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 (*)()'.