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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.
The handler
Fa func
remains installed after a signal has been delivered.
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
The
Fn signal
facility first appeared in
At v4 .
It used to be a system call that implemented a different semantics,
Dq old signals ,
in which a signal was reset to
Dv SIG_DFL
after being caught.
Pp
This
Dq new signals
facility appeared in
Bx 4.0
as jobs library and became the default
Xr signal 3
in
Bx 4.2 .

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