SIGNAL(3) BSD Programmer's Manual SIGNAL(3) NNAAMMEE ssiiggnnaall - simplified software signal facilities SSYYNNOOPPSSIISS ##iinncclluuddee <<ssiiggnnaall..hh>> _v_o_i_d **ssiiggnnaall(_s_i_g, _f_u_n_c()); _v_o_i_d (**ffuunncc)() DDEESSCCRRIIPPTTIIOONN This ssiiggnnaall() facility is a simplified interface to the more general sigaction(2) facility. Signals allow the manipulation of a process from outside its domain as well as allowing the process to manipulate itself or copies of itself (children). There are two general types of signals: those that cause ter- mination of a process and those that do not. Signals which cause termi- nation of a program might result from an irrecoverable error or might be the result of a user at a terminal typing the `interrupt' character. Signals are used when a process is stopped because it wishes to access its control terminal while in the background (see tty(4)). Signals are optionally generated when a process resumes after being stopped, when the status of child processes changes, or when input is ready at the control terminal. Most signals result in the termination of the process receiv- ing them if no action is taken; some signals instead cause the process receiving them to be stopped, or are simply discarded if the process has not requested otherwise. Except for the SIGKILL and SIGSTOP signals, the ssiiggnnaall() function allows for a signal to be caught, to be ignored, or to generate an interupt. These signals are defined in the file <_s_i_g_n_a_l_._h>: NNaammee DDeeffaauulltt AAccttiioonn DDeessccrriippttiioonn SIGHUP terminate process terminal line hangup SIGINT terminate process interrupt program SIGQUIT create core image quit program SIGILL create core image illegal instruction SIGTRAP create core image trace trap SIGABRT create core image abort(2) call (formerly SIGIOT) SIGEMT create core image emulate instruction executed SIGFPE create core image floating-point exception SIGKILL terminate process kill program SIGBUS create core image bus error SIGSEGV create core image segmentation violation SIGSYS create core image system call given invalid argument SIGPIPE terminate process write on a pipe with no reader SIGALRM terminate process real-time timer expired SIGTERM terminate process software termination signal SIGURG discard signal urgent condition present on socket SIGSTOP stop process stop (cannot be caught or ignored) SIGTSTP stop process stop signal generated from keyboard SIGCONT discard signal continue after stop SIGCHLD discard signal child status has changed SIGTTIN stop process background read attempted from control terminal SIGTTOU stop process background write attempted to control terminal SIGIO discard signal I/O is possible on a descriptor (see fcntl(2)) SIGXCPU terminate process cpu time limit exceeded (see setrlimit(2)) SIGXFSZ terminate process file size limit exceeded (see setrlimit(2)) SIGVTALRM terminate process virtual time alarm (see setitimer(2)) SIGPROF terminate process profiling timer alarm (see setitimer(2)) SIGWINCH discard signal Window size change SIGINFO discard signal status request from keyboard SIGUSR1 terminate process User defined signal 1 SIGUSR2 terminate process User defined signal 2 The _f_u_n_c procedure allows a user to choose the action upon receipt of a signal. To set the default action of the signal to occur as listed above, _f_u_n_c should be SIG_DFL. A SIG_DFL resets the default action. To ignore the signal _f_u_n_c should be SIG_IGN. This will cause subsequent in- stances of the signal to be ignored and pending instances to be discard- ed. If SIG_IGN is not used, further occurrences of the signal are auto- matically blocked and _f_u_n_c is called. The handled signal is unblocked with the function returns and the process continues from where it left off when the signal occured. UUnnlliikkee pprreevvii-- oouuss ssiiggnnaall ffaacciilliittiieess,, tthhee hhaannddlleerr ffuunncc(()) rreemmaaiinnss iinnssttaalllleedd aafftteerr aa ssiigg-- nnaall hhaass bbeeeenn ddeelliivveerreedd.. For some system calls, if a signal is caught while the call is executing and the call is permaturely terminated, the call is automatically restarted. (The handler is installed using the SA_RESTART flag with sigaction(2).) The affected system calls include read(2), write(2), sendto(2), recvfrom(2), sendmsg(2) and recvmsg(2) on a communications channel or a low speed device and during a ioctl(2) or wait(2). However, calls that have already committed are not restarted, but instead return a partial success (for example, a short read count). When a process which has installed signal handlers forks, the child pro- cess inherits the signals. All caught signals may be reset to their de- fault action by a call to the execve(2) function; ignored signals remain ignored. RREETTUURRNN VVAALLUUEESS The previous action is returned on a successful call. Otherwise, -1 is returned and the global variable _e_r_r_n_o is set to indicate the error. EERRRROORRSS Signal will fail and no action will take place if one of the following occur: [EINVAL] _S_i_g is not a valid signal number. [EINVAL] An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP. SSEEEE AALLSSOO kill(1), ptrace(2), kill(2), sigaction(2), sigaltstack(2), sigprocmask(2), sigsuspend(2), setjmp(3), tty(4) HHIISSTTOORRYY This ssiiggnnaall facility appeared in 4.0BSD. 4th Berkeley Distribution June 4, 1993 2