SIGNAL(3C) — UNIX Programmer’s Manual

NAME

signal − simplified software signal facilities

SYNOPSIS

#include <signal.h>

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

DESCRIPTION

Signal is a simplified interface to the more general sigvec(2) facility.

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 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 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 cause an 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
SIGIOT6∗IOT instruction
SIGEMT7∗EMT instruction
SIGFPE8∗floating point exception
SIGKILL9kill (cannot be caught or ignored)
SIGBUS10∗bus error
SIGSEGV11∗segmentation violation
SIGSYS12∗bad argument to system call
SIGPIPE13write on a pipe with no one to read it
SIGALRM14alarm clock
SIGTERM15software termination signal
SIGURG16•urgent condition present on socket
SIGSTOP17†stop (cannot be caught or ignored)
SIGTSTP18†stop signal generated from keyboard
SIGCONT19•continue after stop
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(2))
SIGXCPU24cpu time limit exceeded (see setrlimit(2))
SIGXFSZ25file size limit exceeded (see setrlimit(2))
SIGVTALRM26virtual time alarm (see setitimer(2))
SIGPROF27profiling timer alarm (see setitimer(2))
SIGWINCH28•used by PCI to signal a window size change
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 further occurences 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, causing the call to terminate prematurely, the call is automatically restarted. In particular this can occur during a read or write(2) on a slow device (such as a terminal; but not a file) and during a wait(2).

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. Execve(2) resets all caught signals to the default action; ignored signals remain ignored.

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).

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