V10/man/man2/signal.2

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.TH SIGNAL 2
.CT 2 proc_man
.SH NAME
signal, kill \(mi receive and send signals
.SH SYNOPSIS
.nf
.B #include <signal.h>
.PP
.B SIG_TYP signal(sig, func)
.B SIG_TYP func;
.PP
.B int kill(pid, sig)
.fi
.SH DESCRIPTION
A signal
is generated by some abnormal event
initiated by a user at a terminal (quit, interrupt),
by a program error (bus error, etc.),
or by
.I kill
in another process.
Normally, most signals
cause termination of the receiving process,
but
.I signal
allows them either to be ignored
or to be caught by interrupting to a specified function.
The following signal names are defined in
.FR <signal.h> :
.LP
.nf
.ta \w'SIGMMMM 'u +\w'15*  'u
\fLSIGHUP\fP	1	hangup
\fLSIGINT\fP	2	interrupt
\fLSIGQUIT\fP	3*	quit
\fLSIGILL\fP	4*	illegal instruction (not reset when caught)
\fLSIGTRAP\fP	5*	trace trap (not reset when caught)
\fLSIGIOT\fP	6*	IOT instruction
\fLSIGEMT\fP	7*	EMT instruction
\fLSIGFPE\fP	8*	floating point exception
\fLSIGKILL\fP	9	kill (cannot be caught or ignored)
\fLSIGBUS\fP	10*	bus error
\fLSIGSEGV\fP	11*	segmentation violation
\fLSIGSYS\fP	12*	bad argument to system call
\fLSIGPIPE\fP	13	write on a pipe with no one to read it
\fLSIGALRM\fP	14	alarm clock
\fLSIGTERM\fP	15	software termination signal
	16	unassigned
\fLSIGSTOP\fP	17+	stop (cannot be caught or ignored)
\fLSIGCONT\fP	19#	continue a stopped process
\fLSIGCHLD\fP	20#	child has stopped or exited
.sp
.fi
*	places core image in file
.B core
if not caught or ignored
.br
+	suspends process until
.B SIGCONT
or
.BR PIOCRUN ;
see
.IR proc (4)
.br
#	ignored if not caught
.PP
Signals 1 through
.BR NSIG -1,
defined in the include file, exist.
Those not listed above have
no conventional meaning in this system.
(Berkeley systems use 1-15 and 17-25.)
.PP
.I Signal
specifies how signal
.I sig
will be handled.
If
.I func
is
.BR SIG_DFL ,
the default action listed above is reinstated.
If
.I func
is
.BR SIG_IGN ,
the signal will be ignored.
Otherwise, when the signal occurs, it will be caught and
a function, pointed to by
.IR func ,
will be called.  
The type of pointer
.I func
is 
.BR SIG_TYP :
.IP
.B typedef int (*SIG_TYP)();
.LP
It must point to a function such as,
.EX
.L
        int catcher(sig) { ... }
.EE
which will be called with a
signal number as argument.
A return from the catcher function will
continue the process at the point it was interrupted.
.PP
Except as indicated, a signal is reset to
.B SIG_DFL
after being caught.
Thus if it is desired to catch every such signal,
the catching routine must issue another
.I signal
call.
.PP
When a caught signal occurs
during certain system calls, the call terminates prematurely.
In particular this can occur during
.IR read (2)
or
.IR write
on a slow device (like a typewriter, but not a disk),
and during
.IR pause 
and
.IR wait ;
see
.IR alarm (2)
and
.IR exit (2).
The interrupted system call will return error
.BR EINTR .
The user's program may then, if it wishes, re-execute the call.
.PP
.I Signal
returns the previous (or initial)
value of
.I func
for the particular signal.
.PP
After a
.IR  fork (2)
the child inherits all signal settings.
.IR  Exec (2)
resets all caught signals to default action.
.PP
.I Kill
sends signal
.I sig
to the process specified by process id
.I pid.
Signal 0
has no effect on the target process and may be used to
test the existence of a process.
The success of sending a signal is independent of how the receiving
process treats the signal.
.PP
The effective userid of the sending process must be either 0
or the effective userid of the receiving process.
.PP
If
.I pid
is 0, the signal is sent to all other processes in the
sender's process group; see
.IR stream (4).
.PP
If
.I pid
is \-1, and the user is the super-user,
the signal is broadcast universally
except to processes 0 (scheduler),
1 (initialization)
and 2 (pageout); see
.IR init (8).
If
.I pid
is less than \-1,
it is negated
and taken as a process group
whose members should receive the signal.
.PP
Processes may send signals to themselves.
.SH FILES
.F core
.SH "SEE ALSO"
.IR kill (1),
.IR setjmp (3),
.IR stream (4)
.SH DIAGNOSTICS
.IR signal :
.B EINVAL
.br
.IR kill :
.BR EINVAL ,
.BR EPERM ,
.BR ESRCH
.SH BUGS
The reason for a trap should be distinguishable by extra arguments
to the signal handler.
.br
If a repeated signal arrives before the last one can be reset,
there is no chance to catch it.
.br
For historical reasons, the return value of
a catcher function is
.BR int ;
it is 
.B void
in 
.SM ANSI
standard C.