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SIGNAL(2) System Calls Manual SIGNAL(2) NAME signal, kill - receive and send signals SYNOPSIS #include <signal.h> SIG_TYP signal(sig, func) SIG_TYP func; int kill(pid, sig) 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 kill in another process. Normally, most signals cause termination of the receiving process, but signal allows them either to be ignored or to be caught by interrupting to a specified function. The following signal names are defined in 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 or ignored) SIGCONT 19# continue a stopped process SIGCHLD 20# child has stopped or exited * places core image in file core if not caught or ignored + suspends process until SIGCONT or PIOCRUN; see proc(4) # ignored if not caught Signals 1 through NSIG-1, defined in the include file, exist. Those not listed above have no conventional meaning in this system. (Berke- ley systems use 1-15 and 17-25.) Signal specifies how signal sig will be handled. If func is SIG_DFL, the default action listed above is reinstated. If func is SIG_IGN, the signal will be ignored. Otherwise, when the signal occurs, it will be caught and a function, pointed to by func, will be called. The type of pointer func is SIG_TYP: typedef int (*SIG_TYP)(); It must point to a function such as, int catcher(sig) { ... } which will be called with a signal number as argument. A return from the catcher function will continue the process at the point it was in- terrupted. 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 read(2) or write on a slow device (like a typewriter, but not a disk), and during pause and wait; see alarm(2) and exit(2). The interrupted system call will return error EINTR. The user's program may then, if it wishes, re-execute the call. Signal returns the previous (or initial) value of func for the particu- lar signal. After a fork(2) the child inherits all signal settings. Exec(2) resets all caught signals to default action. Kill sends signal sig to the process specified by process id pid. Sig- nal 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. The effective userid of the sending process must be either 0 or the ef- fective userid of the receiving process. If pid is 0, the signal is sent to all other processes in the sender's process group; see stream(4). If 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 init(8). If pid is less than -1, it is negated and taken as a process group whose members should receive the signal. Processes may send signals to themselves. FILES SEE ALSO kill(1), setjmp(3), stream(4) DIAGNOSTICS signal: EINVAL kill: EINVAL, EPERM, ESRCH BUGS The reason for a trap 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. For historical reasons, the return value of a catcher function is int; it is void in ANSI standard C. SIGNAL(2)