libc/sys/sigaction.2

.\" Copyright (c) 1980, 1990 The Regents of the University of California.
.\" All rights reserved.
.\"
.\" %sccs.include.redist.roff%
.\"
.\"	@(#)sigaction.2	6.7 (Berkeley) 03/04/93
.\"
.Dd
.Dt SIGACTION 2
.Os
.Sh NAME
.Nm sigaction
.Nd software signal facilities
.Sh SYNOPSIS
.Fd #include <signal.h>
.Bd -literal
struct sigaction {
	void     (*sa_handler)();
	sigset_t sa_mask;
	int	 sa_flags;
};
.Ed
.Fn sigaction "int sig" "struct sigaction *act" "struct sigaction *oact"
.Sh DESCRIPTION
The system defines a set of signals that may be delivered to a process.
Signal delivery resembles the occurrence of a hardware interrupt:
the signal is blocked from further occurrence, the current process
context is saved, and a new one is built.  A process may specify a
.Em handler
to which a signal is delivered, or specify that a signal is to be
.Em ignored .
A process may also specify that a default action is to be taken
by the system when a signal occurs.
A signal may also be
.Em blocked ,
in which case its delivery is postponed until it is
.Em unblocked .
The action to be taken on delivery is determined at the time
of delivery.
Normally, signal handlers execute on the current stack
of the process.  This may be changed, on a per-handler basis,
so that signals are taken on a special
.Em "signal stack" .
.Pp
Signal routines execute with the signal that caused their
invocation
.Em blocked ,
but other signals may yet occur.
A global
.Em "signal mask"
defines the set of signals currently blocked from delivery
to a process.  The signal mask for a process is initialized
from that of its parent (normally empty).  It
may be changed with a
.Xr sigprocmask 2
call, or when a signal is delivered to the process.
.Pp
When a signal
condition arises for a process, the signal is added to a set of
signals pending for the process.
If the signal is not currently
.Em blocked
by the process then it is delivered to the process.
Signals may be delivered any time a process enters the operating system
(e.g., during a system call, page fault or trap, or clock interrupt).
If multiple signals are ready to be delivered at the same time,
any signals that could be caused by traps are delivered first.
Additional signals may be processed at the same time, with each
appearing to interrupt the handlers for the previous signals
before their first instructions.
The set of pending signals is returned by the
.Xr sigpending 2
function.
When a caught signal
is delivered, the current state of the process is saved,
a new signal mask is calculated (as described below),
and the signal handler is invoked.  The call to the handler
is arranged so that if the signal handling routine returns
normally the process will resume execution in the context
from before the signal's delivery.
If the process wishes to resume in a different context, then it
must arrange to restore the previous context itself.
.Pp
When a signal is delivered to a process a new signal mask is
installed for the duration of the process' signal handler
(or until a
.Xr sigprocmask
call is made).
This mask is formed by taking the union of the current signal mask set,
the signal to be delivered, and
the signal mask associated with the handler to be invoked.
.Pp
.Fn Sigaction
assigns an action for a specific signal.
If
.Fa act
is non-zero, it
specifies an action
.Pf ( Dv SIG_DFL ,
.Dv SIG_IGN ,
or a handler routine) and mask
to be used when delivering the specified signal.
If
.Fa oact
is non-zero, the previous handling information for the signal
is returned to the user.
.Pp
Once a signal handler is installed, it remains installed
until another
.Fn sigaction
call is made, or an
.Xr execve 2
is performed.
A signal-specific default action may be reset by
setting
.Fa sa_handler
to
.Dv SIG_DFL .
The defaults are process termination, possibly with core dump;
no action; stopping the process; or continuing the process.
See the signal list below for each signal's default action.
If
.Fa sa_handler
is
.Dv SIG_DFL ,
the default action for the signal is to discard the signal,
and if a signal is pending,
the pending signal is discarded even if the signal is masked.
If
.Fa sa_handler
is set to
.Dv SIG_IGN
current and pending instances
of the signal are ignored and discarded.
.Pp
Options may be specified by setting
.Em sa_flags .
If the
.Dv SA_NOCLDSTOP
bit is set when installing a catching function
for the
.Dv SIGCHLD
signal,
the
.Dv SIGCHLD
signal will be generated only when a child process exits,
not when a child process stops.
Further, if the
.Dv SA_ONSTACK
bit is set in
.Em sa_flags ,
the system will deliver the signal to the process on a
.Em "signal stack" ,
specified with
.Xr sigstack 2 .
.Pp
If a signal is caught during the system calls listed below,
the call may be forced to terminate
with the error
.Dv EINTR ,
the call may return with a data transfer shorter than requested,
or the call may be restarted.
Restart of pending calls is requested
by setting the
.Dv SA_RESTART
bit in
.Ar sa_flags .
The affected system calls include
.Xr open 2 ,
.Xr read 2 ,
.Xr write 2 ,
.Xr sendto 2 ,
.Xr recvfrom 2 ,
.Xr sendmsg 2
and
.Xr recvmsg 2
on a communications channel or a slow device (such as a terminal,
but not a regular file)
and during a
.Xr wait 2
or
.Xr ioctl 2 .
However, calls that have already committed are not restarted,
but instead return a partial success (for example, a short read count).
.Pp
After a
.Xr fork 2
or
.Xr vfork 2
all signals, the signal mask, the signal stack,
and the restart/interrupt flags are inherited by the child.
.Pp
.Xr Execve 2
reinstates the default
action for all signals which were caught and
resets all signals to be caught on the user stack.
Ignored signals remain ignored;
the signal mask remains the same;
signals that restart pending system calls continue to do so.
.Pp
The following is a list of all signals
with names as in the include file
.Aq Pa signal.h :
.Bl -column SIGVTALARMXX "create core imagexxx"
.It Sy "  NAME  " "	Default Action  " "	Description"
.It Dv SIGHUP No "	terminate process" "	terminal line hangup"
.It Dv SIGINT No "	terminate process" "	interrupt program"
.It Dv SIGQUIT No "	create core image" "	quit program"
.It Dv SIGILL No "	create core image" "	illegal instruction"
.It Dv SIGTRAP No "	create core image" "	trace trap"
.It Dv SIGABRT No "	create core image" Xr 	abort 2
call (formerly
.Dv SIGIOT )
.It Dv SIGEMT No "	create core image" "	emulate instruction executed"
.It Dv SIGFPE No "	create core image" "	floating-point exception"
.It Dv SIGKILL No "	terminate process" "	kill program"
.It Dv SIGBUS No "	create core image" "	bus error"
.It Dv SIGSEGV No "	create core image" "	segmentation violation"
.It Dv SIGSYS No "	create core image" "	system call given invalid argument"
.It Dv SIGPIPE No "	terminate process" "	write on a pipe with no reader"
.It Dv SIGALRM No "	terminate process" "	real-time timer expired"
.It Dv SIGTERM No "	terminate process" "	software termination signal"
.It Dv SIGURG No "	discard signal" "	urgent condition present on socket"
.It Dv SIGSTOP No "	stop process" "	stop (cannot be caught or ignored)"
.It Dv SIGTSTP No "	stop process" "	stop signal generated from keyboard"
.It Dv SIGCONT No "	discard signal" "	continue after stop"
.It Dv SIGCHLD No "	discard signal" "	child status has changed"
.It Dv SIGTTIN No "	stop process" "	background read attempted from control terminal"
.It Dv SIGTTOU No "	stop process" "	background write attempted to control terminal"
.It Dv SIGIO No "	discard signal" Tn "	I/O"
is possible on a descriptor (see
.Xr fcntl 2 )
.It Dv SIGXCPU No "	terminate process" "	cpu time limit exceeded (see"
.Xr setrlimit 2 )
.It Dv SIGXFSZ No "	terminate process" "	file size limit exceeded (see"
.Xr setrlimit 2 )
.It Dv SIGVTALRM No "	terminate process" "	virtual time alarm (see"
.Xr setitimer 2 )
.It Dv SIGPROF No "	terminate process" "	profiling timer alarm (see"
.Xr setitimer 2 )
.It Dv SIGWINCH No "	discard signal" "	Window size change"
.It Dv SIGINFO No "	discard signal" "	status request from keyboard"
.It Dv SIGUSR1 No "	terminate process" "	User defined signal 1"
.It Dv SIGUSR2 No "	terminate process" "	User defined signal 2"
.El
.Sh NOTE
The mask specified in
.Fa act
is not allowed to block
.Dv SIGKILL
or
.Dv SIGSTOP .
This is done silently by the system.
.Sh RETURN VALUES
A 0 value indicated that the call succeeded.  A \-1 return value
indicates an error occurred and
.Va errno
is set to indicated the reason.
.Sh EXAMPLE
The handler routine can be declared:
.Bd -literal -offset indent
void handler(sig, code, scp)
int sig, code;
struct sigcontext *scp;
.Ed
.Pp
Here
.Fa sig
is the signal number, into which the hardware faults and traps are
mapped.
.Fa Code
is a parameter that is either a constant
or the code provided by
the hardware.
.Fa Scp
is a pointer to the
.Fa sigcontext
structure (defined in
.Aq Pa signal.h ) ,
used to restore the context from before the signal.
.Sh ERRORS
.Fn Sigaction
will fail and no new signal handler will be installed if one
of the following occurs:
.Bl -tag -width Er
.It Bq Er EFAULT
Either
.Fa act
or
.Fa oact
points to memory that is not a valid part of the process
address space.
.It Bq Er EINVAL
.Fa Sig
is not a valid signal number.
.It Bq Er EINVAL
An attempt is made to ignore or supply a handler for
.Dv SIGKILL
or
.Dv SIGSTOP .
.El
.Sh STANDARDS
The
.Nm sigaction
function is defined by
.St -p1003.1-88 .
The
.Dv SA_ONSTACK
and
.Dv SA_RESTART
flags are Berkeley extensions,
as are the signals,
.Dv SIGTRAP ,
.Dv SIGEMT ,
.Dv SIGBUS ,
.Dv SIGSYS ,
.Dv SIGURG ,
.Dv SIGIO ,
.Dv SIGXCPU ,
.Dv SIGXFSZ ,
.Dv SIGVTALRM ,
.Dv SIGPROF ,
.Dv SIGWINCH ,
and
.Dv SIGINFO .
Those signals are available on most
.Tn BSD Ns \-derived
systems.
.Sh SEE ALSO
.Xr kill 1 ,
.Xr ptrace 2 ,
.Xr kill 2 ,
.Xr sigaction 2 ,
.Xr sigprocmask 2 ,
.Xr sigsetops 2 ,
.Xr sigsuspend 2 ,
.Xr sigblock 2 ,
.Xr sigsetmask 2 ,
.Xr sigpause 2 ,
.Xr sigstack 2 ,
.Xr sigvec 2 ,
.Xr setjmp 3 ,
.Xr siginterrupt 3 ,
.Xr tty 4