SIGACTION(2)                         SysV                         SIGACTION(2)



NAME
     sigaction - examine and changed signal action

SYNOPSIS
     #include <signal.h>

     int sigaction (signal, action, o_action)
     int signal;
     const struct sigaction *action;
     struct sigaction *o_action;

DESCRIPTION
     The sigaction function allows the calling process to examine and/or
     change the action to be taken when a specific signal is delivered to the
     process issuing this function. signal specifies the signal; acceptable
     values are defined in <signal.h>.

     The sigaction structure, defined in <signal.h>, has the following
     members:

     void (*sa_handler)(int); Signal handler action
     sigset_t sa_mask;             Signal mask
     int sa_flags;            Modification to specified signal

     If action is not null, it points to a sigaction structure specifying the
     action to be associated with the specified signal.  If o_action is not
     null, it points to a sigaction structure into which the action previously
     associated with the signal is stored. If action is null, signal handling
     is unchanged; thus, the call can be used to inquire about the current
     handling of a given signal.

     The sa_handler field identifies the action associated with a specified
     signal.

     If the sa_handler field specifies a signal-catching function, the sa_mask
     field identifies a set of signals that will be added to the process'
     signal mask before the signal-catching function is invoked. Note that the
     SIGKILL and SIGSTOP signals are not added to the signal mask using this
     mechanism.

     The sa_flags field can be used to modify the behavior of the specified
     signal.  The following flag bit, defined in <signal.h> can be set in
     sa_flags:

     SA_NOCLDSTOP     Do not generate SIGCHLD when children stop.

     If signal is SIGCHLD and the SA_NOCLDSTOP flag is not set in sa_flags,
     then a SIGCHLD signal will be generaed for the calling process whenever
     any of its child processes stop. If signal is SIGCHLD and the
     SA_NOCLDSTOP flag is set in sa_flags, then a SIGCHLD signal will not be
     generaed.

     When a signal is caught by a signal-catching function installed by
     sigaction, a new signal mask is calculated and installed for the duration
     of the signal-catching function (or until a call to either sigprocmask(2)
     or sigsuspend(2) is made). This mask is formed by taking the union of the
     current signal mask and the value of the sa_mask for the signal being
     delivered, and then including the signal being delivered. If and when the
     user's signal handler returns normally, the original signal mask is
     restored.

     Once an action is installed for a specific signal, it remains installed
     until another action is explicitly requested (by another call to
     sigaction), or until one of the exec functions is called.

     If sigaction fails, no new signal handler is installed.

NOTES
     The signal(2) function provides a subset of features available with the
     sigaction function.

     The func argument of signal(2) can have the same values that are
     described for the sa_handler field in the sigaction structure of
     sigaction function.  However, no signal handler mask or flags can be
     specified. Refer to signal(2) for details.

     In a multi-threaded environment, sigaction should only be used for
     synchronous signals.

DIAGNOSTICS
     Upon successful completion of sigaction, a value of 0 (zero) is returned.
     Otherwise, a value of -1 is returned and errno is set to indicate the
     error, and no new signal-catching functions are installed.

ERRORS
     sigaction fails if:

     [EINVAL]  The signal argument is not a valid signal number.

     [EINVAL]  An attempt was made to ignore or supply a handler for the
               SIGKILL, SIGSTOP, and SIGCONT signals.

SEE ALSO
     exit(2), kill(2), raise(2) sigsetops(3) signal(2) sigprocmask(2),
     sigsuspend(2), <signal.h>