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 signal arrives. The signal is blocked during the call to the handler, and unblocked when the handler returns. A system call that is interrupted 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 3 extensions.

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

  Signals
MINIX 3 knows about the following signals:

signal	num	notes	description
SIGHUP	1	km	Hangup
SIGINT	2	k	Interrupt (usually DEL or CTRL-C)
SIGQUIT	3	kcm	Quit (usually CTRL-\)
SIGILL	4	Kc	Illegal instruction
SIGTRAP	5	Kc	Trace trap
SIGABRT	6	kcm	Abort program
SIGBUS	7	Kc	Bus error
SIGFPE	8	Kc	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	km	Terminate (default for kill(1))
SIGEMT	16	xKc	Emulator trap
SIGCHLD	17	pi	Child process terminated
SIGCONT	18	pi	Continue if stopped
SIGSTOP	19	ps	Stop signal
SIGTSTP	20	ps	Interactive stop signal
SIGWINCH	21	xi	Window size change
SIGTTIN	22	ps	Background read
SIGTTOU	23	ps	Background write
SIGVTALRM	24	k	Virtual alarm clock
SIGPROF	25	k	Profiler alarm clock

The letters in the notes column indicate:

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

K   The process is killed if the signal is not caught. If the signal is received due to an exception while ignored or masked, the process is killed even if a handler is defined to catch the signal.

c   The signal causes a core dump.

i   The signal is ignored if not caught.

m   The signal is converted to a message for system processes.

x   MINIX 3 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 and SIGSTOP signals 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)