| .\" Copyright (c) 2000 Andries Brouwer <aeb@cwi.nl> |
| .\" and Copyright (c) 2007 Michael Kerrisk <mtk.manpages@gmail.com> |
| .\" and Copyright (c) 2008, Linux Foundation, written by Michael Kerrisk |
| .\" <mtk.manpages@gmail.com> |
| .\" based on work by Rik Faith <faith@cs.unc.edu> |
| .\" and Mike Battersby <mike@starbug.apana.org.au>. |
| .\" |
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| .\" manual under the conditions for verbatim copying, provided that the |
| .\" entire resulting derived work is distributed under the terms of a |
| .\" permission notice identical to this one. |
| .\" |
| .\" Since the Linux kernel and libraries are constantly changing, this |
| .\" manual page may be incorrect or out-of-date. The author(s) assume no |
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| .\" Modified 2004-11-19, mtk: |
| .\" added pointer to sigaction.2 for details of ignoring SIGCHLD |
| .\" 2007-06-03, mtk: strengthened portability warning, and rewrote |
| .\" various sections. |
| .\" 2008-07-11, mtk: rewrote and expanded portability discussion. |
| .\" |
| .TH SIGNAL 2 2016-03-15 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| signal \- ANSI C signal handling |
| .SH SYNOPSIS |
| .B #include <signal.h> |
| .sp |
| .B typedef void (*sighandler_t)(int); |
| .sp |
| .BI "sighandler_t signal(int " signum ", sighandler_t " handler ); |
| .SH DESCRIPTION |
| The behavior of |
| .BR signal () |
| varies across UNIX versions, |
| and has also varied historically across different versions of Linux. |
| \fBAvoid its use\fP: use |
| .BR sigaction (2) |
| instead. |
| See \fIPortability\fP below. |
| |
| .BR signal () |
| sets the disposition of the signal |
| .I signum |
| to |
| .IR handler , |
| which is either |
| .BR SIG_IGN , |
| .BR SIG_DFL , |
| or the address of a programmer-defined function (a "signal handler"). |
| |
| If the signal |
| .I signum |
| is delivered to the process, then one of the following happens: |
| .TP 3 |
| * |
| If the disposition is set to |
| .BR SIG_IGN , |
| then the signal is ignored. |
| .TP |
| * |
| If the disposition is set to |
| .BR SIG_DFL , |
| then the default action associated with the signal (see |
| .BR signal (7)) |
| occurs. |
| .TP |
| * |
| If the disposition is set to a function, |
| then first either the disposition is reset to |
| .BR SIG_DFL , |
| or the signal is blocked (see \fIPortability\fP below), and then |
| .I handler |
| is called with argument |
| .IR signum . |
| If invocation of the handler caused the signal to be blocked, |
| then the signal is unblocked upon return from the handler. |
| .PP |
| The signals |
| .B SIGKILL |
| and |
| .B SIGSTOP |
| cannot be caught or ignored. |
| .SH RETURN VALUE |
| .BR signal () |
| returns the previous value of the signal handler, or |
| .B SIG_ERR |
| on error. |
| In the event of an error, |
| .I errno |
| is set to indicate the cause. |
| .SH ERRORS |
| .TP |
| .B EINVAL |
| .I signum |
| is invalid. |
| .SH CONFORMING TO |
| POSIX.1-2001, POSIX.1-2008, C89, C99. |
| .SH NOTES |
| The effects of |
| .BR signal () |
| in a multithreaded process are unspecified. |
| .PP |
| According to POSIX, the behavior of a process is undefined after it |
| ignores a |
| .BR SIGFPE , |
| .BR SIGILL , |
| or |
| .B SIGSEGV |
| signal that was not generated by |
| .BR kill (2) |
| or |
| .BR raise (3). |
| Integer division by zero has undefined result. |
| On some architectures it will generate a |
| .B SIGFPE |
| signal. |
| (Also dividing the most negative integer by \-1 may generate |
| .BR SIGFPE .) |
| Ignoring this signal might lead to an endless loop. |
| .PP |
| See |
| .BR sigaction (2) |
| for details on what happens when |
| .B SIGCHLD |
| is set to |
| .BR SIG_IGN . |
| .PP |
| See |
| .BR signal (7) |
| for a list of the async-signal-safe functions that can be |
| safely called from inside a signal handler. |
| .PP |
| The use of |
| .I sighandler_t |
| is a GNU extension, exposed if |
| .B _GNU_SOURCE |
| is defined; |
| .\" libc4 and libc5 define |
| .\" .IR SignalHandler ; |
| glibc also defines (the BSD-derived) |
| .I sig_t |
| if |
| .B _BSD_SOURCE |
| (glibc 2.19 and earlier) |
| or |
| .BR _DEFAULT_SOURCE |
| (glibc 2.19 and later) |
| is defined. |
| Without use of such a type, the declaration of |
| .BR signal () |
| is the somewhat harder to read: |
| .in +4n |
| .nf |
| |
| .BI "void ( *" signal "(int " signum ", void (*" handler ")(int)) ) (int);" |
| .fi |
| .in |
| .SS Portability |
| The only portable use of |
| .BR signal () |
| is to set a signal's disposition to |
| .BR SIG_DFL |
| or |
| .BR SIG_IGN . |
| The semantics when using |
| .BR signal () |
| to establish a signal handler vary across systems |
| (and POSIX.1 explicitly permits this variation); |
| .B do not use it for this purpose. |
| |
| POSIX.1 solved the portability mess by specifying |
| .BR sigaction (2), |
| which provides explicit control of the semantics when a |
| signal handler is invoked; use that interface instead of |
| .BR signal (). |
| |
| In the original UNIX systems, when a handler that was established using |
| .BR signal () |
| was invoked by the delivery of a signal, |
| the disposition of the signal would be reset to |
| .BR SIG_DFL , |
| and the system did not block delivery of further instances of the signal. |
| This is equivalent to calling |
| .BR sigaction (2) |
| with the following flags: |
| |
| sa.sa_flags = SA_RESETHAND | SA_NODEFER; |
| |
| System\ V also provides these semantics for |
| .BR signal (). |
| This was bad because the signal might be delivered again |
| before the handler had a chance to reestablish itself. |
| Furthermore, rapid deliveries of the same signal could |
| result in recursive invocations of the handler. |
| |
| BSD improved on this situation, but unfortunately also |
| changed the semantics of the existing |
| .BR signal () |
| interface while doing so. |
| On BSD, when a signal handler is invoked, |
| the signal disposition is not reset, |
| and further instances of the signal are blocked from |
| being delivered while the handler is executing. |
| Furthermore, certain blocking system calls are automatically |
| restarted if interrupted by a signal handler (see |
| .BR signal (7)). |
| The BSD semantics are equivalent to calling |
| .BR sigaction (2) |
| with the following flags: |
| |
| sa.sa_flags = SA_RESTART; |
| |
| The situation on Linux is as follows: |
| .IP * 2 |
| The kernel's |
| .BR signal () |
| system call provides System\ V semantics. |
| .IP * |
| By default, in glibc 2 and later, the |
| .BR signal () |
| wrapper function does not invoke the kernel system call. |
| Instead, it calls |
| .BR sigaction (2) |
| using flags that supply BSD semantics. |
| This default behavior is provided as long as a suitable |
| feature test macro is defined: |
| .B _BSD_SOURCE |
| on glibc 2.19 and earlier or |
| .BR _DEFAULT_SOURCE |
| in glibc 2.19 and later. |
| (By default, these macros are defined; see |
| .BR feature_test_macros (7) |
| for details.) |
| If such a feature test macro is not defined, then |
| .BR signal () |
| provides System\ V semantics. |
| .\" |
| .\" System V semantics are also provided if one uses the separate |
| .\" .BR sysv_signal (3) |
| .\" function. |
| .\" .IP * |
| .\" The |
| .\" .BR signal () |
| .\" function in Linux libc4 and libc5 provide System\ V semantics. |
| .\" If one on a libc5 system includes |
| .\" .I <bsd/signal.h> |
| .\" instead of |
| .\" .IR <signal.h> , |
| .\" then |
| .\" .BR signal () |
| .\" provides BSD semantics. |
| .SH SEE ALSO |
| .BR kill (1), |
| .BR alarm (2), |
| .BR kill (2), |
| .BR killpg (2), |
| .BR pause (2), |
| .BR sigaction (2), |
| .BR signalfd (2), |
| .BR sigpending (2), |
| .BR sigprocmask (2), |
| .BR sigsuspend (2), |
| .BR bsd_signal (3), |
| .BR raise (3), |
| .BR siginterrupt (3), |
| .BR sigqueue (3), |
| .BR sigsetops (3), |
| .BR sigvec (3), |
| .BR sysv_signal (3), |
| .BR signal (7) |