sigaction

Examines and/or changes a signal action.

Syntax

   #include <signal.h>

   int sigaction (int sig, const struct sigaction *act,

    struct sigaction *oact);

Parameters

sig: The signal number of the signal to examine or change. Valid signals are described in Table 3-6 “POSIX/iX Signals”.
act: If not NULL, a pointer to a structure of type sigaction that describes a new signal action to be associated with sig. If NULL, the current signal action is unchanged.
oact: If not NULL, a pointer to a structure of type sigaction that returns the description of the current action for the signal sig (prior to any changes). If NULL, the current action is not returned.

Return Values

0: Success.
-1: An error occurred. The signal action is not changed, and errno is set to indicate the error condition.

Description

The sigaction() function enables the calling process to examine or change (or both) the action associated with the specified signal.

In order to examine the current action associated with a signal without changing the current action, set act to NULL. In order to change an action associated with a signal, define the new signal action in a structure of type sigaction and pass it in act. Refer to the following discussion of the sigaction structure.

The sigaction structure, defined in <signal.h>, includes the following fields:

Member Type	Member Name	Description
`void (*)()`	`sa_handler`	Either `SIG_DFL` for the default action, `SIG_IGN` to ignore the signal, or a pointer to a signal handling function (a signal handler).
`sigset_t`	`sa_mask`;	Additional signals to be blocked during execution of the signal handler specified in `sa_handler`.
`int`	`sa_flags`;	If `sig` specifies `SIGCHLD` and `sa_flags` specifies `SA_NOCLDSTOP`, a `SIGCHLD` signal is not generated for the calling process whenever any of its child processes stop. If `sig` specifies `SIGCHLD` and `sa_flags` does not specify `SA_NOCLDSTOP`, `SIGCHLD` is generated for the calling process whenever any of its child processes stop.

When installing a new signal handler, you must specify in sa_mask any additional signals to be blocked during the execution of the signal handler. When a signal is caught by a signal handler installed by sigaction(), a new signal mask is calculated and installed for the duration of the signal handler (or until a call to either sigprocmask() or sigsuspend()).

This mask is formed by taking a union of the current signal mask and sa_mask for the signal being delivered, then including the signal being delivered. If and when the signal handler returns normally, the original signal mask is restored.

The signals SIGKILL and SIGSTOP, if specified in the sa_mask field, are removed by the system without error.

The structure passed in sa_mask must be initialized by either the sigemptyset() or the sigfillset() functions before adding or removing signals from it using the sigaddset() or sigdelset() functions.

A signal action installed by sigaction() remains in effect until changed by another call to sigaction() or until the next call to one of the exec() functions.

The sigaction() function is incompatible with the ANSI C signal() function. The sigaction() function can return and reinstall a signal action that was originally installed by signal(); however, the structure that sigaction() returns in oact may not reliably be examined by the caller. If this same signal action is later reinstalled, without modification, by another call to sigaction(), the result is as if the original call to signal() were repeated.

Implementation Considerations

Refer to the EFAULT error description below.

Errors

If an error occurs, errno is set to one of the following values:

`EFAULT`	CAUSE	The system detected a bad address in attempting to use the `act` or `oact` parameters.
	ACTION	Make sure that the pointer is correctly initialized.
`EINVAL`	CAUSE	One of the following: The `sig` parameter is not a valid signal number. An attempt was made to handle a signal that cannot be handled or to ignore a signal that cannot be ignored. An attempt was made to change the action from `SIGDFL` for a signal that cannot be handled or ignored.
	ACTION	Refer to Table 3-6 “POSIX/iX Signals” for descriptions of valid and supported signal numbers.

sigaction

Technical documentation

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