signal

Synopsis

#include <signal.h>

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

Status

Partially implemented

Conformance

IEEE Std 1003.1-2017

Description

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:

`(*func)(sig);`

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.1-2017 requires raise() to be async-signal-safe (see Signal Actions). If the process is multithreaded, 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(),orsigqueue()` 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 Signal Concepts. 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 (see exec()). The signal() function shall not change the setting of errno if successful.

Return value

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.

Errors

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).

Tests

Untested

Known bugs

None

See Also

1. Standard library functions
2. Table of Contents