mirror of https://github.com/mkerrisk/man-pages
296 lines
5.9 KiB
Groff
296 lines
5.9 KiB
Groff
.\" Copyright (c) 2000 Andries Brouwer <aeb@cwi.nl>
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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.\" manual page may be incorrect or out-of-date.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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.\"
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.TH SIGNAL 2 2000-04-28 "Linux 2.2" "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
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.BR signal ()
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system call installs a new signal handler for the signal with number
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.IR signum .
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The signal handler is set to
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.I sighandler
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which may be a user specified function, or either
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.B SIG_IGN
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or
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.BR SIG_DFL .
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Upon arrival of a signal with number
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.I signum
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the following happens.
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If the corresponding handler is set to
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.BR SIG_IGN ,
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then the signal is ignored.
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If the handler 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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Finally, if the handler is set to a function
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.I sighandler
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then first either the handler is reset to SIG_DFL
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or an implementation-dependent blocking of the signal
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is performed and next
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.I sighandler
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is called with argument
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.IR signum .
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Using a signal handler function for a signal
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is called "catching the signal".
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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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The
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.BR signal ()
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function 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 PORTABILITY
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The original Unix
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.BR signal ()
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would reset the handler to SIG_DFL, 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 behaviour.
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If one on a libc5 system includes
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.B "<bsd/signal.h>"
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instead of
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.B "<signal.h>"
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then
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.B signal
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is redefined as
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.B __bsd_signal
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and signal has the BSD semantics. 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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.B sysv_signal
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function, one obtains classical behaviour. This is not recommended.
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Trying to change the semantics of this call using
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defines and includes is not a good idea. It is better to avoid
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.B signal
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altogether, and use
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.BR sigaction (2)
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instead.
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.SH NOTES
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The effects of this call in a multi-threaded process are unspecified.
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.PP
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The routine
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.I handler
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must be very careful, since processing elsewhere was interrupted
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at some arbitrary point. POSIX has the concept of "safe function".
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If a signal interrupts an unsafe function, and
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.I handler
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calls an unsafe function, then the behavior is undefined. Safe
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functions are listed explicitly in the various standards.
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The POSIX 1003.1-2003 list is
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_Exit()
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_exit()
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abort()
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accept()
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access()
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aio_error()
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aio_return()
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aio_suspend()
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alarm()
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bind()
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cfgetispeed()
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cfgetospeed()
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cfsetispeed()
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cfsetospeed()
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chdir()
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chmod()
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chown()
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clock_gettime()
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close()
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connect()
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creat()
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dup()
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dup2()
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execle()
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execve()
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fchmod()
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fchown()
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fcntl()
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fdatasync()
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fork()
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fpathconf()
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fstat()
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fsync()
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ftruncate()
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getegid()
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geteuid()
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getgid()
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getgroups()
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getpeername()
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getpgrp()
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getpid()
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getppid()
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getsockname()
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getsockopt()
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getuid()
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kill()
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link()
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listen()
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lseek()
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lstat()
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mkdir()
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mkfifo()
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open()
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pathconf()
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pause()
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pipe()
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poll()
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posix_trace_event()
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pselect()
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raise()
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read()
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readlink()
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recv()
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recvfrom()
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recvmsg()
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rename()
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rmdir()
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select()
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sem_post()
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send()
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sendmsg()
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sendto()
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setgid()
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setpgid()
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setsid()
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setsockopt()
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setuid()
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shutdown()
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sigaction()
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sigaddset()
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sigdelset()
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sigemptyset()
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sigfillset()
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sigismember()
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signal()
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sigpause()
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sigpending()
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sigprocmask()
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sigqueue()
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sigset()
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sigsuspend()
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sleep()
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socket()
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socketpair()
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stat()
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symlink()
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sysconf()
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tcdrain()
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tcflow()
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tcflush()
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tcgetattr()
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tcgetpgrp()
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tcsendbreak()
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tcsetattr()
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tcsetpgrp()
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time()
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timer_getoverrun()
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timer_gettime()
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timer_settime()
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times()
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umask()
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uname()
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unlink()
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utime()
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wait()
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waitpid()
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write().
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.PP
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According to POSIX, the behaviour 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 the
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.BR kill (2)
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or the
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.BR raise (3)
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functions.
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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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The use of
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.B 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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.SH "CONFORMING TO"
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ANSI C
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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 pause (2),
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.BR sigaction (2),
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.BR sigqueue (2),
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.BR sigvec (2),
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.BR killpg (3),
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.BR raise (3),
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.BR sigsetops (3),
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.BR signal (7)
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