signal(2)

signal, sigset, sighold, sigrelse, sigignore, sigpause -- simplified signal management

Synopsis

   #include <signal.h>
   
   void (signal(int sig, void (disp)(int)))(int);
   

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

   int sighold(int sig);
   

   int sigrelse(int sig);
   

   int sigignore(int sig);
   

   int sigpause(int sig);

Description

These functions provide simplified signal management for application processes. See signal(5) for an explanation of general signal concepts.

signal and sigset are used to modify signal dispositions. sig specifies the signal, which may be any signal except SIGKILL and SIGSTOP. disp specifies the signal's disposition, which may be SIG_DFL, SIG_IGN, or the address of a signal handler. If signal is used, disp is the address of a signal handler, and sig is not SIGILL, SIGTRAP, or SIGPWR, the system first sets the signal's disposition to SIG_DFL before executing the signal handler. If sigset is used and disp is the address of a signal handler, the system adds sig to the calling process's signal mask before executing the signal handler; when the signal handler returns, the system restores the calling process's signal mask to its state prior to the delivery of the signal. In addition, if sigset is used and disp is equal to SIG_HOLD, sig is added to the calling process's signal mask and the signal's disposition remains unchanged. However, if sigset is used and disp is not equal to SIG_HOLD, sig will be removed from the calling process's signal mask.

sighold adds sig to the calling process's signal mask.

sigrelse removes sig from the calling process's signal mask.

sigignore sets the disposition of sig to SIG_IGN.

sigpause removes sig from the calling process's signal mask and suspends the calling process until a signal is received.

Return values

On success, signal returns the signal's previous disposition. On failure, signal returns SIG_ERR and sets errno to identify the error.

On success, sigset returns SIG_HOLD if the signal had been blocked or the signal's previous disposition if it had not been blocked. On failure, sigset returns SIG_ERR and sets errno to identify the error.

All other functions return zero on success. On failure, they return -1 and set errno to identify the error.

Errors

In the following conditions, these functions fail and set errno to:

EINVAL: The value of the sig argument is not a valid signal or is equal to SIGKILL or SIGSTOP.
EINTR: A signal was caught during the system call sigpause.

Usage

sighold in conjunction with sigrelse or sigpause may be used to establish critical regions of code that require the delivery of a signal to be temporarily deferred.

If signal or sigset is used to set SIGCHLD's disposition to a signal handler, SIGCHLD will not be sent when the calling process's children are stopped or continued.

If any of the above functions are used to set SIGCHLD's disposition to SIG_IGN, the calling process's child processes will not create zombie processes when they terminate [see exit(2)]. If the calling process subsequently waits for its children, it blocks until all of its children terminate; it then returns a value of -1 with errno set to ECHILD. [see wait(2), waitid(2)].

References

kill(2), pause(2), sigaction(2), signal(5), sigsend(2), wait(2), waitid(2)

Notices

Considerations for threads programming

Signal dispositions (that is, default/ignore/handler) are a process attribute and are shared by all threads. Signal masks, on the other hand, are maintained independently per thread.

See signal(5) for further details.

Considerations for lightweight processes

Each LWP has a its own signal mask. The Threads Library maintains a separate signal mask per thread and adjusts (if necessary) the LWP's mask before a thread begins executing.