.TH SIGACTION 2
.SH NAME
sigaction, signal \- manage signal state and handlers
.SH SYNOPSIS
.ft B
#include <signal.h>

.in +5
.ti -5
int sigaction(int \fIsig\fP, const struct sigaction *\fIact\fP, struct sigaction *\fIoact\fP)
.in -5
.br
void (*signal(int \fIsig\fP, void (*\fIhandler\fP)(int)))(int);
.ft P
.SH DESCRIPTION
.de SP
.if t .sp 0.4
.if n .sp
..
.B Sigaction()
is used to examine, set, or modify the attributes of a signal.  The argument
.I sig
is the signal in question.  The
.I act
argument points to a structure containing the new attributes of the signal,
the structure pointed to by
.I oact
will receive the old attributes that were in effect before the call.
.PP
The
.I act
and
.I oact
arguments may be
.B NULL
to indicate that either no new attributes are to be set, or that the old
attributes are not of interest.
.PP
The structure containing the signal attributes is defined in <signal.h> and
looks like this:
.PP
.RS
.nf
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.ta +4n +12n
struct sigaction {
	void	(*sa_handler)(int sig);
	sigset_t	sa_mask;
	int	sa_flags;
};
.ft R
.fi
.RE
.PP
The
.B 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:
.PP
.TP 12
.B 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.
.TP
.B SIG_IGN
Ignore the signal.
.PP
The
.B sa_mask
field indicates a set of signals that must be blocked when the signal is
being handled.  Whether the signal
.I 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
.BR sigset (3)
functions.
.PP
How the signal is handled precisely is specified by bits in
.BR 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
.B \-1
with
.B errno
set to
.BR EINTR .
The following bit flags can be set to modify this behaviour:
.PP
.TP 15
.B SA_RESETHAND
Reset the signal handler to
.B SIG_DFL
when the signal is caught.
.TP
.B SA_NODEFER
Do not block the signal on entry to the handler.
.TP
.B SA_COMPAT
Handle the signal in a way that is compatible with the the old
.B signal()
call.
.PP
The old
.B 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
.BR "SA_RESETHAND | SA_NODEFER | SA_COMPAT" .
New code should not use
.BR signal() .
Note that
.B signal()
and all of the
.B SA_*
flags are MINIX 3 extensions.
.PP
Signal handlers are reset to
.B SIG_DFL
on an
.BR execve (2).
Signals that are ignored stay ignored.
.SS Signals
MINIX 3 knows about the following signals:
.PP
.nf
.ta +11n +7n +8n
signal	num	notes	description
.SP
SIGHUP	1	k	Hangup
SIGINT	2	k	Interrupt (usually DEL or CTRL\-C)
SIGQUIT	3	kc	Quit (usually CTRL\-\e)
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
.ft R
.fi
.PP
The letters in the notes column indicate:
.PP
.TP 5
.B k
The process is killed if the signal is not caught.
.TP
.B c
The signal causes a core dump.
.TP
.B i
The signal is ignored if not caught.
.TP
.B v
Only Minix-vmd implements this signal.
.TP
.B x
MINIX 3 extension, not defined by \s-2POSIX\s+2.
.TP
.B p
These signals are not implemented, but \s-2POSIX\s+2 requires that they are
defined.
.TP
.B s
The process should be stopped, but is killed instead.
.PP
The
.B SIGKILL
signal cannot be caught or ignored.  The
.B SIGILL
and
.B 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.
.SS Types
\s-2POSIX\s+2 prescribes that <sys/types.h> has the following definition:
.PP
.RS
.B "typedef int (*sighandler_t)(int)"
.RE
.PP
With this type the following declarations can be made:
.PP
.RS
.ft B
.nf
sighandler_t sa_handler;
sighandler_t signal(int \fIsig\fP, sighandler_t \fIhandler\fP);
.fi
.ft R
.RE
.PP
This may help you to understand the earlier declarations better.  The
.B sighandler_t
type is also very useful in old style C code that is compiled by a compiler
for standard C.
.SH "SEE ALSO"
.BR kill (1),
.BR kill (2),
.BR pause (2),
.BR sigprocmask (2),
.BR sigsuspend (2),
.BR sigpending (2),
.BR sigset (3).
.SH DIAGNOSTICS
.B Sigaction()
returns
.B 0
on success or
.B \-1
on error.
.B Signal()
returns the old handler on success or
.B SIG_ERR
on error.  The error code may be:
.PP
.TP 10
.B EINVAL
Bad signal number.
.TP
.B EFAULT
Bad
.I act
or
.I oact
addresses.
.SH AUTHOR
Kees J. Bot (kjb@cs.vu.nl)

.\"
.\" $PchId: sigaction.2,v 1.2 1996/04/11 06:00:28 philip Exp $