SIGACTION(2)

NAME
sigaction, sigprocmask, sigpending, sigsuspend - POSIX signal handling functions.

SYNOPSIS
#include <signal.h>

int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact);

int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);

int sigpending(sigset_t *set);

int sigsuspend(const sigset_t *mask);

DESCRIPTION
The sigaction system call is used to change the action taken by a process on receipt of a specific signal.

signum specifies the signal and can be any valid signal except SIGKILL and SIGSTOP.

If act is non-null, the new action for signal signum is installed from act. If oldact is non-null, the previous action is saved in oldact.

The sigaction structure is defined as

    struct sigaction {
        void (*sa_handler)(int);
        sigset_t sa_mask;
        int sa_flags;
        void (*sa_restorer)(void);
    }

sa_handler specifies the action to be associated with signum and may be SIG_DFL for the default action, SIG_IGN to ignore this signal, or a pointer to a signal handling function.

sa_mask gives a mask of signals which should be blocked during execution of the signal handler. In addition, the signal which triggered the handler will be blocked, unless the SA_NODEFER or SA_NOMASK flags are used.

sa_flags specifies a set of flags which modify the behaviour of the signal handling process. It is formed by the bitwise OR of zero or more of the following:

    SA_NOCLDSTOP
        If signum is SIGCHLD, do not receive notification when child processes stop (i.e., when child processes receive one of SIGSTOP, SIGTSTP, SIGTTIN or SIGTTOU).

    SA_ONESHOT or SA_RESETHAND
        Restore the signal action to the default state once the signal handler has been called. (This is the default behavior of the signal(2) system call.)

    SA_RESTART
        Provide behaviour compatible with BSD signal semantics by making certain system calls restartable across signals.

    SA_NOMASK or SA_NODEFER
        Do not prevent the signal from being received from within its own signal handler.

The sa_restorer element is obsolete and should not be used.

The sigprocmask call is used to change the list of currently blocked signals. The behaviour of the call is dependent on the value of how, as follows.

    SIG_BLOCK
        The set of blocked signals is the union of the current set and the set argument.

    SIG_UNBLOCK
        The signals in set are removed from the current set of blocked signals. It is legal to attempt to unblock a signal which is not blocked.

    SIG_SETMASK
        The set of blocked signals is set to the argument set.

If oldset is non-null, the previous value of the signal mask is stored in oldset.

The sigpending call allows the examination of pending signals (ones which have been raised while blocked). The signal mask of pending signals is stored in set.

The sigsuspend call temporarily replaces the signal mask for the process with that given by mask and then suspends the process until a signal is received.

RETURN VALUES
sigaction, sigprocmask, sigpending and sigsuspend return 0 on success and -1 on error.

ERRORS

EINVAL
    An invalid signal was specified. This will also be generated if an attempt is made to change the action for SIGKILL or SIGSTOP, which cannot be caught.

EFAULT
    act, oldact, set or oldset point to memory which is not a valid part of the process address space.

EINTR
    System call was interrupted.

NOTES
It is not possible to block SIGKILL or SIGSTOP with the sigprocmask call. Attempts to do so will be silently ignored.

According to POSIX, the behaviour of a process is undefined after it ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated by the kill() or the raise() functions. Integer division by zero has undefined result. On some architectures it will generate a SIGFPE signal. Ignoring this signal might lead to an endless loop.

Setting SIGCHLD to SIG_IGN provides automatic reaping of child processes.

The POSIX spec only defines SA_NOCLDSTOP. Use of other sa_flags is non-portable.

The SA_RESETHAND flag is compatible with the SVr4 flag of the same name.

The SA_NODEFER flag is compatible with the SVr4 flag of the same name under kernels 1.3.9 and newer. On older kernels the Linux implementation allowed the receipt of any signal, not just the one we are installing (effectively overriding any sa_mask settings).

The SA_RESETHAND and SA_NODEFER names for SVr4 compatibility are present only in library versions 3.0.9 and greater.

sigaction can be called with a null second argument to query the current signal handler. It can also be used to check whether a given signal is valid for the current machine by calling it with null second and third arguments.

See sigsetops(3) for details on manipulating signal sets.

CONFORMING TO
POSIX, SVr4. SVr4 does not document the EINTR condition.

SEE ALSO
kill(1), kill(2), killpg(2), pause(2), raise(3), siginterrupt(3), signal(2), signal(7), sigsetops(3), sigvec(2)