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.
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 application shall ensure that func points to a
function to be called when that signal occurs. An invocation of such
a function because of a signal, or (recursively) of any further
functions called by that invocation (other than functions in the
standard library), is called a ``signal handler''.
When a signal occurs, and func points to a function, it is
implementation-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 current 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 Section2.4.3, 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
* 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
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.
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 Electrical 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
IEEE/The Open Group 2013 SIGNAL(3P)