@node sigaction, signal
@subheading Syntax

@example
#include <signal.h>

int sigaction (int sig, const struct sigaction *act, struct sigaction *oact);
@end example

@subheading Description

This function allows to examine and/or change the action associated with
a signal @var{sig}.  The @code{struct sigaction} structure, defined by
the header file @file{signal.h}, is declared as follows:

@example
 struct sigaction @{
   int sa_flags;             /* flags for the action; currently ignored */
   void (*sa_handler)(int);  /* the handler for the signal */
   sigset_t sa_mask;         /* additional signals to be blocked */
 @};
@end example

The @code{sa_handler} member is a signal handler, see @ref{signal}.  The
@code{sa_mask} member defines the signals, in addition to @var{sig},
which are to be blocked during the execution of @code{sa_handler}.

The @code{sigaction} function sets the structure pointed to by
@code{oact} to the current action for the signal @var{sig}, and then
sets the new action for @var{sig} as specified by @var{act}.  If the
@var{act} argument is @code{NULL}, @code{sigaction} returns the current
signal action in @var{oact}, but doesn't change it.  If the @var{oact}
argument is a @code{NULL} pointer, it is ignored.  Thus, passing
@code{NULL} pointers for both @code{act} and @code{oact} is a way to
see if @var{sig} is a valid signal number on this system (if not,
@code{sigaction} will return -1 and set @code{errno}).

@subheading Return Value

0 on success, -1 for illegal value of @var{sig}.

@subheading Portability

@portability !ansi, posix