sigset() — Change a signal action or a thread

Standards

Standards / Extensions	C or C++	Dependencies
XPG4.2 Single UNIX Specification, Version 3	both	POSIX(ON)

Format

#define _XOPEN_SOURCE_EXTENDED 1
#include <signal.h>

void (*sigset(int sig, void (*disp)(int)))(int);

General description

The sigset() function provides a simplified method for changing the action associated with a specific signal and unblock the signal, or to block this signal.

sig: The number of a recognized signal. sigset() sets the action associated with this signal and unblock this signal, or adds this signal to the calling thread's signal mask (thus blocking this signal). Refer to Table 1 for a list of the supported values of sig.
The value of sig can be any valid signal type except SIGKILL, SIGSTOP, or SIGTRACE.
disp: There are four possible value that disp can have. Three are actions that can be associated with the signal, sig: SIG_DFL, SIG_IGN, or a pointer to a function. The fourth value is not a signal action, but a flag to sigset() that affects whether the signal action is changed.

The values that disp is permitted to have are:

SIG_DFL

Set the signal action to the signal-specific default.

The default actions for each signal is shown in Table 1.
If disp is set to SIG_DFL, sigset() will change the signal action associated with sig and remove this signal from the calling thread's signal mask (thus unblocking this signal).
If the default action is to stop the process, the execution of that process is temporarily suspended. When a process stops, a SIGCHLD signal will be generated for its parent process, unless the parent process has set the SA_NOCLDSTOP flag. While a process is stopped, any additional signals that are sent to the process will not be delivered until the process is continued, except SIGKILL which always terminates the receiving process. A process that is a member of an orphaned process group will not be allowed to stop in response to the SIGTSTP, SIGTTIN, or SIGTTOU signals. In cases where delivery of one of these signals would stop such a process, the signal will be discarded.
Setting a signal action to SIG_DFL for a signal that is pending, and whose default action is to ignore the signal (for example SIGCHLD), will cause the pending signal to be discarded.

SIG_IGN

Set the signal action to ignore the signal.

Delivery of the signal will have no effect on the process.
If disp is set to SIG_IGN, sigset() will change the signal action associated with sig and remove this signal from the calling thread's signal mask (thus unblocking this signal).
Setting a signal action to SIG_IGN for a signal that is pending will cause the pending signal to be discarded. This provides the ability to discard signals that are found to be blocked and pending by sigpending().
If sig is SIGCHLD, child processes of the calling process will not be transformed into 'zombie' processes when they terminate. If the calling process subsequently waits for its children, and the process has no unwaited from children that were transformed into 'zombie' processes, it will block until all of its children terminate. The wait(), waitid(), or waitpid() function will fail and set errno to ECHILD.

SIG_HOLD

Set the calling thread's signal mask to block signal, sig.

The signal action associated with sig is not changed.

Pointer to function

Set the signal action to catch the signal.

sigset() will change the signal action associated with sig and remove this signal from the calling thread's signal mask (thus unblocking this signal).
On delivery of the signal, the receiving process is to execute the signal-catching function at the specified address. After returning from the signal-catching function, the receiving process will resume execution at the point at which it was interrupted.
The signal-catching function specified by disp is invoked as:
void function(int signo);
Where function is the specified signal-catching function and signo is the signal number of the signal being delivered.

After an action has been specified for a particular signal, using sigset(), it remains installed until it is explicitly changed with another call to sigset(), sigaction(), signal(), one of the exec functions, bsd_signal(), or sigignore().

Special behavior for C++:

The behavior when mixing signal-handling with C++ exception handling is undefined. Also, the use of signal-handling with constructors and destructors is undefined.
C++ and C language linkage conventions are incompatible, and therefore sigaction() cannot receive C++ function pointers. If you attempt to pass a C++ function pointer to sigaction(), the compiler will flag it as an error. Therefore, to use the sigaction() function in the C++ language, you must ensure that signal handler routines established have C linkage, by declaring them as extern "C".

Usage notes

The use of the SIGTHSTOP and SIGTHCONT signal is not supported with this function.

Returned value

If successful, sigset() returns SIG_HOLD if the signal had been blocked and the signal's previous action if it had not been blocked.

If unsuccessful, sigset() returns SIG_ERR and sets errno to one of the following values:

Error Code: Description
EINVAL: The value of the argument sig was not a valid signal type, or it was SIGKILL, SIGSTOP, or SIGTRACE.