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.\" Modified 2004-11-19, mtk:

.\" added pointer to sigaction.2 for details of ignoring SIGCHLD

.\" 2007-06-03, mtk: strengthened portability warning, and rewrote

.\"     various sections.

.\" 2008-07-11, mtk: rewrote and expanded portability discussion.

.\"

.TH SIGNAL 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

signal \- ANSI C signal handling

.SH SYNOPSIS

.B #include <signal.h>

.PP

.B typedef void (*sighandler_t)(int);

.PP

.BI "sighandler_t signal(int " signum ", sighandler_t " handler );

.SH DESCRIPTION

The behavior of

.BR signal ()

varies across UNIX versions,

and has also varied historically across different versions of Linux.

\fBAvoid its use\fP: use

.BR sigaction (2)

instead.

See \fIPortability\fP below.

.PP

.BR signal ()

sets the disposition of the signal

.I signum

to

.IR handler ,

which is either

.BR SIG_IGN ,

.BR SIG_DFL ,

or the address of a programmer-defined function (a "signal handler").

.PP

If the signal

.I signum

is delivered to the process, then one of the following happens:

.TP 3

*

If the disposition is set to

.BR SIG_IGN ,

then the signal is ignored.

.TP

*

If the disposition is set to

.BR SIG_DFL ,

then the default action associated with the signal (see

.BR signal (7))

occurs.

.TP

*

If the disposition is set to a function,

then first either the disposition is reset to

.BR SIG_DFL ,

or the signal is blocked (see \fIPortability\fP below), and then

.I handler

is called with argument

.IR signum .

If invocation of the handler caused the signal to be blocked,

then the signal is unblocked upon return from the handler.

.PP

The signals

.B SIGKILL

and

.B SIGSTOP

cannot be caught or ignored.

.SH RETURN VALUE

.BR signal ()

returns the previous value of the signal handler, or

.B SIG_ERR

on error.

In the event of an error,

.I errno

is set to indicate the cause.

.SH ERRORS

.TP

.B EINVAL

.I signum

is invalid.

.SH CONFORMING TO

POSIX.1-2001, POSIX.1-2008, C89, C99.

.SH NOTES

The effects of

.BR signal ()

in a multithreaded process are unspecified.

.PP

According to POSIX, the behavior of a process is undefined after it

ignores a

.BR SIGFPE ,

.BR SIGILL ,

or

.B SIGSEGV

signal that was not generated by

.BR kill (2)

or

.BR raise (3).

Integer division by zero has undefined result.

On some architectures it will generate a

.B SIGFPE

signal.

(Also dividing the most negative integer by \-1 may generate

.BR SIGFPE .)

Ignoring this signal might lead to an endless loop.

.PP

See

.BR sigaction (2)

for details on what happens when the disposition

.B SIGCHLD

is set to

.BR SIG_IGN .

.PP

See

.BR signal-safety (7)

for a list of the async-signal-safe functions that can be

safely called from inside a signal handler.

.PP

The use of

.I sighandler_t

is a GNU extension, exposed if

.B _GNU_SOURCE

is defined;

.\" libc4 and libc5 define

.\" .IR SignalHandler ;

glibc also defines (the BSD-derived)

.I sig_t

if

.B _BSD_SOURCE

(glibc 2.19 and earlier)

or

.BR _DEFAULT_SOURCE

(glibc 2.19 and later)

is defined.

Without use of such a type, the declaration of

.BR signal ()

is the somewhat harder to read:

.PP

.in +4n

.EX

.BI "void ( *" signal "(int " signum ", void (*" handler ")(int)) ) (int);"

.EE

.in

.SS Portability

The only portable use of

.BR signal ()

is to set a signal's disposition to

.BR SIG_DFL

or

.BR SIG_IGN .

The semantics when using

.BR signal ()

to establish a signal handler vary across systems

(and POSIX.1 explicitly permits this variation);

.B do not use it for this purpose.

.PP

POSIX.1 solved the portability mess by specifying

.BR sigaction (2),

which provides explicit control of the semantics when a

signal handler is invoked; use that interface instead of

.BR signal ().

.PP

In the original UNIX systems, when a handler that was established using

.BR signal ()

was invoked by the delivery of a signal,

the disposition of the signal would be reset to

.BR SIG_DFL ,

and the system did not block delivery of further instances of the signal.

This is equivalent to calling

.BR sigaction (2)

with the following flags:

.PP

.EX

    sa.sa_flags = SA_RESETHAND | SA_NODEFER;

.EE

.PP

System\ V also provides these semantics for

.BR signal ().

This was bad because the signal might be delivered again

before the handler had a chance to reestablish itself.

Furthermore, rapid deliveries of the same signal could

result in recursive invocations of the handler.

.PP

BSD improved on this situation, but unfortunately also

changed the semantics of the existing

.BR signal ()

interface while doing so.

On BSD, when a signal handler is invoked,

the signal disposition is not reset,

and further instances of the signal are blocked from

being delivered while the handler is executing.

Furthermore, certain blocking system calls are automatically

restarted if interrupted by a signal handler (see

.BR signal (7)).

The BSD semantics are equivalent to calling

.BR sigaction (2)

with the following flags:

.PP

.EX

    sa.sa_flags = SA_RESTART;

.EE

.PP

The situation on Linux is as follows:

.IP * 2

The kernel's

.BR signal ()

system call provides System\ V semantics.

.IP *

By default, in glibc 2 and later, the

.BR signal ()

wrapper function does not invoke the kernel system call.

Instead, it calls

.BR sigaction (2)

using flags that supply BSD semantics.

This default behavior is provided as long as a suitable

feature test macro is defined:

.B _BSD_SOURCE

on glibc 2.19 and earlier or

.BR _DEFAULT_SOURCE

in glibc 2.19 and later.

(By default, these macros are defined; see

.BR feature_test_macros (7)

for details.)

If such a feature test macro is not defined, then

.BR signal ()

provides System\ V semantics.

.\"

.\" System V semantics are also provided if one uses the separate

.\" .BR sysv_signal (3)

.\" function.

.\" .IP *

.\" The

.\" .BR signal ()

.\" function in Linux libc4 and libc5 provide System\ V semantics.

.\" If one on a libc5 system includes

.\" .I <bsd/signal.h>

.\" instead of

.\" .IR <signal.h> ,

.\" then

.\" .BR signal ()

.\" provides BSD semantics.

.SH SEE ALSO

.BR kill (1),

.BR alarm (2),

.BR kill (2),

.BR pause (2),

.BR sigaction (2),

.BR signalfd (2),

.BR sigpending (2),

.BR sigprocmask (2),

.BR sigsuspend (2),

.BR bsd_signal (3),

.BR killpg (3),

.BR raise (3),

.BR siginterrupt (3),

.BR sigqueue (3),

.BR sigsetops (3),

.BR sigvec (3),

.BR sysv_signal (3),

.BR signal (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.