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.\"     @(#)signal.3	8.3 (Berkeley) 4/19/94
.\"
.Dd June 5, 2016
.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 \*(lp*
.Fn signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint"
.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.
See
.Xr signal 7
for comprehensive list of supported signals.
.Pp
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
.Fn 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).
.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
Only functions that are async-signal-safe can safely be used in signal
handlers, see
.Xr sigaction 2
for a complete list.
.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
.Fn signal
will fail and no action will take place if one of the following occur:
.Bl -tag -width Er
.It Bq Er EINVAL
Specified
.Em sig
is not a valid signal number;
or 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 bsd_signal 3 ,
.Xr psignal 3 ,
.Xr setjmp 3 ,
.Xr strsignal 3 ,
.Xr tty 4 ,
.Xr signal 7
.Sh HISTORY
This
.Fn signal
facility appeared in
.Bx 4.0 .