mirror of https://github.com/mkerrisk/man-pages
200 lines
4.9 KiB
Groff
200 lines
4.9 KiB
Groff
.\" Copyright (c) 2000 Andries Brouwer <aeb@cwi.nl>
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.\" and Copyright (c) 2007 Michael Kerrisk <mtk.manpages@gmail.com>
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.\" based on work by Rik Faith <faith@cs.unc.edu>
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.\" and Mike Battersby <mike@starbug.apana.org.au>.
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.\" Modified 2004-11-19, mtk:
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.\" added pointer to sigaction.2 for details of ignoring SIGCHLD
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.\" 2007-06-03, mtk: strengthened portability warning, and rerote
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.\" various sections.
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.\"
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.TH SIGNAL 2 2007-06-03 "Linux" "Linux Programmer's Manual"
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.SH NAME
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signal \- ANSI C signal handling
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.SH SYNOPSIS
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.B #include <signal.h>
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.sp
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.B typedef void (*sighandler_t)(int);
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.sp
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.BI "sighandler_t signal(int " signum ", sighandler_t " handler );
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.SH DESCRIPTION
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The behavior of
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.BR signal ()
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varies across Unix versions,
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and has also varied historically across different versions of Linux.
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\fBAvoid its use\fP: use
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.BR sigaction (2)
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instead.
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See \fIPortability\fP below.
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.BR signal ()
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sets the disposition of the signal
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.IR signum
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to
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.IR handler ,
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which is either
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.BR SIG_IGN ,
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.BR SIG_DFL ,
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or the address of a programmer-defined function (a "signal handler").
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If the signal
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.I signum
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is delivered to the process, then one of the following happens:
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.TP 3
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*
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If the disposition is set to
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.BR SIG_IGN ,
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then the signal is ignored.
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.TP
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*
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If the disposition is set to
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.BR SIG_DFL ,
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then the default action associated with the signal (see
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.BR signal (7))
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occurs.
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.TP
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*
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If the disposition is set to a function,
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then first either the disposition is reset to
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.BR SIG_DFL ,
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or the signal is blocked (see \fIPortability\fP below), and then
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.I handler
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is called with argument
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.IR signum .
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If invocation of the handler caused the signal to be blocked,
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then the signal is unblocked upon return from the handler.
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.PP
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The signals
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.B SIGKILL
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and
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.B SIGSTOP
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cannot be caught or ignored.
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.SH "RETURN VALUE"
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.BR signal ()
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returns the previous value of the signal handler, or
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.B SIG_ERR
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on error.
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.SH ERRORS
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.TP
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.B EINVAL
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.I signum
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is invalid.
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.SH "CONFORMING TO"
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C89, C99, POSIX.1-2001.
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.SH NOTES
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The effects of
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.BR signal ()
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in a multi-threaded process are unspecified.
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.PP
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According to POSIX, the behavior of a process is undefined after it
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ignores a
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.BR SIGFPE ,
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.BR SIGILL ,
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or
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.B SIGSEGV
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signal that was not generated by
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.BR kill (2)
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or
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.BR raise (3).
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Integer division by zero has undefined result.
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On some architectures it will generate a
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.B SIGFPE
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signal.
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(Also dividing the most negative integer by \-1 may generate
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.BR SIGFPE .)
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Ignoring this signal might lead to an endless loop.
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.PP
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See
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.BR sigaction (2)
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for details on what happens when
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.B SIGCHLD
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is set to
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.BR SIG_IGN .
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.PP
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See
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.BR signal (7)
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for a list of the async-signal-safe functions that can be
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safely called inside from inside a signal handler.
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.PP
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The use of
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.I sighandler_t
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is a GNU extension.
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Various versions of libc predefine this type; libc4 and libc5 define
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.IR SignalHandler ,
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glibc defines
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.I sig_t
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and, when
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.B _GNU_SOURCE
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is defined, also
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.IR sighandler_t .
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.SS Portability
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The original Unix
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.BR signal ()
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would reset the handler to
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.BR SIG_DFL ,
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and System V
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(and the Linux kernel and libc4,5) does the same.
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On the other hand, BSD does not reset the handler, but blocks
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new instances of this signal from occurring during a call of the handler.
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The glibc2 library follows the BSD behavior.
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If one on a libc5 system includes
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.I <bsd/signal.h>
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instead of
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.I <signal.h>
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then
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.BR signal ()
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is redefined as
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.BR __bsd_signal ()
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and
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.BR signal ()
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has the BSD semantics.
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This is not recommended.
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If one on a glibc2 system defines a feature test
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macro such as
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.B _XOPEN_SOURCE
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or uses a separate
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.BR sysv_signal (3)
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function, one obtains classical behavior.
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This is not recommended.
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.SH "SEE ALSO"
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.BR kill (1),
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.BR alarm (2),
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.BR kill (2),
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.BR killpg (2),
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.BR pause (2),
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.BR sigaction (2),
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.BR sigpending (2),
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.BR sigprocmask (2),
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.BR sigqueue (2),
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.BR sigsuspend (2),
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.BR bsd_signal (3),
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.BR raise (3),
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.BR siginterrupt (3),
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.BR sigsetops (3),
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.BR sigvec (3),
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.BR sysv_signal (3),
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.BR feature_test_macros (7),
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.BR signal (7)
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