1SIGNAL(3P)                 POSIX Programmer's Manual                SIGNAL(3P)
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PROLOG

6       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
7       implementation of this interface may differ (consult the  corresponding
8       Linux  manual page for details of Linux behavior), or the interface may
9       not be implemented on Linux.
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11

NAME

13       signal — signal management
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SYNOPSIS

16       #include <signal.h>
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18       void (*signal(int sig, void (*func)(int)))(int);
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DESCRIPTION

21       The functionality described on this reference page is aligned with  the
22       ISO C  standard.  Any  conflict between the requirements described here
23       and the ISO C standard is unintentional. This  volume  of  POSIX.1‐2008
24       defers to the ISO C standard.
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26       Use of this function is unspecified in a multi-threaded process.
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28       The signal() function chooses one of three ways in which receipt of the
29       signal number sig is to be subsequently handled. If the value  of  func
30       is SIG_DFL, default handling for that signal shall occur.  If the value
31       of func is SIG_IGN, the signal shall be ignored.  Otherwise, the appli‐
32       cation  shall  ensure  that func points to a function to be called when
33       that signal occurs. An invocation of such a function because of a  sig‐
34       nal,  or  (recursively) of any further functions called by that invoca‐
35       tion (other than functions in the standard library), is called a ``sig‐
36       nal handler''.
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38       When  a signal occurs, and func points to a function, it is implementa‐
39       tion-defined whether the equivalent of a:
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41           signal(sig, SIG_DFL);
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43       is executed or the implementation prevents some  implementation-defined
44       set  of  signals (at least including sig) from occurring until the cur‐
45       rent signal handling has completed. (If the value of sig is SIGILL, the
46       implementation  may alternatively define that no action is taken.) Next
47       the equivalent of:
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49           (*func)(sig);
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51       is executed. If and when the function returns, if the value of sig  was
52       SIGFPE,  SIGILL,  or  SIGSEGV or any other implementation-defined value
53       corresponding to a computational exception, the behavior is  undefined.
54       Otherwise,  the  program  shall  resume  execution  at the point it was
55       interrupted. The ISO C standard places a  restriction  on  applications
56       relating  to the use of raise() from signal handlers.  This restriction
57       does not apply to POSIX applications, as POSIX.1‐2008 requires  raise()
58       to be async-signal-safe (see Section 2.4.3, Signal Actions).
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60       If  the process is multi-threaded, or if the process is single-threaded
61       and a signal handler is executed other than as the result of:
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63        *  The process calling abort(), raise(),  kill(),  pthread_kill(),  or
64           sigqueue() to generate a signal that is not blocked
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66        *  A  pending  signal  being  unblocked and being delivered before the
67           call that unblocked it returns
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69       the behavior is undefined if the signal handler refers  to  any  object
70       other than errno with static storage duration other than by assigning a
71       value to an object declared as volatile sig_atomic_t, or if the  signal
72       handler  calls  any function defined in this standard other than one of
73       the functions listed in Section 2.4, Signal Concepts.
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75       At program start-up, the equivalent of:
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77           signal(sig, SIG_IGN);
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79       is executed for some signals, and the equivalent of:
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81           signal(sig, SIG_DFL);
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83       is executed for all other signals (see exec).
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85       The signal() function shall not change the setting of errno if success‐
86       ful.
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RETURN VALUE

89       If  the request can be honored, signal() shall return the value of func
90       for the most recent call to signal()  for  the  specified  signal  sig.
91       Otherwise,  SIG_ERR  shall  be  returned  and a positive value shall be
92       stored in errno.
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ERRORS

95       The signal() function shall fail if:
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97       EINVAL The sig argument is not a valid signal number or an  attempt  is
98              made  to catch a signal that cannot be caught or ignore a signal
99              that cannot be ignored.
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101       The signal() function may fail if:
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103       EINVAL An attempt was made to set the action to SIG_DFL  for  a  signal
104              that cannot be caught or ignored (or both).
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106       The following sections are informative.
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EXAMPLES

109       None.
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APPLICATION USAGE

112       The  sigaction()  function  provides  a more comprehensive and reliable
113       mechanism for controlling signals; new applications should  use  sigac‐
114       tion() rather than signal().
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RATIONALE

117       None.
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FUTURE DIRECTIONS

120       None.
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SEE ALSO

123       Section 2.4, Signal Concepts, exec, pause(), raise(), sigaction(), sig‐
124       suspend(), waitid()
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126       The Base Definitions volume of POSIX.1‐2008, <signal.h>
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129       Portions of this text are reprinted and reproduced in  electronic  form
130       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
131       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
132       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
133       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
134       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) In the
135       event of any discrepancy between this version and the original IEEE and
136       The  Open Group Standard, the original IEEE and The Open Group Standard
137       is the referee document. The original Standard can be  obtained  online
138       at http://www.unix.org/online.html .
139
140       Any  typographical  or  formatting  errors that appear in this page are
141       most likely to have been introduced during the conversion of the source
142       files  to  man page format. To report such errors, see https://www.ker
143       nel.org/doc/man-pages/reporting_bugs.html .
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147IEEE/The Open Group                  2013                           SIGNAL(3P)
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