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signal(2) [redhat man page]

SIGNAL(2)						     Linux Programmer's Manual							 SIGNAL(2)

signal - ANSI C signal handling SYNOPSIS
#include <signal.h> typedef void (*sighandler_t)(int); sighandler_t signal(int signum, sighandler_t handler); DESCRIPTION
The signal() system call installs a new signal handler for the signal with number signum. The signal handler is set to sighandler which may be a user specified function, or either SIG_IGN or SIG_DFL. Upon arrival of a signal with number signum the following happens. If the corresponding handler is set to SIG_IGN, then the signal is ignored. If the handler is set to SIG_DFL, then the default action associated to the signal (see signal(7)) occurs. Finally, if the han- dler is set to a function sighandler then first either the handler is reset to SIG_DFL or an implementation-dependent blocking of the sig- nal is performed and next sighandler is called with argument signum. Using a signal handler function for a signal is called "catching the signal". The signals SIGKILL and SIGSTOP cannot be caught or ignored. RETURN VALUE
The signal() function returns the previous value of the signal handler, or SIG_ERR on error. PORTABILITY
The original Unix signal() would reset the handler to SIG_DFL, and System V (and the Linux kernel and libc4,5) does the same. On the other hand, BSD does not reset the handler, but blocks new instances of this signal from occurring during a call of the handler. The glibc2 library follows the BSD behaviour. If one on a libc5 system includes <bsd/signal.h> instead of <signal.h> then signal is redefined as __bsd_signal and signal has the BSD semantics. This is not recommended. If one on a glibc2 system defines a feature test macro such as _XOPEN_SOURCE or uses a separate sysv_signal function, one obtains classical behaviour. This is not recommended. Trying to change the semantics of this call using defines and includes is not a good idea. It is better to avoid signal altogether, and use sigaction(2) instead. NOTES
According to POSIX, the behaviour of a process is undefined after it ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated by the kill(2) or the raise(3) functions. Integer division by zero has undefined result. On some architectures it will generate a SIGFPE signal. (Also dividing the most negative integer by -1 may generate SIGFPE.) Ignoring this signal might lead to an endless loop. According to POSIX ( it is unspecified what happens when SIGCHLD is set to SIG_IGN. Here the BSD and SYSV behaviours differ, caus- ing BSD software that sets the action for SIGCHLD to SIG_IGN to fail on Linux. The use of sighandler_t is a GNU extension. Various versions of libc predefine this type; libc4 and libc5 define SignalHandler, glibc defines sig_t and, when _GNU_SOURCE is defined, also sighandler_t. CONFORMING TO
kill(1), kill(2), killpg(2), pause(2), raise(3), sigaction(2), signal(7), sigsetops(3), sigvec(2), alarm(2) Linux 2.2 2000-04-28 SIGNAL(2)

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BSD_SIGNAL(3)						     Linux Programmer's Manual						     BSD_SIGNAL(3)

bsd_signal - signal handling with BSD semantics SYNOPSIS
#define _XOPEN_SOURCE /* See feature_test_macros(7) */ #include <signal.h> typedef void (*sighandler_t)(int); sighandler_t bsd_signal(int signum, sighandler_t handler); DESCRIPTION
The bsd_signal() function takes the same arguments, and performs the same task, as signal(2). The difference between the two is that bsd_signal() is guaranteed to provide reliable signal semantics, that is: a) the disposition of the signal is not reset to the default when the handler is invoked; b) delivery of further instances of the signal is blocked while the signal handler is executing; and c) if the handler interrupts a blocking system call, then the system call is automatically restarted. A portable application cannot rely on signal(2) to provide these guarantees. RETURN VALUE
The bsd_signal() function returns the previous value of the signal handler, or SIG_ERR on error. ERRORS
As for signal(2). CONFORMING TO
4.2BSD, POSIX.1-2001. POSIX.1-2008 removes the specification of bsd_signal(), recommending the use of sigaction(2) instead. NOTES
Use of bsd_signal() should be avoided; use sigaction(2) instead. On modern Linux systems, bsd_signal() and signal(2) are equivalent. But on older systems, signal(2) provided unreliable signal semantics; see signal(2) for details. The use of sighandler_t is a GNU extension; this type is defined only if the _GNU_SOURCE feature test macro is defined. SEE ALSO
sigaction(2), signal(2), sysv_signal(3), signal(7) COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at 2009-03-15 BSD_SIGNAL(3)
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