.TH SIGNAL 2 .CT 2 proc_man .SH NAME signal, kill \(mi receive and send signals .SH SYNOPSIS .nf .B #include .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 : .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 \fLSIGLAB\fP 26# file descriptor label change (not reset when caught) .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 the signal will be caught; when the signal occurs, the function pointed to by .IR func , say .IR catcher (), defined thus, .EX .L int catcher(sig) { ... } .EE will be called with the signal number as argument. A return from the 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 or .IR write (2) on a slow device (like a typewriter, but not a disc); and during .I pause and .IR wait (2). The interrupted system call will return error .BR EINTR . The user's program may then, if it wishes, re-execute the call. .PP Core images are made exactly as if by .IR creat (2) and .IR write (2). However, no core image will be made for a process that ever had capability .BR T_NOCHK or for a child of such a process until the child successfully performs .IR exec (2). .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) .SH DIAGNOSTICS .IR signal : .B EINVAL .br .IR kill : .BR EINVAL , .BR EPERM , .BR ESRCH .SH BUGS Unadvertised and unsupported .I func and .I sig arguments can radically and permanently change the behavior of signals. .br 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.