signal() is a simplified interface to the more general sigvec(2) facility. Programs that use signal() in preference to sigvec() are more likely to be portable to all systems.

A signal is generated by some abnormal event, initiated by a user at a terminal (quit, interrupt, stop), by a program error (bus error, etc.), by request of another program (kill), or when a process is stopped because it wishes to access its control terminal while in the background (see termio(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 cause termination of the receiving process 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 signal() call allows signals either to be ignored or to interrupt to a specified location. The following is a list of all signals with names as in the include file <signal.h>:

SIGHUP 1 hangup SIGINT 2 interrupt SIGQUIT 3* quit SIGILL 4* illegal instruction SIGTRAP 5* trace trap SIGABRT 6* abort (generated by abort(3) routine) SIGEMT 7* emulator trap SIGFPE 8* arithmetic exception SIGKILL 9 kill (cannot be caught, blocked, or ignored) SIGBUS 10* bus error SIGSEGV 11* segmentation violation SIGSYS 12* bad argument to system call SIGPIPE 13 write on a pipe or other socket with no one to read it SIGALRM 14 alarm clock SIGTERM 15 software termination signal SIGURG 16b urgent condition present on socket SIGSTOP 17d stop (cannot be caught, blocked, or ignored) SIGTSTP 18d stop signal generated from keyboard SIGCONT 19b continue after stop SIGCHLD 20b child status has changed SIGTTIN 21d background read attempted from control terminal SIGTTOU 22d background write attempted to control terminal SIGIO 23b I/O is possible on a descriptor (see fcntl(2V)) SIGXCPU 24 cpu time limit exceeded (see getrlimit(2)) SIGXFSZ 25 file size limit exceeded (see getrlimit(2)) SIGVTALRM26 virtual time alarm (see getitimer(2)) SIGPROF 27 profiling timer alarm (see getitimer(2)) SIGWINCH28 window changed (see termio(4) and win(4S)) SIGLOST 29* resource lost (see lockd(8C)) SIGUSR1 30 user-defined signal 1 SIGUSR2 31 user-defined signal 2

The starred signals in the list above cause a core image if not caught or ignored.

If func is SIG_DFL, the default action for signal sig is reinstated; this default is termination (with a core image for starred signals) except for signals marked with b or d. Signals marked with b are discarded if the action is SIG_DFL; signals marked with d cause the process to stop. If func is SIG_IGN the signal is subsequently ignored and pending instances of the signal are discarded. Otherwise, when the signal occurs further occurrences of the signal are automatically blocked and func is called.

A return from the function unblocks the handled signal and continues the process at the point it was interrupted. Unlike previous signal facilities, the handler func remains installed after a signal has been delivered.

If a caught signal occurs during certain system calls, terminating the call prematurely, the call is automatically restarted. In particular this can occur during a read(2V) or write(2V) on a slow device (such as a terminal; but not a file) and during a wait(2V).

The value of signal() is the previous (or initial) value of func for the particular signal.

After a fork(2V) or vfork(2) the child inherits all signals. An execve(2V) resets all caught signals to the default action; ignored signals remain ignored.

If func is SIG_IGN the signal is subsequently ignored and pending instances of the signal are discarded. Otherwise, when the signal occurs, func is called. Further occurrences of the signal are not automatically blocked. The value of func for the caught signal is reset to SIG_DFL before func is called, unless the signal is SIGILL or SIGTRAP.

A return from the function continues the process at the point at which it was interrupted. The handler func does not remain installed after a signal has been delivered.

If a caught signal occurs during certain system calls, causing the call to terminate prematurely, the call is interrupted. In particular this can occur during a read(2V) or write(2V) on a slow device (such as a terminal; but not a file) and during a wait(2V). After the signal catching function returns, the interrupted system call may return a -1 to the calling process with errno set to EINTR.

signal() returns the previous action on success. On failure, it returns -1 and sets errno to indicate the error.

signal() will fail and no action will take place if one of the following occurs:

EINVAL

sig was not a valid signal number. An attempt was made to ignore or supply a handler for SIGKILL or SIGSTOP.

The handler routine can be declared:

void handler(sig, code, scp, addr) int sig, code; struct sigcontext **scp; char **addr;

Here sig is the signal number; code is a parameter of certain signals that provides additional detail; scp is a pointer to the sigcontext structure (defined in <signal.h>), used to restore the context from before the signal; and addr is additional address information. See sigvec(2) for more details.