signal
Defined in header <signal.h>
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void (*signal( int sig, void (*handler) (int))) (int); |
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Sets the error handler for signal sig
. The signal handler can be set so that default handling will occur, signal is ignored, or a user-defined function is called.
When signal handler is set to a function and a signal occurs, it is implementation defined whether signal(sig, SIG_DFL) will be executed immediately before the start of signal handler. Also, the implementation can prevent some implementation-defined set of signals from occurring while the signal handler runs.
Parameters
sig | - | the signal to set the signal handler to. It can be an implementation-defined value or one of the following values:
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handler | - | the signal handler. This must be one of the following:
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Return value
Previous signal handler on success or SIG_ERR on failure (setting a signal handler can be disabled on some implementations).
Signal handler
The following limitations are imposed on the user-defined function that is installed as a signal handler.
If the user defined function returns when handling SIGFPE, SIGILL or SIGSEGV, the behavior is undefined.
If the signal handler is called as a result of abort or raise, the behavior is undefined if the signal handler calls raise.
If the signal handler is called NOT as a result of abort or raise (in other words, the signal handler is asynchronous), the behavior is undefined if
- the signal handler calls any function within the standard library, except
- abort
- _Exit
- quick_exit
-
signal
with the first argument being the number of the signal currently handled (async handler can re-register itself, but not other signals). - atomic functions from stdatomic.h if the atomic arguments are lock-free
- atomic_is_lock_free (with any kind of atomic arguments)
- the signal handler refers to any object with static or thread-local (since C11) storage duration that is not a lock-free atomic (since C11) other than by assigning to a static volatile sig_atomic_t.
On entry to the signal handler, the state of the floating-point environment and the values of all objects is unspecified, except for
- objects of type volatile sig_atomic_t
- objects of lock-free atomic types (since C11)
- side effects made visible through atomic_signal_fence (since C11)
On return from a signal handler, the value of any object modified by the signal handler that is not volatile sig_atomic_t or lock-free atomic(since C11) is undefined.
The behavior is undefined if signal is used in a multithreaded program. It is not required to be thread-safe.
Notes
POSIX requires that signal
is thread-safe, and specifies a list of async-signal-safe library functions that may be called from any signal handler.
Besides abort
and raise
, POSIX specifies that kill
, pthread_kill
, and sigqueue
generate synchronous signals.
POSIX recommends sigaction instead of signal
, due to its underspecified behavior and significant implementation variations, regarding signal delivery while a signal handler is executed.
Example
#include <signal.h> #include <stdio.h> volatile sig_atomic_t gSignalStatus; void signal_handler(int signal) { gSignalStatus = signal; } int main(void) { signal(SIGINT, signal_handler); printf("SignalValue: %d\n", gSignalStatus); printf("Sending signal: %d\n", SIGINT); raise(SIGINT); printf("SignalValue: %d\n", gSignalStatus); }
Output:
SignalValue: 0 Sending signal: 2 SignalValue: 2
References
- C11 standard (ISO/IEC 9899:2011):
- 7.14.1.1 The signal function (p: 266-267)
- C99 standard (ISO/IEC 9899:1999):
- 7.14.1.1 The signal function (p: 247-248)
- C89/C90 standard (ISO/IEC 9899:1990):
- 4.7.1.1 The signal function
See also
runs the signal handler for particular signal (function) |