1SIGNAL(2)                  Linux Programmer's Manual                 SIGNAL(2)
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3
4

NAME

6       signal - ANSI C signal handling
7

SYNOPSIS

9       #include <signal.h>
10
11       typedef void (*sighandler_t)(int);
12
13       sighandler_t signal(int signum, sighandler_t handler);
14

DESCRIPTION

16       WARNING:  the behavior of signal() varies across UNIX versions, and has
17       also varied historically across different versions of Linux.  Avoid its
18       use: use sigaction(2) instead.  See Portability below.
19
20       signal() sets the disposition of the signal signum to handler, which is
21       either SIG_IGN, SIG_DFL, or the address of a  programmer-defined  func‐
22       tion (a "signal handler").
23
24       If  the signal signum is delivered to the process, then one of the fol‐
25       lowing happens:
26
27       *  If the disposition is set to SIG_IGN, then the signal is ignored.
28
29       *  If the disposition is set to SIG_DFL, then the default action  asso‐
30          ciated with the signal (see signal(7)) occurs.
31
32       *  If  the disposition is set to a function, then first either the dis‐
33          position is reset to SIG_DFL, or the signal is blocked  (see  Porta‐
34          bility  below), and then handler is called with argument signum.  If
35          invocation of the handler caused the signal to be blocked, then  the
36          signal is unblocked upon return from the handler.
37
38       The signals SIGKILL and SIGSTOP cannot be caught or ignored.
39

RETURN VALUE

41       signal()  returns  the previous value of the signal handler On failure,
42       it returns SIG_ERR, and errno is set to indicate the error.
43

ERRORS

45       EINVAL signum is invalid.
46

CONFORMING TO

48       POSIX.1-2001, POSIX.1-2008, C89, C99.
49

NOTES

51       The effects of signal() in a multithreaded process are unspecified.
52
53       According to POSIX, the behavior of a process is undefined after it ig‐
54       nores  a  SIGFPE,  SIGILL,  or SIGSEGV signal that was not generated by
55       kill(2) or raise(3).  Integer division by zero  has  undefined  result.
56       On some architectures it will generate a SIGFPE signal.  (Also dividing
57       the most negative integer by -1 may generate  SIGFPE.)   Ignoring  this
58       signal might lead to an endless loop.
59
60       See  sigaction(2)  for  details  on  what  happens when the disposition
61       SIGCHLD is set to SIG_IGN.
62
63       See signal-safety(7) for a list of the async-signal-safe functions that
64       can be safely called from inside a signal handler.
65
66       The  use  of sighandler_t is a GNU extension, exposed if _GNU_SOURCE is
67       defined; glibc also defines  (the  BSD-derived)  sig_t  if  _BSD_SOURCE
68       (glibc  2.19  and earlier) or _DEFAULT_SOURCE (glibc 2.19 and later) is
69       defined.  Without use of such a type, the declaration  of  signal()  is
70       the somewhat harder to read:
71
72           void ( *signal(int signum, void (*handler)(int)) ) (int);
73
74   Portability
75       The  only  portable use of signal() is to set a signal's disposition to
76       SIG_DFL or SIG_IGN.  The semantics when using signal() to  establish  a
77       signal handler vary across systems (and POSIX.1 explicitly permits this
78       variation); do not use it for this purpose.
79
80       POSIX.1 solved the portability mess by specifying  sigaction(2),  which
81       provides explicit control of the semantics when a signal handler is in‐
82       voked; use that interface instead of signal().
83
84       In the original UNIX systems, when a handler that was established using
85       signal()  was  invoked  by the delivery of a signal, the disposition of
86       the signal would be reset to SIG_DFL, and the system did not block  de‐
87       livery of further instances of the signal.  This is equivalent to call‐
88       ing sigaction(2) with the following flags:
89
90           sa.sa_flags = SA_RESETHAND | SA_NODEFER;
91
92       System V also provides these semantics for signal().  This was bad  be‐
93       cause  the  signal  might  be  delivered again before the handler had a
94       chance to reestablish itself.  Furthermore,  rapid  deliveries  of  the
95       same signal could result in recursive invocations of the handler.
96
97       BSD  improved on this situation, but unfortunately also changed the se‐
98       mantics of the existing signal() interface while  doing  so.   On  BSD,
99       when  a signal handler is invoked, the signal disposition is not reset,
100       and further instances of the signal are blocked  from  being  delivered
101       while  the  handler is executing.  Furthermore, certain blocking system
102       calls are automatically restarted if interrupted by  a  signal  handler
103       (see  signal(7)).   The  BSD semantics are equivalent to calling sigac‐
104       tion(2) with the following flags:
105
106           sa.sa_flags = SA_RESTART;
107
108       The situation on Linux is as follows:
109
110       * The kernel's signal() system call provides System V semantics.
111
112       * By default, in glibc 2 and later, the signal() wrapper function  does
113         not  invoke  the  kernel system call.  Instead, it calls sigaction(2)
114         using flags that supply BSD semantics.  This default behavior is pro‐
115         vided   as  long  as  a  suitable  feature  test  macro  is  defined:
116         _BSD_SOURCE on glibc 2.19 and earlier  or  _DEFAULT_SOURCE  in  glibc
117         2.19  and  later.   (By  default,  these macros are defined; see fea‐
118         ture_test_macros(7) for details.)  If such a feature  test  macro  is
119         not defined, then signal() provides System V semantics.
120

SEE ALSO

122       kill(1),  alarm(2),  kill(2), pause(2), sigaction(2), signalfd(2), sig‐
123       pending(2), sigprocmask(2),  sigsuspend(2),  bsd_signal(3),  killpg(3),
124       raise(3),   siginterrupt(3),   sigqueue(3),   sigsetops(3),  sigvec(3),
125       sysv_signal(3), signal(7)
126

COLOPHON

128       This page is part of release 5.13 of the Linux  man-pages  project.   A
129       description  of  the project, information about reporting bugs, and the
130       latest    version    of    this    page,    can     be     found     at
131       https://www.kernel.org/doc/man-pages/.
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135Linux                             2021-03-22                         SIGNAL(2)
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