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signal() - Unix, Linux System Call

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signal - ANSI C signal handling


#include <signal.h>

typedef void (*sighandler_t)(int);

sighandler_t signal(int signum, sighandler_t handler);


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 with the signal (see signal(7)) occurs. Finally, if the handler is set to a function sighandler then first either the handler is reset to SIG_DFL or an implementation-dependent blocking of the signal 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.


The signal() function returns the previous value of the signal handler, or SIG_ERR on error.


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.


The effects of this call in a multi-threaded process are unspecified.

The routine handler must be very careful, since processing elsewhere was interrupted at some arbitrary point. POSIX has the concept of "safe function". If a signal interrupts an unsafe function, and handler calls an unsafe function, then the behavior is undefined. Safe functions are listed explicitly in the various standards. The POSIX.1-2003 list is

_Exit() _exit() abort() accept() access() aio_error() aio_return() aio_suspend() alarm() bind() cfgetispeed() cfgetospeed() cfsetispeed() cfsetospeed() chdir() chmod() chown() clock_gettime() close() connect() creat() dup() dup2() execle() execve() fchmod() fchown() fcntl() fdatasync() fork() fpathconf() fstat() fsync() ftruncate() getegid() geteuid() getgid() getgroups() getpeername() getpgrp() getpid() getppid() getsockname() getsockopt() getuid() kill() link() listen() lseek() lstat() mkdir() mkfifo() open() pathconf() pause() pipe() poll() posix_trace_event() pselect() raise() read() readlink() recv() recvfrom() recvmsg() rename() rmdir() select() sem_post() send() sendmsg() sendto() setgid() setpgid() setsid() setsockopt() setuid() shutdown() sigaction() sigaddset() sigdelset() sigemptyset() sigfillset() sigismember() signal() sigpause() sigpending() sigprocmask() sigqueue() sigset() sigsuspend() sleep() socket() socketpair() stat() symlink() sysconf() tcdrain() tcflow() tcflush() tcgetattr() tcgetpgrp() tcsendbreak() tcsetattr() tcsetpgrp() time() timer_getoverrun() timer_gettime() timer_settime() times() umask() uname() unlink() utime() wait() waitpid() write().

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.

See sigaction(2) for details on what happens when SIGCHLD is set to SIG_IGN.

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.


C89, POSIX.1-2001.


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