SIGNAL(2) SIGNAL(2)
NAME
signal - catch or ignore signals
SYNOPSIS
#include <signal.h>
int (*signal(sig, func))()
int (*func)();
DESCRIPTION
A signal is generated by some abnormal event, initiated
either by user at a terminal (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 location.
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 with no one to read it
SIGALRM 14 alarm clock
SIGTERM 15 software termination signal
16 unassigned
SIGSTOP 17+ stop (cannot be caught, held or ignored)
SIGCONT 19# continue a stopped process
SIGCHLD 20# child has stopped or exited
* causes core image if not caught or ignored
+ suspends process until SIGCONT or PIOCRUN, see proc(4)
# ignored if not caught
Signals 1 through NSIG, 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).
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.
SIGNAL(2) SIGNAL(2)
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 terminates prematurely. In particular this can occur
during read or write(2) on a slow device (like a typewriter;
but not a file); and during pause and wait(2). When such a
signal occurs, the saved user status is arranged so that
when return from the signal-catching takes place, it will
appear that the system call returned an error status. 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), setjmp(3), proc(4)
DIAGNOSTICS
The value (int)-1 is returned if the given signal is out of
range.
BUGS
Unadvertised and unsupported func arguments can radically
and permanently change the behavior of signals.
Trap syndromes should be distinguishable by extra arguments
to the signal handler.
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 problematical. At the very least the type should be
`void (*)()'.