SIGNAL(3V) — SYSTEM V LIBRARY

NAME

signal − simplified software signal facilities

SYNOPSIS

#include <signal.h>

void (∗signal(sig, func))()
void (∗func)();

DESCRIPTION

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>:

SIGHUP1hangup
SIGINT2interrupt
SIGQUIT3∗quit
SIGILL4∗illegal instruction
SIGTRAP5∗trace trap
SIGABRT6∗abort (generated by abort(3) routine)
SIGEMT7∗emulator trap
SIGFPE8∗arithmetic exception
SIGKILL9kill (cannot be caught, blocked, or ignored)
SIGBUS10∗bus error
SIGSEGV11∗segmentation violation
SIGSYS12∗bad argument to system call
SIGPIPE13write on a pipe or other socket with no one to read it
SIGALRM14alarm clock
SIGTERM15software termination signal
SIGURG16•urgent condition present on socket
SIGSTOP17†stop (cannot be caught, blocked, or ignored)
SIGTSTP18†stop signal generated from keyboard
SIGCONT19•continue after stop (cannot be blocked)
SIGCHLD20•child status has changed
SIGTTIN21†background read attempted from control terminal
SIGTTOU22†background write attempted to control terminal
SIGIO23•I/O is possible on a descriptor (see fcntl(2V))
SIGXCPU24cpu time limit exceeded (see
getrlimit(2))
SIGXFSZ25file size limit exceeded (see
getrlimit(2))
SIGVTALRM26 virtual time alarm (see
getitimer(2))
SIGPROF27profiling timer alarm (see
getitimer(2))
SIGWINCH28•window changed (see termio(4) and win(4S))
SIGLOST29∗resource lost (see
lockd(8C))
SIGUSR130user-defined signal 1
SIGUSR231user-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 • or †. Signals marked with • are discarded if the action is SIG_DFL; signals marked with † 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 func is called. 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 it was interrupted.

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(2). After the signal catching function returns, the interrupted system call may return a −1 to the calling process with errno set to EINTR.

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

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

NOTES

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.

RETURN VALUE

The previous action is returned on a successful call. Otherwise, −1 is returned and errno is set to indicate the error.

ERRORS

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

EINVAL sig is not a valid signal number.

EINVAL An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP.

EINVAL An attempt is made to ignore SIGCONT (by default SIGCONT is ignored).

Museum

Related Articles