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.\"     @(#)signal.3	8.3 (Berkeley) 4/19/94
.\" $FreeBSD: src/lib/libc/gen/signal.3,v 1.17.2.9 2003/03/13 18:05:37 trhodes Exp $
.\"
.Dd April 19, 1994
.Dt SIGNAL 3
.Os
.Sh NAME
.Nm signal
.Nd simplified software signal facilities
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In signal.h
.\" The following is Quite Ugly, but syntactically correct.  Don't try to
.\" fix it.
.Ft void
.Fn (*signal "int sig" "void (*func)(int)))(int"
.Pp
or in
.Dx Ns 's
equivalent but easier to read typedef'd version:
.Ft typedef "void (*sig_t) (int)" ;
.Ft sig_t
.Fn signal "int sig" "sig_t func"
.Sh DESCRIPTION
This
.Fn signal
facility
is a simplified interface to the more general
.Xr sigaction 2
facility.
.Pp
Signals allow the manipulation of a process from outside its
domain as well as allowing the process to manipulate itself or
copies of itself (children).
There are two general types of signals:
those that cause termination of a process and those that do not.
Signals which cause termination of a program might result from
an irrecoverable error or might be the result of a user at a terminal
typing the `interrupt' character.
Signals are used when a process is stopped because it wishes to access
its control terminal while in the background (see
.Xr tty 4 ) .
Signals are optionally generated
when a process resumes after being stopped,
when the status of child processes changes,
or when input is ready at the control terminal.
Most signals result in the termination of the process receiving them
if no action
is taken; some signals instead cause the process receiving them
to be stopped, or are simply discarded if the process has not
requested otherwise.
Except for the
.Dv SIGKILL
and
.Dv SIGSTOP
signals, the
.Fn signal
function allows for a signal to be caught, to be ignored, or to generate
an interrupt.
These signals are defined in the file
.In signal.h :
.Bl -column SIGCKPTEXITXX "create core imagexxx"
.It Sy NAME Ta Sy Default Action Ta Sy Description
.It Dv SIGHUP Ta terminate process Ta terminal line hangup
.It Dv SIGINT Ta terminate process Ta interrupt program
.It Dv SIGQUIT Ta create core image Ta quit program
.It Dv SIGILL Ta create core image Ta illegal instruction
.It Dv SIGTRAP Ta create core image Ta trace trap
.It Dv SIGABRT Ta create core image Ta abort program (formerly Dv SIGIOT )
.It Dv SIGEMT Ta create core image Ta emulate instruction executed
.It Dv SIGFPE Ta create core image Ta floating-point exception
.It Dv SIGKILL Ta terminate process Ta kill program
.It Dv SIGBUS Ta create core image Ta bus error
.It Dv SIGSEGV Ta create core image Ta segmentation violation
.It Dv SIGSYS Ta create core image Ta non-existent system call invoked
.It Dv SIGPIPE Ta terminate process Ta write on a pipe with no reader
.It Dv SIGALRM Ta terminate process Ta real-time timer expired
.It Dv SIGTERM Ta terminate process Ta software termination signal
.It Dv SIGURG Ta discard signal Ta urgent condition present on socket
.It Dv SIGSTOP Ta stop process Ta stop (cannot be caught or ignored)
.It Dv SIGTSTP Ta stop process Ta stop signal generated from keyboard
.It Dv SIGCONT Ta discard signal Ta continue after stop
.It Dv SIGCHLD Ta discard signal Ta child status has changed
.It Dv SIGTTIN Ta stop process Ta background read attempted from control terminal
.It Dv SIGTTOU Ta stop process Ta background write attempted to control terminal
.It Dv SIGIO Ta discard signal Ta I/O is possible on a descriptor (see Xr fcntl 2 )
.It Dv SIGXCPU Ta terminate process Ta cpu time limit exceeded (see Xr setrlimit 2 )
.It Dv SIGXFSZ Ta terminate process Ta file size limit exceeded (see Xr setrlimit 2 )
.It Dv SIGVTALRM Ta terminate process Ta virtual time alarm (see Xr setitimer 2 )
.It Dv SIGPROF Ta terminate process Ta profiling timer alarm (see Xr setitimer 2 )
.It Dv SIGWINCH Ta discard signal Ta window size change
.It Dv SIGINFO Ta discard signal Ta status request from keyboard
.It Dv SIGUSR1 Ta terminate process Ta user defined signal 1
.It Dv SIGUSR2 Ta terminate process Ta user defined signal 2
.It Dv SIGCKPT Ta checkpoint process Ta checkpoint
.It Dv SIGCKPTEXIT Ta terminate process Ta checkpoint and exit
.El
.Pp
The
.Fa sig
argument specifies which signal was received.
The
.Fa func
procedure allows a user to choose the action upon receipt of a signal.
To set the default action of the signal to occur as listed above,
.Fa func
should be
.Dv SIG_DFL .
A
.Dv SIG_DFL
resets the default action.
To ignore the signal
.Fa func
should be
.Dv SIG_IGN .
This will cause subsequent instances of the signal to be ignored
and pending instances to be discarded.
If
.Dv SIG_IGN
is not used,
further occurrences of the signal are
automatically blocked and
.Fa func
is called.
.Pp
The handled signal is unblocked when the
function returns and
the process continues from where it left off when the signal occurred.
.Bf -symbolic
Unlike previous signal facilities, the handler
func() remains installed after a signal has been delivered.
.Ef
.Pp
For some system calls, if a signal is caught while the call is
executing and the call is prematurely terminated,
the call is automatically restarted.
(The handler is installed using the
.Dv SA_RESTART
flag with
.Xr sigaction 2 . )
The affected system calls include
.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 low speed device
and during a
.Xr ioctl 2
or
.Xr wait 2 .
However, calls that have already committed are not restarted,
but instead return a partial success (for example, a short read count).
These semantics could be changed with
.Xr siginterrupt 3 .
.Pp
When a process which has installed signal handlers forks,
the child process inherits the signals.
All caught signals may be reset to their default action by a call
to the
.Xr execve 2
function;
ignored signals remain ignored.
.Pp
See
.Xr sigaction 2
for a list of functions
that are considered safe for use in signal handlers.
.Sh RETURN VALUES
The previous action is returned on a successful call.
Otherwise,
.Dv SIG_ERR
is returned and  the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn signal
function
will fail and no action will take place if one of the
following occur:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa sig
argument
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 SEE ALSO
.Xr kill 1 ,
.Xr kill 2 ,
.Xr ptrace 2 ,
.Xr sigaction 2 ,
.Xr sigaltstack 2 ,
.Xr sigprocmask 2 ,
.Xr sigsuspend 2 ,
.Xr fpsetmask 3 ,
.Xr setjmp 3 ,
.Xr siginterrupt 3 ,
.Xr tty 4
.Sh HISTORY
This
.Fn signal
facility appeared in
.Bx 4.0 .