SIGNAL(3P) POSIX Programmer's Manual SIGNAL(3P)PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAMEsignal — signal management
void (*signal(int sig, void (*func)(int)))(int);
The functionality described on this reference page is aligned with the
ISO C standard. Any conflict between the requirements described here
and the ISO C standard is unintentional. This volume of POSIX.1‐2008
defers to the ISO C standard.
Use of this function is unspecified in a multi-threaded process.
The signal() function chooses one of three ways in which receipt of the
signal number sig is to be subsequently handled. If the value of func
is SIG_DFL, default handling for that signal shall occur. If the value
of func is SIG_IGN, the signal shall be ignored. Otherwise, the appli‐
cation shall ensure that func points to a function to be called when
that signal occurs. An invocation of such a function because of a sig‐
nal, or (recursively) of any further functions called by that invoca‐
tion (other than functions in the standard library), is called a ``sig‐
When a signal occurs, and func points to a function, it is implementa‐
tion-defined whether the equivalent of a:
is executed or the implementation prevents some implementation-defined
set of signals (at least including sig) from occurring until the cur‐
rent signal handling has completed. (If the value of sig is SIGILL, the
implementation may alternatively define that no action is taken.) Next
the equivalent of:
is executed. If and when the function returns, if the value of sig was
SIGFPE, SIGILL, or SIGSEGV or any other implementation-defined value
corresponding to a computational exception, the behavior is undefined.
Otherwise, the program shall resume execution at the point it was
interrupted. The ISO C standard places a restriction on applications
relating to the use of raise() from signal handlers. This restriction
does not apply to POSIX applications, as POSIX.1‐2008 requires raise()
to be async-signal-safe (see Section 2.4.3, Signal Actions).
If the process is multi-threaded, or if the process is single-threaded
and a signal handler is executed other than as the result of:
* The process calling abort(), raise(), kill(), pthread_kill(), or
sigqueue() to generate a signal that is not blocked
* A pending signal being unblocked and being delivered before the
call that unblocked it returns
the behavior is undefined if the signal handler refers to any object
other than errno with static storage duration other than by assigning a
value to an object declared as volatile sig_atomic_t, or if the signal
handler calls any function defined in this standard other than one of
the functions listed in Section 2.4, Signal Concepts.
At program start-up, the equivalent of:
is executed for some signals, and the equivalent of:
is executed for all other signals (see exec).
The signal() function shall not change the setting of errno if success‐
If the request can be honored, signal() shall return the value of func
for the most recent call to signal() for the specified signal sig.
Otherwise, SIG_ERR shall be returned and a positive value shall be
stored in errno.
The signal() function shall fail if:
EINVAL The sig argument is not a valid signal number or an attempt is
made to catch a signal that cannot be caught or ignore a signal
that cannot be ignored.
The signal() function may fail if:
EINVAL An attempt was made to set the action to SIG_DFL for a signal
that cannot be caught or ignored (or both).
The following sections are informative.
The sigaction() function provides a more comprehensive and reliable
mechanism for controlling signals; new applications should use sigac‐
tion() rather than signal().
Section 2.4, Signal Concepts, exec, pause(), raise(), sigaction(), sig‐
The Base Definitions volume of POSIX.1‐2008, <signal.h>
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker‐
IEEE/The Open Group 2013 SIGNAL(3P)