler (int signo)
@{
  int old_errno = errno;

  while (1) @{
    register int pid;
    int w;
    struct process *p;

    /* @r{Keep asking for a status until we get a definitive result.}  */
    do
      @{
        errno = 0;
        pid = waitpid (WAIT_ANY, &w, WNOHANG | WUNTRACED);
      @}
    while (pid <= 0 && errno == EINTR);

    if (pid <= 0) @{
      /* @r{A real failure means there are no more}
         @r{stopped or terminated child processes, so return.}  */
      errno = old_errno;
      return;
    @}

    /* @r{Find the process that signaled us, and record its status.}  */

    for (p = process_list; p; p = p->next)
      if (p->pid == pid) @{
        p->status = w;
        /* @r{Indicate that the @code{status} field}
           @r{has data to look at.  We do this only after storing it.}  */
        p->have_status = 1;

        /* @r{If process has terminated, stop waiting for its output.}  */
        if (WIFSIGNALED (w) || WIFEXITED (w))
          if (p->input_descriptor)
            FD_CLR (p->input_descriptor, &input_wait_mask);

        /* @r{The program should check this flag from time to time}
           @r{to see if there is any news in @code{process_list}.}  */
        ++process_status_change;
      @}

    /* @r{Loop around to handle all the processes}
       @r{that have something to tell us.}  */
  @}
@}
@end smallexample

Here is the proper way to check the flag @code{process_status_change}:

@smallexample
if (process_status_change) @{
  struct process *p;
  process_status_change = 0;
  for (p = process_list; p; p = p->next)
    if (p->have_status) @{
      @dots{} @r{Examine @code{p->status}} @dots{}
    @}
@}
@end smallexample

@noindent
It is vital to clear the flag before examining the list; otherwise, if a
signal were delivered just before the clearing of the flag, and after
the appropriate element of the process list had been checked, the status
change would go unnoticed until the next signal arrived to set the flag
again.  You could, of course, avoid this problem by blocking the signal
while scanning the list, but it is much more elegant to guarantee
correctness by doing things in the right order.

The loop which checks process status avoids examining @code{p->status}
until it sees that status has been validly stored.  This is to make sure
that the status cannot change in the middle of accessing it.  Once
@code{p->have_status} is set, it means that the child process is stopped
or terminated, and in either case, it cannot stop or terminate again
until the program has taken notice.  @xref{Atomic Usage}, for more
information about coping with interruptions during accessings of a
variable.

Here is another way you can test whether the handler has run since the
last time you checked.  This technique uses a counter which is never
changed outside the handler.  Instead of clearing the count, the program
remembers the previous value and sees whether it has changed since the
previous check.  The advantage of this method is that different parts of
the program can check independently, each part checking whether there
has been a signal since that part last checked.

@smallexample
sig_atomic_t process_status_change;

sig_atomic_t last_process_status_change;

@dots{}
@{
  sig_atomic_t prev = last_process_status_change;
  last_process_status_change = process_status_change;
  if (last_process_status_change != prev) @{
    struct process *p;
    for (p = process_list; p; p = p->next)
      if (p->have_status) @{
        @dots{} @r{Examine @code{p->status}} @dots{}
      @}
  @}
@}
@end smallexample

@node Nonreentrancy
@subsection Signal Handling and Nonreentrant Functions
@cindex restrictions on signal handler functions

Handler functions usually don't do very much.  The best practice is to
write a handler that does nothing but set an external variable that the
program checks regularly, and leave all serious work to the program.
This is best because the handler can be called at asynchronously, at
unpredictable times---perhaps in the middle of a primitive function, or
even between the beginning and the end of a C operator that requires
multiple instructions.  The data structures being manipulated might
therefore be in an inconsistent state when the handler function is
invoked.  Even copying one @code{int} variable into another can take two
instructions on most machines.

This means you have to be very careful about what you do in a signal
handler.

@itemize @bullet
@item
@cindex @code{volatile} declarations
If your handler needs to access any global variables from your program,
declare those variables @code{volatile}.  This tells the compiler that
the value of the variable might change asynchronously, and inhibits
certain optimizations that would be invalidated by such modifications.

@item
@cindex reentrant functions
If you call a function in the handler, make sure it is @dfn{reentrant}
with respect to signals, or else make sure that the signal cannot
interrupt a call to a related function.
@end itemize

A function can be non-reentrant if it uses memory that is not on the
stack.

@itemize @bullet
@item
If a function uses a static variable or a global variable, or a
dynamically-allocated object that it finds for itself, then it is
non-reentrant and any two calls to the function can interfere.

For example, suppose that the signal handler uses @code{gethostbyname}.
This function returns its value in a static object, reusing the same
object each time.  If the signal happens to arrive during a call to
@code{gethostbyname}, or even after one (while the program is still
using the value), it will clobber the value that the program asked for.

However, if the program does not use @code{gethostbyname} or any other
function that returns information in the same object, or if it always
blocks signals around each use, then you are safe.

There are a large number of library functions that return values in a
fixed object, always reusing the same object in this fashion, and all of
them cause the same problem.  The description of a function in this
manual always mentions this behavior.

@item
If a function uses and modifies an object that you supply, then it is
potentially non-reentrant; two calls can interfere if they use the same
object.

This case arises when you do I/O using streams.  Suppose that the
signal handler prints a message with @code{fprintf}.  Suppose that the
program was in the middle of an @code{fprintf} call using the same
stream when the signal was delivered.  Both the signal handler's message
and the program's data could be corrupted, because both calls operate on
the same data structure---the stream itself.

However, if you know that the stream that the handler uses cannot
possibly be used by the program at a time when signals can arrive, then
you are safe.  It is no problem if the program uses some other stream.

@item
On most systems, @code{malloc} and @code{free} are not reentrant,
because they use a static data structure which records what memory
blocks are free.  As a result, no library functions that allocate or
free memory are reentrant.  This includes functions that allocate space
to store a result.

The best way to avoid the need to allocate memory in a handler is to
allocate in advance space for signal handlers to use.

The best way to avoid freeing memory in a handler is to flag or record
the objects to be freed, and have the program check from time to time
whether anything is waiting to be freed.  But this must be done with
care, because placing an object on a chain is not atomic, and if it is
interrupted by another signal handler that does the same thing, you
could ``lose'' one of the objects.

@ignore
!!! not true
On the GNU system, @code{malloc} and @code{free} are safe to use in
signal handlers because they block signals.  As a result, the library
functions that allocate space for a result are also safe in signal
handlers.  The obstack allocation functions are safe as long as you
don't use the same obstack both inside and outside of a signal handler.
@end ignore

The relocating allocation functions (@pxref{Relocating Allocator})
are certainly not safe to use in a signal handler.

@item
Any function that modifies @code{errno} is non-reentrant, but you can
correct for this: in the handler, save the original value of
@code{errno} and restore it before returning normally.  This prevents
errors that occur within the signal handler from being confused with
errors from system calls at the point the program is interrupted to run
the handler.

This technique is generally applicable; if you want to call in a handler
a function that modifies a particular object in memory, you can make
this safe by saving and restoring that object.

@item
Merely reading from a memory object is safe provided that you can deal
with any of the values that might appear in the object at a time when
the signal can be delivered.  Keep in mind that assignment to some data
types requires more than one instruction, which means that the handler
could run ``in the middle of'' an assignment to the variable if its type
is not atomic.  @xref{Atomic Data Access}.

@item
Merely writing into a memory object is safe as long as a sudden change
in the value, at any time when the handler might run, will not disturb
anything.
@end itemize

@node Atomic Data Access
@subsection Atomic Data Access and Signal Handling

Whether the data in your application concerns atoms, or mere text, you
have to be careful about the fact that access to a single datum is not
necessarily @dfn{atomic}.  This means that it can take more than one
instruction to read or write a single object.  In such cases, a signal
handler might in the middle of reading or writing the object.

There are three ways you can cope with this problem.  You can use data
types that are always accessed atomically; you can carefully arrange
that nothing untoward happens if an access is interrupted, or you can
block all signals around any access that had better not be interrupted
(@pxref{Blocking Signals}).

@menu
* Non-atomic Example::		A program illustrating interrupted access.
* Types: Atomic Types.		Data types that guarantee no interruption.
* Usage: Atomic Usage.		Proving that interruption is harmless.
@end menu

@node Non-atomic Example
@subsubsection Problems with Non-Atomic Access

Here is an example which shows what can happen if a signal handler runs
in the middle of modifying a variable.  (Interrupting the reading of a
variable can also lead to paradoxical results, but here we only show
writing.)

@smallexample
#include <signal.h>
#include <stdio.h>

struct two_words @{ int a, b; @} memory;

void
handler(int signum)
@{
   printf ("%d,%d\n", memory.a, memory.b);
   alarm (1);
@}

@group
int
main (void)
@{
   static struct two_words zeros = @{ 0, 0 @}, ones = @{ 1, 1 @};
   signal (SIGALRM, handler);
   memory = zeros;
   alarm (1);
   while (1)
     @{
       memory = zeros;
       memory = ones;
     @}
@}
@end group
@end smallexample

This program fills @code{memory} with zeros, ones, zeros, ones,
alternating forever; meanwhile, once per second, the alarm signal handler
prints the current contents.  (Calling @code{printf} in the handler is
safe in this program because it is certainly not being called outside
the handler when the signal happens.)

Clearly, this program can print a pair of zeros or a pair of ones.  But
that's not all it can do!  On most machines, it takes several
instructions to store a new value in @code{memory}, and the value is
stored one word at a time.  If the signal is delivered in between these
instructions, the handler might find that @code{memory.a} is zero and
@code{memory.b} is one (or vice versa).

On some machines it may be possible to store a new value in
@code{memory} with just one instruction that cannot be interrupted.  On
these machines, the handler will always print two zeros or two ones.

@node Atomic Types
@subsubsection Atomic Types

To avoid uncertainty about interrupting access to a variable, you can
use a particular data type for which access is always atomic:
@code{sig_atomic_t}.  Reading and writing this data type is guaranteed
to happen in a single instruction, so there's no way for a handler to
run ``in the middle'' of an access.

The type @code{sig_atomic_t} is always an integer data type, but which
one it is, and how many bits it contains, may vary from machine to
machine.

@comment signal.h
@comment ISO
@deftp {Data Type} sig_atomic_t
This is an integer data type.  Objects of this type are always accessed
atomically.
@end deftp

In practice, you can assume that @code{int} and other integer types no
longer than @code{int} are atomic.  You can also assume that pointer
types are atomic; that is very convenient.  Both of these are true on
all of the machines that the GNU C library supports, and on all POSIX
systems we know of.
@c ??? This might fail on a 386 that uses 64-bit pointers.

@node Atomic Usage
@subsubsection Atomic Usage Patterns

Certain patterns of access avoid any problem even if an access is
interrupted.  For example, a flag which is set by the handler, and
tested and cleared by the main program from time to time, is always safe
even if access actually requires two instructions.  To show that this is
so, we must consider each access that could be interrupted, and show
that there is no problem if it is interrupted.

An interrupt in the middle of testing the flag is safe because either it's
recognized to be nonzero, in which case the precise value doesn't
matter, or it will be seen to be nonzero the next time it's tested.

An interrupt in the middle of clearing the flag is no problem because
either the value ends up zero, which is what happens if a signal comes
in just before the flag is cleared, or the value ends up nonzero, and
subsequent events occur as if the signal had come in just after the flag
was cleared.  As long as the code handles both of these cases properly,
it can also handle a signal in the middle of clearing the flag.  (This
is an example of the sort of reasoning you need to do to figure out
whether non-atomic usage is safe.)

Sometimes you can insure uninterrupted access to one object by
protecting its use with another object, perhaps one whose type
guarantees atomicity.  @xref{Merged Signals}, for an example.

@node Interrupted Primitives
@section Primitives Interrupted by Signals

A signal can arrive and be handled while an I/O primitive such as
@code{open} or @code{read} is waiting for an I/O device.  If the signal
handler returns, the system faces the question: what should happen next?

POSIX specifies one approach: make the primitive fail right away.  The
error code for this kind of failure is @code{EINTR}.  This is flexible,
but usually inconvenient.  Typically, POSIX applications that use signal
handlers must check for @code{EINTR} after each library function that
can return it, in order to try the call again.  Often programmers forget
to check, which is a common source of error.

The GNU library provides a convenient way to retry a call after a
temporary failure, with the macro @code{TEMP_FAILURE_RETRY}:

@comment unistd.h
@comment GNU
@defmac TEMP_FAILURE_RETRY (@var{expression})
This macro evaluates @var{expression} once.  If it fails and reports
error code @code{EINTR}, @code{TEMP_FAILURE_RETRY} evaluates it again,
and over and over until the result is not a temporary failure.

The value returned by @code{TEMP_FAILURE_RETRY} is whatever value
@var{expression} produced.
@end defmac

BSD avoids @code{EINTR} entirely and provides a more convenient
approach: to restart the interrupted primitive, instead of making it
fail.  If you choose this approach, you need not be concerned with
@code{EINTR}.

You can choose either approach with the GNU library.  If you use
@code{sigaction} to establish a signal handler, you can specify how that
handler should behave.  If you specify the @code{SA_RESTART} flag,
return from that handler will resume a primitive; otherwise, return from
that handler will cause @code{EINTR}.  @xref{Flags for Sigaction}.

Another way to specify the choice is with the @code{siginterrupt}
function.  @xref{BSD Handler}.

@c !!! not true now about _BSD_SOURCE
When you don't specify with @code{sigaction} or @code{siginterrupt} what
a particular handler should do, it uses a default choice.  The default
choice in the GNU library depends on the feature test macros you have
defined.  If you define @code{_BSD_SOURCE} or @code{_GNU_SOURCE} before
calling @code{signal}, the default is to resume primitives; otherwise,
the default is to make them fail with @code{EINTR}.  (The library
contains alternate versions of the @code{signal} function, and the
feature test macros determine which one you really call.)  @xref{Feature
Test Macros}.
@cindex EINTR, and restarting interrupted primitives
@cindex restarting interrupted primitives
@cindex interrupting primitives
@cindex primitives, interrupting
@c !!! want to have @cindex system calls @i{see} primitives [no page #]

The description of each primitive affected by this issue
lists @code{EINTR} among the error codes it can return.

There is one situation where resumption never happens no matter which
choice you make: when a data-transfer function such as @code{read} or
@code{write} is interrupted by a signal after transferring part of the
data.  In this case, the function returns the number of bytes already
transferred, indicating partial success.

This might at first appear to cause unreliable behavior on
record-oriented devices (including datagram sockets; @pxref{Datagrams}),
where splitting one @code{read} or @code{write} into two would read or
write two records.  Actually, there is no problem, because interruption
after a partial transfer cannot happen on such devices; they always
transfer an entire record in one burst, with no waiting once data
transfer has started.

@node Generating Signals
@section Generating Signals
@cindex sending signals
@cindex raising signals
@cindex signals, generating

Besides signals that are generated as a result of a hardware trap or
interrupt, your program can explicitly send signals to itself or to
another process.

@menu
* Signaling Yourself::          A process can send a signal to itself.
* Signaling Another Process::   Send a signal to another process.
* Permission for kill::         Permission for using @code{kill}.
* Kill Example::                Using @code{kill} for Communication.
@end menu

@node Signaling Yourself
@subsection Signaling Yourself

A process can send itself a signal with the @code{raise} function.  This
function is declared in @file{signal.h}.
@pindex signal.h

@comment signal.h
@comment ISO
@deftypefun int raise (int @var{signum})
The @code{raise} function sends the signal @var{signum} to the calling
process.  It returns zero if successful and a nonzero value if it fails.
About the only reason for failure would be if the value of @var{signum}
is invalid.
@end deftypefun

@comment signal.h
@comment SVID
@deftypefun int gsignal (int @var{signum})
The @code{gsignal} function does the same thing as @code{raise}; it is
provided only for compatibility with SVID.
@end deftypefun

One convenient use for @code{raise} is to reproduce the default behavior
of a signal that you have trapped.  For instance, suppose a user of your
program types the SUSP character (usually @kbd{C-z}; @pxref{Special
Characters}) to send it an interactive stop signal
(@code{SIGTSTP}), and you want to clean up some internal data buffers
before stopping.  You might set this up like this:

@comment RMS suggested getting rid of the handler for SIGCONT in this function.
@comment But that would require that the handler for SIGTSTP unblock the
@comment signal before doing the call to raise.  We haven't covered that
@comment topic yet, and I don't want to distract from the main point of
@comment the example with a digression to explain what is going on.  As
@comment the example is written, the signal that is raise'd will be delivered
@comment as soon as the SIGTSTP handler returns, which is fine.

@smallexample
#include <signal.h>

/* @r{When a stop signal arrives, set the action back to the default
   and then resend the signal after doing cleanup actions.} */

void
tstp_handler (int sig)
@{
  signal (SIGTSTP, SIG_DFL);
  /* @r{Do cleanup actions here.} */
  @dots{}
  raise (SIGTSTP);
@}

/* @r{When the process is continued again, restore the signal handler.} */

void
cont_handler (int sig)
@{
  signal (SIGCONT, cont_handler);
  signal (SIGTSTP, tstp_handler);
@}

@group
/* @r{Enable both handlers during program initialization.} */

int
main (void)
@{
  signal (SIGCONT, cont_handler);
  signal (SIGTSTP, tstp_handler);
  @dots{}
@}
@end group
@end smallexample

@strong{Portability note:} @code{raise} was invented by the @w{ISO C}
committee.  Older systems may not support it, so using @code{kill} may
be more portable.  @xref{Signaling Another Process}.

@node Signaling Another Process
@subsection Signaling Another Process

@cindex killing a process
The @code{kill} function can be used to send a signal to another process.
In spite of its name, it can be used for a lot of things other than
causing a process to terminate.  Some examples of situations where you
might want to send signals between processes are:

@itemize @bullet
@item
A parent process starts a child to perform a task---perhaps having the
child running an infinite loop---and then terminates the child when the
task is no longer needed.

@item
A process executes as part of a group, and needs to terminate or notify
the other processes in the group when an error or other event occurs.

@item
Two processes need to synchronize while working together.
@end itemize

This section assumes that you know a little bit about how processes
work.  For more information on this subject, see @ref{Processes}.

The @code{kill} function is declared in @file{signal.h}.
@pindex signal.h

@comment signal.h
@comment POSIX.1
@deftypefun int kill (pid_t @var{pid}, int @var{signum})
The @code{kill} function sends the signal @var{signum} to the process
or process group specified by @var{pid}.  Besides the signals listed in
@ref{Standard Signals}, @var{signum} can also have a value of zero to
check the validity of the @var{pid}.

The @var{pid} specifies the process or process group to receive the
signal:

@table @code
@item @var{pid} > 0
The process whose identifier is @var{pid}.

@item @var{pid} == 0
All processes in the same process group as the sender.

@item @var{pid} < -1
The process group whose identifier is @minus{}@var{pid}.

@item @var{pid} == -1
If the process is privileged, send the signal to all processes except
for some special system processes.  Otherwise, send the signal to all
processes with the same effective user ID.
@end table

A process can send a signal to itself with a call like @w{@code{kill
(getpid(), @var{signum})}}.  If @code{kill} is used by a process to send
a signal to itself, and the signal is not blocked, then @code{kill}
delivers at least one signal (which might be some other pending
unblocked signal instead of the signal @var{signum}) to that process
before it returns.

The return value from @code{kill} is zero if the signal can be sent
successfully.  Otherwise, no signal is sent, and a value of @code{-1} is
returned.  If @var{pid} specifies sending a signal to several processes,
@code{kill} succeeds if it can send the signal to at least one of them.
There's no way you can tell which of the processes got the signal
or whether all of them did.

The following @code{errno} error conditions are defined for this function:

@table @code
@item EINVAL
The @var{signum} argument is an invalid or unsupported number.

@item EPERM
You do not have the privilege to send a signal to the process or any of
the processes in the process group named by @var{pid}.

@item ESCRH
The @var{pid} argument does not refer to an existing process or group.
@end table
@end deftypefun

@comment signal.h
@comment BSD
@deftypefun int killpg (int @var{pgid}, int @var{signum})
This is similar to @code{kill}, but sends signal @var{signum} to the
process group @var{pgid}.  This function is provided for compatibility
with BSD; using @code{kill} to do this is more portable.
@end deftypefun

As a simple example of @code{kill}, the call @w{@code{kill (getpid (),
@var{sig})}} has the same effect as @w{@code{raise (@var{sig})}}.

@node Permission for kill
@subsection Permission for using @code{kill}

There are restrictions that prevent you from using @code{kill} to send
signals to any random process.  These are intended to prevent antisocial
behavior such as arbitrarily killing off processes belonging to another
user.  In typical use, @code{kill} is used to pass signals between
parent, child, and sibling processes, and in these situations you
normally do have permission to send signals.  The only common exception
is when you run a setuid program in a child process; if the program
changes its real UID as well as its effective UID, you may not have
permission to send a signal.  The @code{su} program does this.

Whether a process has permission to send a signal to another process
is determined by the user IDs of the two processes.  This concept is
discussed in detail in @ref{Process Persona}.

Generally, for a process to be able to send a signal to another process,
either the sending process must belong to a privileged user (like
@samp{root}), or the real or effective user ID of the sending process
must match the real or effective user ID of the receiving process.  If
the receiving process has changed its effective user ID from the
set-user-ID mode bit on its process image file, then the owner of the
process image file is used in place of its current effective user ID.
In some implementations, a parent process might be able to send signals
to a child process even if the user ID's don't match, and other
implementations might enforce other restrictions.

The @code{SIGCONT} signal is a special case.  It can be sent if the
sender is part of the same session as the receiver, regardless of
user IDs.

@node Kill Example
@subsection Using @code{kill} for Communication
@cindex interprocess communication, with signals
Here is a longer example showing how signals can be used for
interprocess communication.  This is what the @code{SIGUSR1} and
@code{SIGUSR2} signals are provided for.  Since these signals are fatal
by default, the process that is supposed to receive them must trap them
through @code{signal} or @code{sigaction}.

In this example, a parent process forks a child process and then waits
for the child to complete its initialization.  The child process tells
the parent when it is ready by sending it a @code{SIGUSR1} signal, using
the @code{kill} function.

@smallexample
@include sigusr.c.texi
@end smallexample

This example uses a busy wait, which is bad, because it wastes CPU
cycles that other programs could otherwise use.  It is better to ask the
system to wait until the signal arrives.  See the example in
@ref{Waiting for a Signal}.

@node Blocking Signals
@section Blocking Signals
@cindex blocking signals

Blocking a signal means telling the operating system to hold it and
deliver it later.  Generally, a program does not block signals
indefinitely---it might as well ignore them by setting their actions to
@code{SIG_IGN}.  But it is useful to block signals briefly, to prevent
them from interrupting sensitive operations.  For instance:

@itemize @bullet
@item
You can use the @code{sigprocmask} function to block signals while you
modify global variables that are also modified by the handlers for these
signals.

@item
You can set @code{sa_mask} in your @code{sigaction} call to block
certain signals while a particular signal handler runs.  This way, the
signal handler can run without being interrupted itself by signals.
@end itemize

@menu
* Why Block::                           The purpose of blocking signals.
* Signal Sets::                         How to specify which signals to
                                         block.
* Process Signal Mask::                 Blocking delivery of signals to your
				         process during normal execution.
* Testing for Delivery::                Blocking to Test for Delivery of
                                         a Signal.
* Blocking for Handler::                Blocking additional signals while a
				         handler is being run.
* Checking for Pending Signals::        Checking for Pending Signals
* Remembering a Signal::                How you can get almost the same
                                         effect as blocking a signal, by
                                         handling it and setting a flag
                                         to be tested later.
@end menu

@node Why Block
@subsection Why Blocking Signals is Useful

Temporary blocking of signals with @code{sigprocmask} gives you a way to
prevent interrupts during critical parts of your code.  If signals
arrive in that part of the program, they are delivered later, after you
unblock them.

One example where this is useful is for sharing data between a signal
handler and the rest of the program.  If the type of the data is not
@code{sig_atomic_t} (@pxref{Atomic Data Access}), then the signal
handler could run when the rest of the program has only half finished
reading or writing the data.  This would lead to confusing consequences.

To make the program reliable, you can prevent the signal handler from
running while the rest of the program is examining or modifying that
data---by blocking the appropriate signal around the parts of the
program that touch the data.

Blocking signals is also necessary when you want to perform a certain
action only if a signal has not arrived.  Suppose that the handler for
the signal sets a flag of type @code{sig_atomic_t}; you would like to
test the flag and perform the action if the flag is not set.  This is
unreliable.  Suppose the signal is delivered immediately after you test
the flag, but before the consequent action: then the program will
perform the action even though the signal has arrived.

The only way to test reliably for whether a signal has yet arrived is to
test while the signal is blocked.

@node Signal Sets
@subsection Signal Sets

All of the signal blocking functions use a data structure called a
@dfn{signal set} to specify what signals are affected.  Thus, every
activity involves two stages: creating the signal set, and then passing
it as an argument to a library function.
@cindex signal set

These facilities are declared in the header file @file{signal.h}.
@pindex signal.h

@comment signal.h
@comment POSIX.1
@deftp {Data Type} sigset_t
The @code{sigset_t} data type is used to represent a signal set.
Internally, it may be implemented as either an integer or structure
type.

For portability, use only the functions described in this section to
initialize, change, and retrieve information from @code{sigset_t}
objects---don't try to manipulate them directly.
@end deftp

There are two ways to initialize a signal set.  You can initially
specify it to be empty with @code{sigemptyset} and then add specified
signals individually.  Or you can specify it to be full with
@code{sigfillset} and then delete specified signals individually.

You must always initialize the signal set with one of these two
functions before using it in any other way.  Don't try to set all the
signals explicitly because the @code{sigset_t} object might include some
other information (like a version field) that needs to be initialized as
well.  (In addition, it's not wise to put into your program an
assumption that the system has no signals aside from the ones you know
about.)

@comment signal.h
@comment POSIX.1
@deftypefun int sigemptyset (sigset_t *@var{set})
This function initializes the signal set @var{set} to exclude all of the
defined signals.  It always returns @code{0}.
@end deftypefun

@comment signal.h
@comment POSIX.1
@deftypefun int sigfillset (sigset_t *@var{set})
This function initializes the signal set @var{set} to include
all of the defined signals.  Again, the return value is @code{0}.
@end deftypefun

@comment signal.h
@comment POSIX.1
@deftypefun int sigaddset (sigset_t *@var{set}, int @var{signum})
This function adds the signal @var{signum} to the signal set @var{set}.
All @code{sigaddset} does is modify @var{set}; it does not block or
unblock any signals.

The return value is @code{0} on success and @code{-1} on failure.
The following @code{errno} error condition is defined for this function:

@table @code
@item EINVAL
The @var{signum} argument doesn't specify a valid signal.
@end table
@end deftypefun

@comment signal.h
@comment POSIX.1
@deftypefun int sigdelset (sigset_t *@var{set}, int @var{signum})
This function removes the signal @var{signum} from the signal set
@var{set}.  All @code{sigdelset} does is modify @var{set}; it does not
block or unblock any signals.  The return value and error conditions are
the same as for @code{sigaddset}.
@end deftypefun

Finally, there is a function to test what signals are in a signal set:

@comment signal.h
@comment POSIX.1
@deftypefun int sigismember (const sigset_t *@var{set}, int @var{signum})
The @code{sigismember} function tests whether the signal @var{signum} is
a member of the signal set @var{set}.  It returns @code{1} if the signal
is in the set, @code{0} if not, and @code{-1} if there is an error.

The following @code{errno} error condition is defined for this function:

@table @code
@item EINVAL
The @var{signum} argument doesn't specify a valid signal.
@end table
@end deftypefun

@node Process Signal Mask
@subsection Process Signal Mask
@cindex signal mask
@cindex process signal mask

The collection of signals that are currently blocked is called the
@dfn{signal mask}.  Each process has its own signal mask.  When you
create a new process (@pxref{Creating a Process}), it inherits its
parent's mask.  You can block or unblock signals with total flexibility
by modifying the signal mask.

The prototype for the @code{sigprocmask} function is in @file{signal.h}.
@pindex signal.h

@comment signal.h
@comment POSIX.1
@deftypefun int sigprocmask (int @var{how}, const sigset_t *@var{set}, sigset_t *@var{oldset})
The @code{sigprocmask} function is used to examine or change the calling
process's signal mask.  The @var{how} argument determines how the signal
mask is changed, and must be one of the following values:

@table @code
@comment signal.h
@comment POSIX.1
@vindex SIG_BLOCK
@item SIG_BLOCK
Block the signals in @code{set}---add them to the existing mask.  In
other words, the new mask is the union of the existing mask and
@var{set}.

@comment signal.h
@comment POSIX.1
@vindex SIG_UNBLOCK
@item SIG_UNBLOCK
Unblock the signals in @var{set}---remove them from the existing mask.

@comment signal.h
@comment POSIX.1
@vindex SIG_SETMASK
@item SIG_SETMASK
Use @var{set} for the mask; ignore the previous value of the mask.
@end table

The last argument, @var{oldset}, is used to return information about the
old process signal mask.  If you just want to change the mask without
looking at it, pass a null pointer as the @var{oldset} argument.
Similarly, if you want to know what's in the mask without changing it,
pass a null pointer for @var{set} (in this case the @var{how} argument
is not significant).  The @var{oldset} argument is often used to
remember the previous signal mask in order to restore it later.  (Since
the signal mask is inherited over @code{fork} and @code{exec} calls, you
can't predict what its contents are when your program starts running.)

If invoking @code{sigprocmask} causes any pending signals to be
unblocked, at least one of those signals is delivered to the process
before @code{sigprocmask} returns.  The order in which pending signals
are delivered is not specified, but you can control the order explicitly
by making multiple @code{sigprocmask} calls to unblock various signals
one at a time.

The @code{sigprocmask} function returns @code{0} if successful, and @code{-1}
to indicate an error.  The following @code{errno} error conditions are
defined for this function:

@table @code
@item EINVAL
The @var{how} argument is invalid.
@end table

You can't block the @code{SIGKILL} and @code{SIGSTOP} signals, but
if the signal set includes these, @code{sigprocmask} just ignores
them instead of returning an error status.

Remember, too, that blocking program error signals such as @code{SIGFPE}
leads to undesirable results for signals generated by an actual program
error (as opposed to signals sent with @code{raise} or @code{kill}).
This is because your program may be too broken to be able to continue
executing to a point where the signal is unblocked again.
@xref{Program Error Signals}.
@end deftypefun

@node Testing for Delivery
@subsection Blocking to Test for Delivery of a Signal

Now for a simple example.  Suppose you establish a handler for
@code{SIGALRM} signals that sets a flag whenever a signal arrives, and
your main program checks this flag from time to time and then resets it.
You can prevent additional @code{SIGALRM} signals from arriving in the
meantime by wrapping the critical part of the code with calls to
@code{sigprocmask}, like this:

@smallexample
/* @r{This variable is set by the SIGALRM signal handler.} */
volatile sig_atomic_t flag = 0;

int
main (void)
@{
  sigset_t block_alarm;

  @dots{}

  /* @r{Initialize the signal mask.} */
  sigemptyset (&block_alarm);
  sigaddset (&block_alarm, SIGALRM);

@group
  while (1)
    @{
      /* @r{Check if a signal has arrived; if so, reset the flag.} */
      sigprocmask (SIG_BLOCK, &block_alarm, NULL);
      if (flag)
        @{
          @var{actions-if-not-arrived}
          flag = 0;
        @}
      sigprocmask (SIG_UNBLOCK, &block_alarm, NULL);

      @dots{}
    @}
@}
@end group
@end smallexample

@node Blocking for Handler
@subsection Blocking Signals for a Handler
@cindex blocking signals, in a handler

When a signal handler is invoked, you usually want it to be able to
finish without being interrupted by another signal.  From the moment the
handler starts until the moment it finishes, you must block signals that
might confuse it or corrupt its data.

When a handler function is invoked on a signal, that signal is
automatically blocked (in addition to any other signals that are already
in the process's signal mask) during the time the handler is running.
If you set up a handler for @code{SIGTSTP}, for instance, then the
arrival of that signal forces further @code{SIGTSTP} signals to wait
during the execution of the handler.

However, by default, other kinds of signals are not blocked; they can
arrive during handler execution.

The reliable way to block other kinds of signals during the execution of
the handler is to use the @code{sa_mask} member of the @code{sigaction}
structure.

Here is an example:

@smallexample
#include <signal.h>
#include <stddef.h>

void catch_stop ();

void
install_handler (void)
@{
  struct sigaction setup_action;
  sigset_t block_mask;

  sigemptyset (&block_mask);
  /* @r{Block other terminal-generated signals while handler runs.} */
  sigaddset (&block_mask, SIGINT);
  sigaddset (&block_mask, SIGQUIT);
  setup_action.sa_handler = catch_stop;
  setup_action.sa_mask = block_mask;
  setup_action.sa_flags = 0;
  sigaction (SIGTSTP, &setup_action, NULL);
@}
@end smallexample

This is more reliable than blocking the other signals explicitly in the
code for the handler.  If you block signals explicitly in the handler,
you can't avoid at least a short interval at the beginning of the
handler where they are not yet blocked.

You cannot remove signals from the process's current mask using this
mechanism.  However, you can make calls to @code{sigprocmask} within
your handler to block or unblock signals as you wish.

In any case, when the handler returns, the system restores the mask that
was in place before the handler was entered.  If any signals that become
unblocked by this restoration are pending, the process will receive
those signals immediately, before returning to the code that was
interrupted.

@node Checking for Pending Signals
@subsection Checking for Pending Signals
@cindex pending signals, checking for
@cindex blocked signals, checking for
@cindex checking for pending signals

You can find out which signals are pending at any time by calling
@code{sigpending}.  This function is declared in @file{signal.h}.
@pindex signal.h

@comment signal.h
@comment POSIX.1
@deftypefun int sigpending (sigset_t *@var{set})
The @code{sigpending} function stores information about pending signals
in @var{set}.  If there is a pending signal that is blocked from
delivery, then that signal is a member of the returned set.  (You can
test whether a particular signal is a member of this set using
@code{sigismember}; see @ref{Signal Sets}.)

The return value is @code{0} if successful, and @code{-1} on failure.
@end deftypefun

Testing whether a signal is pending is not often useful.  Testing when
that signal is not blocked is almost certainly bad design.

Here is an example.

@smallexample
#include <signal.h>
#include <stddef.h>

sigset_t base_mask, waiting_mask;

sigemptyset (&base_mask);
sigaddset (&base_mask, SIGINT);
sigaddset (&base_mask, SIGTSTP);

/* @r{Block user interrupts while doing other processing.} */
sigprocmask (SIG_SETMASK, &base_mask, NULL);
@dots{}

/* @r{After a while, check to see whether any signals are pending.} */
sigpending (&waiting_mask);
if (sigismember (&waiting_mask, SIGINT)) @{
  /* @r{User has tried to kill the process.} */
@}
else if (sigismember (&waiting_mask, SIGTSTP)) @{
  /* @r{User has tried to stop the process.} */
@}
@end smallexample

Remember that if there is a particular signal pending for your process,
additional signals of that same type that arrive in the meantime might
be discarded.  For example, if a @code{SIGINT} signal is pending when
another @code{SIGINT} signal arrives, your program will probably only
see one of them when you unblock this signal.

@strong{Portability Note:} The @code{sigpending} function is new in
POSIX.1.  Older systems have no equivalent facility.

@node Remembering a Signal
@subsection Remembering a Signal to Act On Later

Instead of blocking a signal using the library facilities, you can get
almost the same results by making the handler set a flag to be tested
later, when you ``unblock''.  Here is an example:

@smallexample
/* @r{If this flag is nonzero, don't handle the signal right away.} */
volatile sig_atomic_t signal_pending;

/* @r{This is nonzero if a signal arrived and was not handled.} */
volatile sig_atomic_t defer_signal;

void
handler (int signum)
@{
  if (defer_signal)
    signal_pending = signum;
  else
    @dots{} /* @r{``Really'' handle the signal.} */
@}

@dots{}

void
update_mumble (int frob)
@{
  /* @r{Prevent signals from having immediate effect.} */
  defer_signal++;
  /* @r{Now update @code{mumble}, without worrying about interruption.} */
  mumble.a = 1;
  mumble.b = hack ();
  mumble.c = frob;
  /* @r{We have updated @code{mumble}.  Handle any signal that came in.} */
  defer_signal--;
  if (defer_signal == 0 && signal_pending != 0)
    raise (signal_pending);
@}
@end smallexample

Note how the particular signal that arrives is stored in
@code{signal_pending}.  That way, we can handle several types of
inconvenient signals with the same mechanism.

We increment and decrement @code{defer_signal} so that nested critical
sections will work properly; thus, if @code{update_mumble} were called
with @code{signal_pending} already nonzero, signals would be deferred
not only within @code{update_mumble}, but also within the caller.  This
is also why we do not check @code{signal_pending} if @code{defer_signal}
is still nonzero.

The incrementing and decrementing of @code{defer_signal} require more
than one instruction; it is possible for a signal to happen in the
middle.  But that does not cause any problem.  If the signal happens
early enough to see the value from before the increment or decrement,
that is equivalent to a signal which came before the beginning of the
increment or decrement, which is a case that works properly.

It is absolutely vital to decrement @code{defer_signal} before testing
@code{signal_pending}, because this avoids a subtle bug.  If we did
these things in the other order, like this,

@smallexample
  if (defer_signal == 1 && signal_pending != 0)
    raise (signal_pending);
  defer_signal--;
@end smallexample

@noindent
then a signal arriving in between the @code{if} statement and the decrement
would be effectively ``lost'' for an indefinite amount of time.  The
handler would merely set @code{defer_signal}, but the program having
already tested this variable, it would not test the variable again.

@cindex timing error in signal handling
Bugs like these are called @dfn{timing errors}.  They are especially bad
because they happen only rarely and are nearly impossible to reproduce.
You can't expect to find them with a debugger as you would find a
reproducible bug.  So it is worth being especially careful to avoid
them.

(You would not be tempted to write the code in this order, given the use
of @code{defer_signal} as a counter which must be tested along with
@code{signal_pending}.  After all, testing for zero is cleaner than
testing for one.  But if you did not use @code{defer_signal} as a
counter, and gave it values of zero and one only, then either order
might seem equally simple.  This is a further advantage of using a
counter for @code{defer_signal}: it will reduce the chance you will
write the code in the wrong order and create a subtle bug.)

@node Waiting for a Signal
@section Waiting for a Signal
@cindex waiting for a signal
@cindex @code{pause} function

If your program is driven by external events, or uses signals for
synchronization, then when it has nothing to do it should probably wait
until a signal arrives.

@menu
* Using Pause::                 The simple way, using @code{pause}.
* Pause Problems::              Why the simple way is often not very good.
* Sigsuspend::                  Reliably waiting for a specific signal.
@end menu

@node Using Pause
@subsection Using @code{pause}

The simple way to wait until a signal arrives is to call @code{pause}.
Please read about its disadvantages, in the following section, before
you use it.

@comment unistd.h
@comment POSIX.1
@deftypefun int pause ()
The @code{pause} function suspends program execution until a signal
arrives whose action is either to execute a handler function, or to
terminate the process.

If the signal causes a handler function to be executed, then
@code{pause} returns.  This is considered an unsuccessful return (since
``successful'' behavior would be to suspend the program forever), so the
return value is @code{-1}.  Even if you specify that other primitives
should resume when a system handler returns (@pxref{Interrupted
Primitives}), this has no effect on @code{pause}; it always fails when a
signal is handled.

The following @code{errno} error conditions are defined for this function:

@table @code
@item EINTR
The function was interrupted by delivery of a signal.
@end table

If the signal causes program termination, @code{pause} doesn't return
(obviously).

This function is a cancelation point in multi-threaded programs.  This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time @code{pause} is
called.  If the thread gets canceled these resources stay allocated
until the program ends.  To avoid this calls to @code{pause} should be
protected using cancelation handlers.
@c ref pthread_cleanup_push / pthread_cleanup_pop

The @code{pause} function is declared in  @file{unistd.h}.
@end deftypefun

@node Pause Problems
@subsection Problems with @code{pause}

The simplicity of @code{pause} can conceal serious timing errors that
can make a program hang mysteriously.

It is safe to use @code{pause} if the real work of your program is done
by the signal handlers themselves, and the ``main program'' does nothing
but call @code{pause}.  Each time a signal is delivered, the handler
will do the next batch of work that is to be done, and then return, so
that the main loop of the program can call @code{pause} again.

You can't safely use @code{pause} to wait until one more signal arrives,
and then resume real work.  Even if you arrange for the signal handler
to cooperate by setting a flag, you still can't use @code{pause}
reliably.  Here is an example of this problem:

@smallexample
/* @r{@code{usr_interrupt} is set by the signal handler.}  */
if (!usr_interrupt)
  pause ();

/* @r{Do work once the signal arrives.}  */
@dots{}
@end smallexample

@noindent
This has a bug: the signal could arrive after the variable
@code{usr_interrupt} is checked, but before the call to @code{pause}.
If no further signals arrive, the process would never wake up again.

You can put an upper limit on the excess waiting by using @code{sleep}
in a loop, instead of using @code{pause}.  (@xref{Sleeping}, for more
about @code{sleep}.)  Here is what this looks like:

@smallexample
/* @r{@code{usr_interrupt} is set by the signal handler.}
while (!usr_interrupt)
  sleep (1);

/* @r{Do work once the signal arrives.}  */
@dots{}
@end smallexample

For some purposes, that is good enough.  But with a little more
complexity, you can wait reliably until a particular signal handler is
run, using @code{sigsuspend}.
@ifinfo
@xref{Sigsuspend}.
@end ifinfo

@node Sigsuspend
@subsection Using @code{sigsuspend}

The clean and reliable way to wait for a signal to arrive is to block it
and then use @code{sigsuspend}.  By using @code{sigsuspend} in a loop,
you can wait for certain kinds of signals, while letting other kinds of
signals be handled by their handlers.

@comment signal.h
@comment POSIX.1
@deftypefun int sigsuspend (const sigset_t *@var{set})
This function replaces the process's signal mask with @var{set} and then
suspends the process until a signal is delivered whose action is either
to terminate the process or invoke a signal handling function.  In other
words, the program is effectively suspended until one of the signals that
is not a member of @var{set} arrives.

If the process is woken up by deliver of a signal that invokes a handler
function, and the handler function returns, then @code{sigsuspend} also
returns.

The mask remains @var{set} only as long as @code{sigsuspend} is waiting.
The function @code{sigsuspend} always restores the previous signal mask
when it returns.

The return value and error conditions are the same as for @code{pause}.
@end deftypefun

With @code{sigsuspend}, you can replace the @code{pause} or @code{sleep}
loop in the previous section with something completely reliable:

@smallexample
sigset_t mask, oldmask;

@dots{}

/* @r{Set up the mask of signals to temporarily block.} */
sigemptyset (&mask);
sigaddset (&mask, SIGUSR1);

@dots{}

/* @r{Wait for a signal to arrive.} */
sigprocmask (SIG_BLOCK, &mask, &oldmask);
while (!usr_interrupt)
  sigsuspend (&oldmask);
sigprocmask (SIG_UNBLOCK, &mask, NULL);
@end smallexample

This last piece of code is a little tricky.  The key point to remember
here is that when @code{sigsuspend} returns, it resets the process's
signal mask to the original value, the value from before the call to
@code{sigsuspend}---in this case, the @code{SIGUSR1} signal is once
again blocked.  The second call to @code{sigprocmask} is
necessary to explicitly unblock this signal.

One other point: you may be wondering why the @code{while} loop is
necessary at all, since the program is apparently only waiting for one
@code{SIGUSR1} signal.  The answer is that the mask passed to
@code{sigsuspend} permits the process to be woken up by the delivery of
other kinds of signals, as well---for example, job control signals.  If
the process is woken up by a signal that doesn't set
@code{usr_interrupt}, it just suspends itself again until the ``right''
kind of signal eventually arrives.

This technique takes a few more lines of preparation, but that is needed
just once for each kind of wait criterion you want to use.  The code
that actually waits is just four lines.

@node Signal Stack
@section Using a Separate Signal Stack

A signal stack is a special area of memory to be used as the execution
stack during signal handlers.  It should be fairly large, to avoid any
danger that it will overflow in turn; the macro @code{SIGSTKSZ} is
defined to a canonical size for signal stacks.  You can use
@code{malloc} to allocate the space for the stack.  Then call
@code{sigaltstack} or @code{sigstack} to tell the system to use that
space for the signal stack.

You don't need to write signal handlers differently in order to use a
signal stack.  Switching from one stack to the other happens
automatically.  (Some non-GNU debuggers on some machines may get
confused if you examine a stack trace while a handler that uses the
signal stack is running.)

There are two interfaces for telling the system to use a separate signal
stack.  @code{sigstack} is the older interface, which comes from 4.2
BSD.  @code{sigaltstack} is the newer interface, and comes from 4.4
BSD.  The @code{sigaltstack} interface has the advantage that it does
not require your program to know which direction the stack grows, which
depends on the specific machine and operating system.

@comment signal.h
@comment BSD
@deftp {Data Type} {struct sigaltstack}
This structure describes a signal stack.  It contains the following members:

@table @code
@item void *ss_sp
This points to the base of the signal stack.

@item size_t ss_size
This is the size (in bytes) of the signal stack which @samp{ss_sp} points to.
You should set this to however much space you allocated for the stack.

There are two macros defined in @file{signal.h} that you should use in
calculating this size:

@vtable @code
@item SIGSTKSZ
This is the canonical size for a signal stack.  It is judged to be
sufficient for normal uses.

@item MINSIGSTKSZ
This is the amount of signal stack space the operating system needs just
to implement signal delivery.  The size of a signal stack @strong{must}
be greater than this.

For most cases, just using @code{SIGSTKSZ} for @code{ss_size} is
sufficient.  But if you know how much stack space your program's signal
handlers will need, you may want to use a different size.  In this case,
you should allocate @code{MINSIGSTKSZ} additional bytes for the signal
stack and increase @code{ss_size} accordingly.
@end vtable

@item int ss_flags
This field contains the bitwise @sc{or} of these flags:

@vtable @code
@item SA_DISABLE
This tells the system that it should not use the signal stack.

@item SA_ONSTACK
This is set by the system, and indicates that the signal stack is
currently in use.  If this bit is not set, then signals will be
delivered on the normal user stack.
@end vtable
@end table
@end deftp

@comment signal.h
@comment BSD
@deftypefun int sigaltstack (const struct sigaltstack *@var{stack}, struct sigaltstack *@var{oldstack})
The @code{sigaltstack} function specifies an alternate stack for use
during signal handling.  When a signal is received by the process and
its action indicates that the signal stack is used, the system arranges
a switch to the currently installed signal stack while the handler for
that signal is executed.

If @var{oldstack} is not a null pointer, information about the currently
installed signal stack is returned in the location it points to.  If
@var{stack} is not a null pointer, then this is installed as the new
stack for use by signal handlers.

The return value is @code{0} on success and @code{-1} on failure.  If
@code{sigaltstack} fails, it sets @code{errno} to one of these values:

@table @code
@item EINVAL
You tried to disable a stack that was in fact currently in use.

@item ENOMEM
The size of the alternate stack was too small.
It must be greater than @code{MINSIGSTKSZ}.
@end table
@end deftypefun

Here is the older @code{sigstack} interface.  You should use
@code{sigaltstack} instead on systems that have it.

@comment signal.h
@comment BSD
@deftp {Data Type} {struct sigstack}
This structure describes a signal stack.  It contains the following members:

@table @code
@item void *ss_sp
This is the stack pointer.  If the stack grows downwards on your
machine, this should point to the top of the area you allocated.  If the
stack grows upwards, it should point to the bottom.

@item int ss_onstack
This field is true if the process is currently using this stack.
@end table
@end deftp

@comment signal.h
@comment BSD
@deftypefun int sigstack (const struct sigstack *@var{stack}, struct sigstack *@var{oldstack})
The @code{sigstack} function specifies an alternate stack for use during
signal handling.  When a signal is received by the process and its
action indicates that the signal stack is used, the system arranges a
switch to the currently installed signal stack while the handler for
that signal is executed.

If @var{oldstack} is not a null pointer, information about the currently
installed signal stack is returned in the location it points to.  If
@var{stack} is not a null pointer, then this is installed as the new
stack for use by signal handlers.

The return value is @code{0} on success and @code{-1} on failure.
@end deftypefun

@node BSD Signal Handling
@section BSD Signal Handling

This section describes alternative signal handling functions derived
from BSD Unix.  These facilities were an advance, in their time; today,
they are mostly obsolete, and supported mainly for compatibility with
BSD Unix.

There are many similarities between the BSD and POSIX signal handling
facilities, because the POSIX facilities were inspired by the BSD
facilities.  Besides having different names for all the functions to
avoid conflicts, the main differences between the two are:

@itemize @bullet
@item
BSD Unix represents signal masks as an @code{int} bit mask, rather than
as a @code{sigset_t} object.

@item
The BSD facilities use a different default for whether an interrupted
primitive should fail or resume.  The POSIX facilities make system
calls fail unless you specify that they should resume.  With the BSD
facility, the default is to make system calls resume unless you say they
should fail.  @xref{Interrupted Primitives}.
@end itemize

The BSD facilities are declared in @file{signal.h}.
@pindex signal.h

@menu
* BSD Handler::                 BSD Function to Establish a Handler.
* Blocking in BSD::             BSD Functions for Blocking Signals.
@end menu

@node BSD Handler
@subsection BSD Function to Establish a Handler

@comment signal.h
@comment BSD
@deftp {Data Type} {struct sigvec}
This data type is the BSD equivalent of @code{struct sigaction}
(@pxref{Advanced Signal Handling}); it is used to specify signal actions
to the @code{sigvec} function.  It contains the following members:

@table @code
@item sighandler_t sv_handler
This is the handler function.

@item int sv_mask
This is the mask of additional signals to be blocked while the handler
function is being called.

@item int sv_flags
This is a bit mask used to specify various flags which affect the
behavior of the signal.  You can also refer to this field as
@code{sv_onstack}.
@end table
@end deftp

These symbolic constants can be used to provide values for the
@code{sv_flags} field of a @code{sigvec} structure.  This field is a bit
mask value, so you bitwise-OR the flags of interest to you together.

@comment signal.h
@comment BSD
@deftypevr Macro int SV_ONSTACK
If this bit is set in the @code{sv_flags} field of a @code{sigvec}
structure, it means to use the signal stack when delivering the signal.
@end deftypevr

@comment signal.h
@comment BSD
@deftypevr Macro int SV_INTERRUPT
If this bit is set in the @code{sv_flags} field of a @code{sigvec}
structure, it means that system calls interrupted by this kind of signal
should not be restarted if the handler returns; instead, the system
calls should return with a @code{EINTR} error status.  @xref{Interrupted
Primitives}.
@end deftypevr

@comment signal.h
@comment Sun
@deftypevr Macro int SV_RESETHAND
If this bit is set in the @code{sv_flags} field of a @code{sigvec}
structure, it means to reset the action for the signal back to
@code{SIG_DFL} when the signal is received.
@end deftypevr

@comment signal.h
@comment BSD
@deftypefun int sigvec (int @var{signum}, const struct sigvec *@var{action},struct sigvec *@var{old-action})
This function is the equivalent of @code{sigaction} (@pxref{Advanced Signal
Handling}); it installs the action @var{action} for the signal @var{signum},
returning information about the previous action in effect for that signal
in @var{old-action}.
@end deftypefun

@comment signal.h
@comment BSD
@deftypefun int siginterrupt (int @var{signum}, int @var{failflag})
This function specifies which approach to use when certain primitives
are interrupted by handling signal @var{signum}.  If @var{failflag} is
false, signal @var{signum} restarts primitives.  If @var{failflag} is
true, handling @var{signum} causes these primitives to fail with error
code @code{EINTR}.  @xref{Interrupted Primitives}.
@end deftypefun

@node Blocking in BSD
@subsection BSD Functions for Blocking Signals

@comment signal.h
@comment BSD
@deftypefn Macro int sigmask (int @var{signum})
This macro returns a signal mask that has the bit for signal @var{signum}
set.  You can bitwise-OR the results of several calls to @code{sigmask}
together to specify more than one signal.  For example,

@smallexample
(sigmask (SIGTSTP) | sigmask (SIGSTOP)
 | sigmask (SIGTTIN) | sigmask (SIGTTOU))
@end smallexample

@noindent
specifies a mask that includes all the job-control stop signals.
@end deftypefn

@comment signal.h
@comment BSD
@deftypefun int sigblock (int @var{mask})
This function is equivalent to @code{sigprocmask} (@pxref{Process Signal
Mask}) with a @var{how} argument of @code{SIG_BLOCK}: it adds the
signals specified by @var{mask} to the calling process's set of blocked
signals.  The return value is the previous set of blocked signals.
@end deftypefun

@comment signal.h
@comment BSD
@deftypefun int sigsetmask (int @var{mask})
This function equivalent to @code{sigprocmask} (@pxref{Process
Signal Mask}) with a @var{how} argument of @code{SIG_SETMASK}: it sets
the calling process's signal mask to @var{mask}.  The return value is
the previous set of blocked signals.
@end deftypefun

@comment signal.h
@comment BSD
@deftypefun int sigpause (int @var{mask})
This function is the equivalent of @code{sigsuspend} (@pxref{Waiting
for a Signal}):  it sets the calling process's signal mask to @var{mask},
and waits for a signal to arrive.  On return the previous set of blocked
signals is restored.
@end deftypefun