.\" Copyright (c) 1980 Regents of the University of California.
.\" All rights reserved. The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\" @(#)signal.3 6.5 (Berkeley) 7/1/90
.TH SIGNAL 3C "July 1, 1990"
signal \- simplified software signal facilities
.B void (*signal(sig, func))()
is a simplified interface to the more general
is generated by some abnormal event,
initiated by a user at a terminal (quit, interrupt, stop),
by a program error (bus error, etc.),
by request of another program (kill),
or when a process is stopped because it wishes to access
its control terminal while in the background (see
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 cause termination of the receiving process 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
Except for the SIGKILL and SIGSTOP
call allows signals either to be ignored
or to cause an interrupt to a specified location.
The following is a list of all signals with
names as in the include file
.ta \w'SIGVTALRM 'u +\w'15* 'u
SIGILL 4* illegal instruction
SIGABRT 6* \fIabort\fP() call (formerly SIGIOT)
SIGEMT 7* EMT instruction
SIGFPE 8* floating point exception
SIGKILL 9 kill (cannot be caught or ignored)
SIGSEGV 11* segmentation violation
SIGSYS 12* bad argument to system call
SIGPIPE 13 write on a pipe with no one to read it
SIGTERM 15 software termination signal
SIGURG 16\*b urgent condition present on socket
SIGSTOP 17\*d stop (cannot be caught or ignored)
SIGTSTP 18\*d stop signal generated from keyboard
SIGCONT 19\*b continue after stop
SIGCHLD 20\*b child status has changed
SIGTTIN 21\*d background read attempted from control terminal
SIGTTOU 22\*d background write attempted to control terminal
SIGIO 23\*b i/o is possible on a descriptor (see \fIfcntl\fP(2))
SIGXCPU 24 cpu time limit exceeded (see \fIsetrlimit\fP(2))
SIGXFSZ 25 file size limit exceeded (see \fIsetrlimit\fP(2))
SIGVTALRM 26 virtual time alarm (see \fIsetitimer\fP(2))
SIGPROF 27 profiling timer alarm (see \fIsetitimer\fP(2))
SIGWINCH 28\*b Window size change
SIGINFO 29\*b status request from keyboard
SIGUSR1 30 User defined signal 1
SIGUSR2 31 User defined signal 2
The starred signals in the list above cause a core image
if not caught or ignored.
is SIG_DFL, the default action
is reinstated; this default is termination
(with a core image for starred signals)
except for signals marked with \*b or \*d.
Signals marked with \*b are discarded if the action
is SIG_DFL; signals marked
with \*d cause the process to stop.
is SIG_IGN the signal is subsequently ignored
and pending instances of the signal are discarded.
Otherwise, when the signal occurs
further occurrences of the signal are
automatically blocked and
A return from the function unblocks
continues the process at the point it was interrupted.
\fBUnlike previous signal facilities, the handler \fIfunc\fP
remains installed after a signal has been delivered.\fP
If a caught signal occurs
during certain system calls, causing
the call to terminate prematurely, the call
is automatically restarted
(the handler is installed using the SA_RESTART flag with
The affected system calls include
on a communications channel or a slow device (such as a terminal,
However, calls that have already committed are not restarted,
but instead return a partial success (for example, a short read count).
is the previous (or initial)
for the particular signal.
resets all caught signals to the default action;
ignored signals remain ignored.
The previous action is returned on a successful call.
Otherwise, \-1 is returned and
is set to indicate the error.
will fail and no action will take place if one of the
is not a valid signal number.
An attempt is made to ignore or supply a handler for SIGKILL
kill(1), ptrace(2), kill(2),
sigaction(2), sigprocmask(2), sigsuspend(2),
sigstack(2), setjmp(3), tty(4)
The handler routine can be declared:
void handler(sig, code, scp)
is the signal number, into which the hardware faults and traps are
mapped as defined below. Code is a parameter which is either a constant
as given below or, for compatibility mode faults, the code provided by
used by the system to restore the process context from before
Compatibility mode faults are distinguished from the
other SIGILL traps by having PSL_CM set in the psl.
The following defines the mapping of hardware traps to signals
and codes. All of these symbols are defined in
.ta \w' Floating/decimal divide by zero 'u +\w'15* 'u +8n
Hardware condition Signal Code
Integer overflow SIGFPE FPE_INTOVF_TRAP
Integer division by zero SIGFPE FPE_INTDIV_TRAP
Floating overflow trap SIGFPE FPE_FLTOVF_TRAP
Floating/decimal division by zero SIGFPE FPE_FLTDIV_TRAP
Floating underflow trap SIGFPE FPE_FLTUND_TRAP
Decimal overflow trap SIGFPE FPE_DECOVF_TRAP
Subscript-range SIGFPE FPE_SUBRNG_TRAP
Floating overflow fault SIGFPE FPE_FLTOVF_FAULT
Floating divide by zero fault SIGFPE FPE_FLTDIV_FAULT
Floating underflow fault SIGFPE FPE_FLTUND_FAULT
Length access control SIGSEGV
Protection violation SIGBUS
Reserved instruction SIGILL ILL_RESAD_FAULT
Customer-reserved instr. SIGEMT
Reserved operand SIGILL ILL_PRIVIN_FAULT
Reserved addressing SIGILL ILL_RESOP_FAULT
Compatibility-mode SIGILL hardware supplied code
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