Name

signal - simplified software signal facilities

Library

Standard C Library (libc, -lc)

Synopsis

#include <signal.h>

void (*
signal(int sig, void (*func)(int)))(int);

or in FreeBSD's equivalent but easier to read typedef'd version:

typedef void (*sig_t) (int);

sig_t
signal(int sig, sig_t func);

Description

This signal() 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 termination of a process and those that do not. Signals which cause termination 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 receiving 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 signal() function allows for a signal to be caught, to be ignored, or to generate an interrupt. These signals are defined in the file <signal.h>:

Name

Default Action Description

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 program (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 non-existent system call invoked

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 sig argument specifies which signal was received. The func 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, func should be SIG_DFL. A SIG_DFL resets the default action. To ignore the signal func should be SIG_IGN. This will cause subsequent instances of the signal to be ignored and pending instances to be discarded. If SIG_IGN is not used, further occurrences of the signal are automatically blocked and func is called.

The handled signal is unblocked when the function returns and the process continues from where it left off when the signal occurred. Unlike previous signal facilities, the handler func() remains installed after a signal has been delivered.

For some system calls, if a signal is caught while the call is executing and the call is prematurely 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). These semantics could be changed with siginterrupt(3) .

When a process which has installed signal handlers forks, the child process inherits the signals. All caught signals may be reset to their default action by a call to the execve(2) function; ignored signals remain ignored.

See sigaction(2) for a list of functions that are considered safe for use in signal handlers.

Return Values

The previous action is returned on a successful call. Otherwise, SIG_ERR is returned and the global variable errno is set to indicate the error.

Errors

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

[EINVAL]

The sig argument is not a valid signal number.

[EINVAL]

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

See Also

kill(1) , kill(2) , ptrace(2) , sigaction(2) , sigaltstack(2) , sigprocmask(2) , sigsuspend(2) , fpsetmask(3) , setjmp(3) , siginterrupt(3) , tty(4)

History

This signal() facility appeared in 4.0BSD.

Table of Contents

• Name
• Library
• Synopsis
• Description
• Return Values
• Errors
• See Also
• History