SIGACTION(2)							System Calls Manual						      SIGACTION(2)

NAME

       sigaction, signal - manage signal state and handlers

SYNOPSIS

       #include <signal.h>

       int sigaction(int sig, const struct sigaction *act, struct sigaction *oact)
       void (*signal(int sig, void (*handler)(int)))(int);

DESCRIPTION

       Sigaction()  is	used to examine, set, or modify the attributes of a signal.  The argument sig is the signal in question.  The act argument
       points to a structure containing the new attributes of the signal, the structure pointed to by oact will receive the  old  attributes  that
       were in effect before the call.

       The  act  and  oact  arguments  may  be NULL to indicate that either no new attributes are to be set, or that the old attributes are not of
       interest.

       The structure containing the signal attributes is defined in <signal.h> and looks like this:

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

       The sa_handler field contains the address of a signal handler, a function that is called when the process is signalled,	or  one  of  these
       special constants:

       SIG_DFL	   Default  signal handling is to be performed.  This usually means that the process is killed, but some signals may be ignored by
		   default.

       SIG_IGN	   Ignore the signal.

       The sa_mask field indicates a set of signals that must be blocked when the signal is being handled.   Whether  the  signal  sig	itself	is
       blocked	when  being handled is not controlled by this mask.  The mask is of a "signal set" type that is to be manipulated by the sigset(3)
       functions.

       How the signal is handled precisely is specified by bits in sa_flags.  If none of the flags is set then the handler is called when the sig-
       nal  arrives.   The signal is blocked during the call to the handler, and unblocked when the handler returns.  A system call that is inter-
       rupted returns -1 with errno set to EINTR.  The following bit flags can be set to modify this behaviour:

       SA_RESETHAND   Reset the signal handler to SIG_DFL when the signal is caught.

       SA_NODEFER     Do not block the signal on entry to the handler.

       SA_COMPAT      Handle the signal in a way that is compatible with the the old signal() call.

       The old signal() signal system call sets a signal handler for a given signal and returns the old signal handler.  No signals  are  blocked,
       the  flags  are SA_RESETHAND | SA_NODEFER | SA_COMPAT.  New code should not use signal().  Note that signal() and all of the SA_* flags are
       Minix extensions.

       Signal handlers are reset to SIG_DFL on an execve(2).  Signals that are ignored stay ignored.

   Signals
       Minix knows about the following signals:

       signal	  num	 notes	 description

       SIGHUP	  1	 k	 Hangup
       SIGINT	  2	 k	 Interrupt (usually DEL or CTRL-C)
       SIGQUIT	  3	 kc	 Quit (usually CTRL-)
       SIGILL	  4	 kc	 Illegal instruction
       SIGTRAP	  5	 xkc	 Trace trap
       SIGABRT	  6	 kc	 Abort program
       SIGFPE	  8	 k	 Floating point exception
       SIGKILL	  9	 k	 Kill
       SIGUSR1	  10	 k	 User defined signal #1
       SIGSEGV	  11	 kc	 Segmentation fault
       SIGUSR2	  12	 k	 User defined signal #2
       SIGPIPE	  13	 k	 Write to a pipe with no reader
       SIGALRM	  14	 k	 Alarm clock
       SIGTERM	  15	 k	 Terminate (default for kill(1))
       SIGCHLD	  17	 pvi	 Child process terminated
       SIGCONT	  18	 p	 Continue if stopped
       SIGSTOP	  19	 ps	 Stop signal
       SIGTSTP	  20	 ps	 Interactive stop signal
       SIGTTIN	  21	 ps	 Background read
       SIGTTOU	  22	 ps	 Background write
       SIGWINCH   23	 xvi	 Window size change

       The letters in the notes column indicate:

       k    The process is killed if the signal is not caught.

       c    The signal causes a core dump.

       i    The signal is ignored if not caught.

       v    Only Minix-vmd implements this signal.

       x    Minix extension, not defined by POSIX.

       p    These signals are not implemented, but POSIX requires that they are defined.

       s    The process should be stopped, but is killed instead.

       The SIGKILL signal cannot be caught or ignored.	The SIGILL and SIGTRAP	signals  cannot  be  automatically  reset.   The  system  silently
       enforces these restrictions.  This may or may not be reflected by the attributes of these signals and the signal masks.

   Types
       POSIX prescribes that <sys/types.h> has the following definition:

	      typedef int (*sighandler_t)(int)

       With this type the following declarations can be made:

	      sighandler_t sa_handler;
	      sighandler_t signal(int sig, sighandler_t handler);

       This  may  help	you  to understand the earlier declarations better.  The sighandler_t type is also very useful in old style C code that is
       compiled by a compiler for standard C.

SEE ALSO

       kill(1), kill(2), pause(2), sigprocmask(2), sigsuspend(2), sigpending(2), sigset(3).

DIAGNOSTICS

       Sigaction() returns 0 on success or -1 on error.  Signal() returns the old handler on success or SIG_ERR on error.  The error code may be:

       EINVAL	 Bad signal number.

       EFAULT	 Bad act or oact addresses.

AUTHOR

       Kees J. Bot (kjb@cs.vu.nl)

																      SIGACTION(2)