signal(2)

1signal(2)                     System Calls Manual                    signal(2)
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NAME

6       signal - ANSI C signal handling
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <signal.h>
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14       typedef void (*sighandler_t)(int);
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16       sighandler_t signal(int signum, sighandler_t handler);
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DESCRIPTION

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

44       signal() returns the previous value of the signal handler.  On failure,
45       it returns SIG_ERR, and errno is set to indicate the error.
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ERRORS

48       EINVAL signum is invalid.
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VERSIONS

51       The use of sighandler_t is a GNU extension, exposed if  _GNU_SOURCE  is
52       defined;  glibc  also  defines  (the  BSD-derived) sig_t if _BSD_SOURCE
53       (glibc 2.19 and earlier) or _DEFAULT_SOURCE (glibc 2.19 and  later)  is
54       defined.   Without  use  of such a type, the declaration of signal() is
55       the somewhat harder to read:
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57           void ( *signal(int signum, void (*handler)(int)) ) (int);
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59   Portability
60       The only portable use of signal() is to set a signal's  disposition  to
61       SIG_DFL  or  SIG_IGN.  The semantics when using signal() to establish a
62       signal handler vary across systems (and POSIX.1 explicitly permits this
63       variation); do not use it for this purpose.
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65       POSIX.1  solved  the portability mess by specifying sigaction(2), which
66       provides explicit control of the semantics when a signal handler is in‐
67       voked; use that interface instead of signal().
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STANDARDS

70       C11, POSIX.1-2008.
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HISTORY

73       C89, POSIX.1-2001.
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75       In the original UNIX systems, when a handler that was established using
76       signal() was invoked by the delivery of a signal,  the  disposition  of
77       the  signal would be reset to SIG_DFL, and the system did not block de‐
78       livery of further instances of the signal.  This is equivalent to call‐
79       ing sigaction(2) with the following flags:
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81           sa.sa_flags = SA_RESETHAND | SA_NODEFER;
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83       System V  also provides these semantics for signal().  This was bad be‐
84       cause the signal might be delivered again  before  the  handler  had  a
85       chance  to  reestablish  itself.   Furthermore, rapid deliveries of the
86       same signal could result in recursive invocations of the handler.
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88       BSD improved on this situation, but unfortunately also changed the  se‐
89       mantics  of  the  existing  signal() interface while doing so.  On BSD,
90       when a signal handler is invoked, the signal disposition is not  reset,
91       and  further  instances  of the signal are blocked from being delivered
92       while the handler is executing.  Furthermore, certain  blocking  system
93       calls  are  automatically  restarted if interrupted by a signal handler
94       (see signal(7)).  The BSD semantics are equivalent  to  calling  sigac‐
95       tion(2) with the following flags:
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97           sa.sa_flags = SA_RESTART;
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99       The situation on Linux is as follows:
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101       •  The kernel's signal() system call provides System V semantics.
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103       •  By default, in glibc 2 and later, the signal() wrapper function does
104          not invoke the kernel system call.  Instead, it  calls  sigaction(2)
105          using  flags  that  supply  BSD semantics.  This default behavior is
106          provided as long as  a  suitable  feature  test  macro  is  defined:
107          _BSD_SOURCE  on  glibc  2.19 and earlier or _DEFAULT_SOURCE in glibc
108          2.19 and later.  (By default, these macros  are  defined;  see  fea‐
109          ture_test_macros(7)  for  details.)  If such a feature test macro is
110          not defined, then signal() provides System V semantics.
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NOTES

113       The effects of signal() in a multithreaded process are unspecified.
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115       According to POSIX, the behavior of a process is undefined after it ig‐
116       nores  a  SIGFPE,  SIGILL,  or SIGSEGV signal that was not generated by
117       kill(2) or raise(3).  Integer division by zero  has  undefined  result.
118       On some architectures it will generate a SIGFPE signal.  (Also dividing
119       the most negative integer by -1 may generate  SIGFPE.)   Ignoring  this
120       signal might lead to an endless loop.
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122       See  sigaction(2)  for  details  on  what  happens when the disposition
123       SIGCHLD is set to SIG_IGN.
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125       See signal-safety(7) for a list of the async-signal-safe functions that
126       can be safely called from inside a signal handler.
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