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.Dd December 1, 2017
.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 \*(lp*signal "int sig" "void \*(lp*func\*(rp\*(lpint\*(rp\*(rp\*(rp\*(lpint"
.Pp
or in
.Fx Ap s
equivalent but easier to read typedef'd version:
.Ft typedef "void \*(lp*sig_t\*(rp \*(lpint\*(rp" ;
.Pp
.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 No ".Dv SIGVTALRM" "create core image"
.It Sy "Num" Ta Sy "Name" Ta Sy "Default Action" Ta Sy "Description"
.It 1 Ta Dv SIGHUP Ta "terminate process" Ta "terminal line hangup"
.It 2 Ta Dv SIGINT Ta "terminate process" Ta "interrupt program"
.It 3 Ta Dv SIGQUIT Ta "create core image" Ta "quit program"
.It 4 Ta Dv SIGILL Ta "create core image" Ta "illegal instruction"
.It 5 Ta Dv SIGTRAP Ta "create core image" Ta "trace trap"
.It 6 Ta Dv SIGABRT Ta "create core image" Ta "abort program"
(formerly
.Dv SIGIOT )
.It 7 Ta Dv SIGEMT Ta "create core image" Ta "emulate instruction executed"
.It 8 Ta Dv SIGFPE Ta "create core image" Ta "floating-point exception"
.It 9 Ta Dv SIGKILL Ta "terminate process" Ta "kill program"
.It 10 Ta Dv SIGBUS Ta "create core image" Ta "bus error"
.It 11 Ta Dv SIGSEGV Ta "create core image" Ta "segmentation violation"
.It 12 Ta Dv SIGSYS Ta "create core image" Ta "non-existent system call invoked"
.It 13 Ta Dv SIGPIPE Ta "terminate process" Ta "write on a pipe with no reader"
.It 14 Ta Dv SIGALRM Ta "terminate process" Ta "real-time timer expired"
.It 15 Ta Dv SIGTERM Ta "terminate process" Ta "software termination signal"
.It 16 Ta Dv SIGURG Ta "discard signal" Ta "urgent condition present on socket"
.It 17 Ta Dv SIGSTOP Ta "stop process" Ta "stop (cannot be caught or ignored)"
.It 18 Ta Dv SIGTSTP Ta "stop process" Ta "stop signal generated from keyboard"
.It 19 Ta Dv SIGCONT Ta "discard signal" Ta "continue after stop"
.It 20 Ta Dv SIGCHLD Ta "discard signal" Ta "child status has changed"
.It 21 Ta Dv SIGTTIN Ta "stop process" Ta "background read attempted from"
control terminal
.It 22 Ta Dv SIGTTOU Ta "stop process" Ta "background write attempted to"
control terminal
.It 23 Ta Dv SIGIO Ta "discard signal" Ta Tn "I/O"
is possible on a descriptor (see
.Xr fcntl 2 )
.It 24 Ta Dv SIGXCPU Ta "terminate process" Ta "cpu time limit exceeded (see"
.Xr setrlimit 2 )
.It 25 Ta Dv SIGXFSZ Ta "terminate process" Ta "file size limit exceeded (see"
.Xr setrlimit 2 )
.It 26 Ta Dv SIGVTALRM Ta "terminate process" Ta "virtual time alarm (see"
.Xr setitimer 2 )
.It 27 Ta Dv SIGPROF Ta "terminate process" Ta "profiling timer alarm (see"
.Xr setitimer 2 )
.It 28 Ta Dv SIGWINCH Ta "discard signal" Ta "Window size change"
.It 29 Ta Dv SIGINFO Ta "discard signal" Ta "status request from keyboard"
.It 30 Ta Dv SIGUSR1 Ta "terminate process" Ta "User defined signal 1"
.It 31 Ta Dv SIGUSR2 Ta "terminate process" Ta "User defined signal 2"
.It 32 Ta Dv SIGTHR Ta "terminate process" Ta "thread interrupt"
.It 33 Ta Dv SIGLIBRT Ta "terminate process" Ta "real-time library interrupt"
.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.
Any handler installed with
.Xr signal 3
will have the
.Dv SA_RESTART
flag set, meaning that any restartable system call will not return on
receipt of a signal.
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
If a process explicitly specifies
.Dv SIG_IGN
as the action for the signal
.Dv SIGCHLD ,
the system will not create zombie processes when children
of the calling process exit.
As a consequence, the system will discard the exit status
from the child processes.
If the calling process subsequently issues a call to
.Xr wait 2
or equivalent, it will block until all of the calling process's
children terminate, and then return a value of \-1 with
.Va errno
set to
.Er ECHILD .
.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, 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 wait 2 ,
.Xr fpsetmask 3 ,
.Xr setjmp 3 ,
.Xr siginterrupt 3 ,
.Xr tty 4
.Sh HISTORY
The
.Fn signal
function appeared in
.At v4 .
The current
.Nm
facility appeared in
.Bx 4.0 .
The option to avoid the creation of child zombies through ignoring
.Dv SIGCHLD
appeared in
.Fx 5.0 .