term% cat index.txt SIGNAL(2) System Calls Manual SIGNAL(2)
NAME
signal - catch or ignore signals
SYNOPSIS
#include <signal.h>
(*signal(sig, func))()
(*func)();
DESCRIPTION
A signal is generated by some abnormal event, initiated either by user
at a typewriter (quit, interrupt), by a program error (bus error,
etc.), or by request of another program (kill). Normally all signals
cause termination of the receiving process, but a signal call allows
them either to be ignored or to cause an interrupt to a specified loca‐
tion. Here is the list of signals with names as in the include file.
SIGHUP 1 hangup
SIGINT 2 interrupt
SIGQUIT 3* quit
SIGILL 4* illegal instruction (not reset when caught)
SIGTRAP 5* trace trap (not reset when caught)
SIGIOT 6* IOT instruction
SIGEMT 7* EMT instruction
SIGFPE 8* floating point exception
SIGKILL 9 kill (cannot be caught or ignored)
SIGBUS 10* bus error
SIGSEGV 11* segmentation violation
SIGSYS 12* bad argument to system call
SIGPIPE 13 write on a pipe or link with no one to read it
SIGALRM 14 alarm clock
SIGTERM 15 software termination signal
16 unassigned
The starred signals in the list above cause a core image if not caught
or ignored.
If func is SIG_DFL, the default action for signal sig is reinstated;
this default is termination, sometimes with a core image. If func is
SIG_IGN the signal is ignored. Otherwise when the signal occurs func
will be called with the signal number as argument. A return from the
function will continue the process at the point it was interrupted.
Except as indicated, a signal is reset to SIG_DFL after being caught.
Thus if it is desired to catch every such signal, the catching routine
must issue another signal call.
When a caught signal occurs during certain system calls, the call ter‐
minates prematurely. In particular this can occur during a read or
write(2) on a slow device (like a typewriter; but not a file); and dur‐
ing pause or wait(2). When such a signal occurs, the saved user status
is arranged in such a way that when return from the signal-catching
takes place, it will appear that the system call returned an error sta‐
tus. The user's program may then, if it wishes, re-execute the call.
The value of signal is the previous (or initial) value of func for the
particular signal.
After a fork(2) the child inherits all signals. Exec(2) resets all
caught signals to default action.
SEE ALSO
kill(1), kill(2), ptrace(2), setjmp(3)
DIAGNOSTICS
The value (int)-1 is returned if the given signal is out of range.
BUGS
If a repeated signal arrives before the last one can be reset, there is
no chance to catch it.
The type specification of the routine and its func argument are prob‐
lematical.
ASSEMBLER
(signal = 48.)
sys signal; sig; label
(old label in r0)
If label is 0, default action is reinstated. If label is odd, the sig‐
nal is ignored. Any other even label specifies an address in the
process where an interrupt is simulated. An RTI or RTT instruction
will return from the interrupt.
SIGNAL(2)