The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


signal - signal management


#include <signal.h>

void (*signal(int
sig, void (*func)(int)))(int);


[CX] [Option Start] 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 IEEE Std 1003.1-2001 defers to the ISO C standard. [Option End]

[CX] [Option Start] Use of this function is unspecified in a multi-threaded process. [Option End]

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:

signal(sig, SIG_DFL);

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. If the signal occurs as the result of calling the abort(), raise(), [CX] [Option Start] kill(), pthread_kill(), or sigqueue() [Option End] function, the signal handler shall not call the raise() function.

If the signal occurs other than as the result of calling abort(), raise(), [CX] [Option Start] kill(), pthread_kill(), or sigqueue(), [Option End]  the behavior is undefined if the signal handler refers to any object 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 in the standard library other than one of the functions listed in Signal Concepts. Furthermore, if such a call fails, the value of errno is unspecified.

At program start-up, the equivalent of:

signal(sig, SIG_IGN);

is executed for some signals, and the equivalent of:

signal(sig, SIG_DFL);

is executed for all other signals [CX] [Option Start]  (see exec). [Option End]


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:

[CX] [Option Start] 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. [Option End]

The signal() function may fail if:

[CX] [Option Start] An attempt was made to set the action to SIG_DFL for a signal that cannot be caught or ignored (or both). [Option End]

The following sections are informative.




The sigaction() function provides a more comprehensive and reliable mechanism for controlling signals; new applications should use sigaction() rather than signal().






Signal Concepts, exec(), pause(), sigaction(), sigsuspend(), waitid(), the Base Definitions volume of IEEE Std 1003.1-2001, <signal.h>


First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

The DESCRIPTION is updated to indicate that the sigpause() function restores the process' signal mask to its original state before returning.

The RETURN VALUE section is updated to indicate that the sigpause() function suspends execution of the process until a signal is received, whereupon it returns -1 and sets errno to [EINTR].

Issue 6

Extensions beyond the ISO C standard are marked.

The DESCRIPTION is updated to avoid use of the term "must" for application requirements.

The DESCRIPTION is updated for alignment with the ISO/IEC 9899:1999 standard.

References to the wait3() function are removed.

The sighold(), sigignore(), sigrelse(), and sigset() functions are split out onto their own reference page.

End of informative text.

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