Section (2) signal


Linux manual pages Section 2  

Name

signal — ANSI C signal handling

Synopsis

#include <signal.h>

typedef void (*sighandler_t)(int);
sighandler_t signal( int signum,
  sighandler_t handler);
 

DESCRIPTION

The behavior of signal() varies across UNIX versions, and has also varied historically across different versions of Linux. Avoid its use: use sigaction(2) instead. See Portability below.

signal() sets the disposition of the signal signum to handler, which is either SIG_IGN, SIG_DFL, or the address of a programmer-defined function (a signal handler).

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

  • If the disposition is set to SIG_IGN, then the signal is ignored.

  • If the disposition is set to SIG_DFL, then the default action associated with the signal (see signal(7)) occurs.

  • If the disposition is set to a function, then first either the disposition is reset to SIG_DFL, or the signal is blocked (see Portability below), and then handler is called with argument signum. If invocation of the handler caused the signal to be blocked, then the signal is unblocked upon return from the handler.

The signals SIGKILL and SIGSTOP cannot be caught or ignored.

RETURN VALUE

signal() returns the previous value of the signal handler, or SIG_ERR on error. In the event of an error, errno is set to indicate the cause.

ERRORS

EINVAL

signum is invalid.

CONFORMING TO

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

NOTES

The effects of signal() in a multithreaded process are unspecified.

According to POSIX, the behavior of a process is undefined after it ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated by kill(2) or raise(3). Integer division by zero has undefined result. On some architectures it will generate a SIGFPE signal. (Also dividing the most negative integer by −1 may generate SIGFPE.) Ignoring this signal might lead to an endless loop.

See sigaction(2) for details on what happens when the disposition SIGCHLD is set to SIG_IGN.

See signal-safety(7) for a list of the async-signal-safe functions that can be safely called from inside a signal handler.

The use of sighandler_t is a GNU extension, exposed if _GNU_SOURCE is defined; glibc also defines (the BSD-derived) sig_t if _BSD_SOURCE (glibc 2.19 and earlier) or _DEFAULT_SOURCE (glibc 2.19 and later) is defined. Without use of such a type, the declaration of signal() is the somewhat harder to read:

  void( *signal(int signum, void (*handler)(int)) ) (int);

Portability

The only portable use of signal() is to set a signal_zsingle_quotesz_s disposition to SIG_DFL or SIG_IGN. The semantics when using signal() to establish a signal handler vary across systems (and POSIX.1 explicitly permits this variation); do not use it for this purpose.

POSIX.1 solved the portability mess by specifying sigaction(2), which provides explicit control of the semantics when a signal handler is invoked; use that interface instead of signal().

In the original UNIX systems, when a handler that was established using signal() was invoked by the delivery of a signal, the disposition of the signal would be reset to SIG_DFL, and the system did not block delivery of further instances of the signal. This is equivalent to calling sigaction(2) with the following flags:

    sa.sa_flags = SA_RESETHAND | SA_NODEFER;

System V also provides these semantics for 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.

BSD improved on this situation, but unfortunately also changed the semantics of the existing 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 signal(7)). The BSD semantics are equivalent to calling sigaction(2) with the following flags:

    sa.sa_flags = SA_RESTART;

The situation on Linux is as follows:

  • The kernel_zsingle_quotesz_s signal() system call provides System V semantics.

  • By default, in glibc 2 and later, the signal() wrapper function does not invoke the kernel system call. Instead, it calls sigaction(2) using flags that supply BSD semantics. This default behavior is provided as long as a suitable feature test macro is defined: _BSD_SOURCE on glibc 2.19 and earlier or _DEFAULT_SOURCE in glibc 2.19 and later. (By default, these macros are defined; see feature_test_macros(7) for details.) If such a feature test macro is not defined, then signal() provides System V semantics.

SEE ALSO

kill(1), alarm(2), kill(2), pause(2), sigaction(2), signalfd(2), sigpending(2), sigprocmask(2), sigsuspend(2), bsd_signal(3), killpg(3), raise(3), siginterrupt(3), sigqueue(3), sigsetops(3), sigvec(3), sysv_signal(3), signal(7)

COLOPHON

This page is part of release 4.16 of the Linux 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/.


  Copyright (c) 2000 Andries Brouwer <aebcwi.nl>
and Copyright (c) 2007 Michael Kerrisk <mtk.manpagesgmail.com>
and Copyright (c) 2008, Linux Foundation, written by Michael Kerrisk
     <mtk.manpagesgmail.com>
based on work by Rik Faith <faithcs.unc.edu>
and Mike Battersby <mikestarbug.apana.org.au>.

%%%LICENSE_START(VERBATIM)
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
%%%LICENSE_END

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.

Section (7) signal


Linux manual pages Section 7  

Name

signal — overview of signals

DESCRIPTION

Linux supports both POSIX reliable signals (hereinafter standard signals) and POSIX real-time signals.

Signal dispositions

Each signal has a current disposition, which determines how the process behaves when it is delivered the signal.

The entries in the Action column of the table below specify the default disposition for each signal, as follows:

Term

Default action is to terminate the process.

Ign

Default action is to ignore the signal.

Core

Default action is to terminate the process and dump core (see core(5)).

Stop

Default action is to stop the process.

Cont

Default action is to continue the process if it is currently stopped.

A process can change the disposition of a signal using sigaction(2) or signal(2). (The latter is less portable when establishing a signal handler; see signal(2) for details.) Using these system calls, a process can elect one of the following behaviors to occur on delivery of the signal: perform the default action; ignore the signal; or catch the signal with a signal handler, a programmer-defined function that is automatically invoked when the signal is delivered.

By default, a signal handler is invoked on the normal process stack. It is possible to arrange that the signal handler uses an alternate stack; see sigaltstack(2) for a discussion of how to do this and when it might be useful.

The signal disposition is a per-process attribute: in a multithreaded application, the disposition of a particular signal is the same for all threads.

A child created via fork(2) inherits a copy of its parent_zsingle_quotesz_s signal dispositions. During an execve(2), the dispositions of handled signals are reset to the default; the dispositions of ignored signals are left unchanged.

Sending a signal

The following system calls and library functions allow the caller to send a signal:

raise(3)

Sends a signal to the calling thread.

kill(2)

Sends a signal to a specified process, to all members of a specified process group, or to all processes on the system.

killpg(3)

Sends a signal to all of the members of a specified process group.

pthread_kill(3)

Sends a signal to a specified POSIX thread in the same process as the caller.

tgkill(2)

Sends a signal to a specified thread within a specific process. (This is the system call used to implement pthread_kill(3).)

sigqueue(3)

Sends a real-time signal with accompanying data to a specified process.

Waiting for a signal to be caught

The following system calls suspend execution of the calling thread until a signal is caught (or an unhandled signal terminates the process):

pause(2)

Suspends execution until any signal is caught.

sigsuspend(2)

Temporarily changes the signal mask (see below) and suspends execution until one of the unmasked signals is caught.

Synchronously accepting a signal

Rather than asynchronously catching a signal via a signal handler, it is possible to synchronously accept the signal, that is, to block execution until the signal is delivered, at which point the kernel returns information about the signal to the caller. There are two general ways to do this:

  • sigwaitinfo(2), sigtimedwait(2), and sigwait(3) suspend execution until one of the signals in a specified set is delivered. Each of these calls returns information about the delivered signal.

  • signalfd(2) returns a file descriptor that can be used to read information about signals that are delivered to the caller. Each read(2) from this file descriptor blocks until one of the signals in the set specified in the signalfd(2) call is delivered to the caller. The buffer returned by read(2) contains a structure describing the signal.

Signal mask and pending signals

A signal may be blocked, which means that it will not be delivered until it is later unblocked. Between the time when it is generated and when it is delivered a signal is said to be pending.

Each thread in a process has an independent signal mask, which indicates the set of signals that the thread is currently blocking. A thread can manipulate its signal mask using pthread_sigmask(3). In a traditional single-threaded application, sigprocmask(2) can be used to manipulate the signal mask.

A child created via fork(2) inherits a copy of its parent_zsingle_quotesz_s signal mask; the signal mask is preserved across execve(2).

A signal may be process-directed or thread-directed. A process-directed signal is one that is targeted at (and thus pending for) the process as a whole. A signal may be process-directed because it was generated by the kernel for reasons other than a hardware exception, or because it was sent using kill(2) or sigqueue(3). A thread-directed signal is one that is targeted at a specific thread. A signal may be thread-directed because it was generated as a consequence of executing a specific machine-language instruction that triggered a hardware exception (e.g., SIGSEGV for an invalid memory access, or SIGFPE for a math error), or because it was it was targeted at a specific thread using interfaces such as tgkill(2) or pthread_kill(3).

A process-directed signal may be delivered to any one of the threads that does not currently have the signal blocked. If more than one of the threads has the signal unblocked, then the kernel chooses an arbitrary thread to which to deliver the signal.

A thread can obtain the set of signals that it currently has pending using sigpending(2). This set will consist of the union of the set of pending process-directed signals and the set of signals pending for the calling thread.

A child created via fork(2) initially has an empty pending signal set; the pending signal set is preserved across an execve(2).

Standard signals

Linux supports the standard signals listed below. The second column of the table indicates which standard (if any) specified the signal: P1990 indicates that the signal is described in the original POSIX.1-1990 standard; P2001 indicates that the signal was added in SUSv2 and POSIX.1-2001.

Signal Standard Action Comment
SIGABRT P1990 Core Abort signal from abort(3)
SIGALRM P1990 Term Timer signal from alarm(2)
SIGBUS P2001 Core Bus error (bad memory access)
SIGCHLD P1990 Ign Child stopped or terminated
SIGCLD Ign A synonym for SIGCHLD
SIGCONT P1990 Cont Continue if stopped
SIGEMT Term Emulator trap
SIGFPE P1990 Core Floating-point exception
SIGHUP P1990 Term Hangup detected on controlling terminal or death of controlling process
SIGILL P1990 Core Illegal Instruction
SIGINFO   A synonym for SIGPWR
SIGINT P1990 Term Interrupt from keyboard
SIGIO Term I/O now possible (4.2BSD)
SIGIOT Core IOT trap. A synonym for SIGABRT
SIGKILL P1990 Term Kill signal
SIGLOST Term File lock lost (unused)
SIGPIPE P1990 Term Broken pipe: write to pipe with no readers; see pipe(7)
SIGPOLL P2001 Term Pollable event (Sys V). Synonym for SIGIO
SIGPROF P2001 Term Profiling timer expired
SIGPWR Term Power failure (System V)
SIGQUIT P1990 Core Quit from keyboard
SIGSEGV P1990 Core Invalid memory reference
SIGSTKFLT Term Stack fault on coprocessor (unused)
SIGSTOP P1990 Stop Stop process
SIGTSTP P1990 Stop Stop typed at terminal
SIGSYS P2001 Core Bad system call (SVr4); see also seccomp(2)
SIGTERM P1990 Term Termination signal
SIGTRAP P2001 Core Trace/breakpoint trap
SIGTTIN P1990 Stop Terminal input for background process
SIGTTOU P1990 Stop Terminal output for background process
SIGUNUSED Core Synonymous with SIGSYS
SIGURG P2001 Ign Urgent condition on socket (4.2BSD)
SIGUSR1 P1990 Term User-defined signal 1
SIGUSR2 P1990 Term User-defined signal 2
SIGVTALRM P2001 Term Virtual alarm clock (4.2BSD)
SIGXCPU P2001 Core CPU time limit exceeded (4.2BSD); see setrlimit(2)
SIGXFSZ P2001 Core File size limit exceeded (4.2BSD); see setrlimit(2)
SIGWINCH Ign Window resize signal (4.3BSD, Sun)

The signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored.

Up to and including Linux 2.2, the default behavior for SIGSYS, SIGXCPU, SIGXFSZ, and (on architectures other than SPARC and MIPS) SIGBUS was to terminate the process (without a core dump). (On some other UNIX systems the default action for SIGXCPU and SIGXFSZ is to terminate the process without a core dump.) Linux 2.4 conforms to the POSIX.1-2001 requirements for these signals, terminating the process with a core dump.

SIGEMT is not specified in POSIX.1-2001, but nevertheless appears on most other UNIX systems, where its default action is typically to terminate the process with a core dump.

SIGPWR (which is not specified in POSIX.1-2001) is typically ignored by default on those other UNIX systems where it appears.

SIGIO (which is not specified in POSIX.1-2001) is ignored by default on several other UNIX systems.

Queueing and delivery semantics for standard signals

If multiple standard signals are pending for a process, the order in which the signals are delivered is unspecified.

Standard signals do not queue. If multiple instances of a standard signal are generated while that signal is blocked, then only one instance of the signal is marked as pending (and the signal will be delivered just once when it is unblocked). In the case where a standard signal is already pending, the siginfo_t structure (see sigaction(2)) associated with that signal is not overwritten on arrival of subsequent instances of the same signal. Thus, the process will receive the information associated with the first instance of the signal.

Signal numbering for standard signals

The numeric value for each signal is given in the table below. As shown in the table, many signals have different numeric values on different architectures. The first numeric value in each table row shows the signal number on x86, ARM, and most other architectures; the second value is for Alpha and SPARC; the third is for MIPS; and the last is for PARISC. A dash (−) denotes that a signal is absent on the corresponding architecture.

Signal x86/ARM Alpha/ MIPS PARISC Notes
  most others SPARC      
SIGHUP  1  1  1  1  
SIGINT  2  2  2  2  
SIGQUIT  3  3  3  3  
SIGILL  4  4  4  4  
SIGTRAP  5  5  5  5  
SIGABRT  6  6  6  6  
SIGIOT  6  6  6  6  
SIGBUS  7 10 10 10  
SIGEMT  7  7 -  
SIGFPE  8  8  8  8  
SIGKILL  9  9  9  9  
SIGUSR1 10 30 16 16  
SIGSEGV 11 11 11 11  
SIGUSR2 12 31 17 17  
SIGPIPE 13 13 13 13  
SIGALRM 14 14 14 14  
SIGTERM 15 15 15 15  
SIGSTKFLT 16  7  
SIGCHLD 17 20 18 18  
SIGCLD 18  
SIGCONT 18 19 25 26  
SIGSTOP 19 17 23 24  
SIGTSTP 20 18 24 25  
SIGTTIN 21 21 26 27  
SIGTTOU 22 22 27 28  
SIGURG 23 16 21 29  
SIGXCPU 24 24 30 12  
SIGXFSZ 25 25 31 30  
SIGVTALRM 26 26 28 20  
SIGPROF 27 27 29 21  
SIGWINCH 28 28 20 23  
SIGIO 29 23 22 22  
SIGPOLL         Same as SIGIO
SIGPWR 30 29/− 19 19  
SIGINFO 29/−  
SIGLOST −/29  
SIGSYS 31 12 12 31  
SIGUNUSED 31 31  

Note the following:

  • Where defined, SIGUNUSED is synonymous with SIGSYS. Since glibc 2.26, SIGUNUSED is no longer defined on any architecture.

  • Signal 29 is SIGINFO/SIGPWR (synonyms for the same value) on Alpha but SIGLOST on SPARC.

Real-time signals

Starting with version 2.2, Linux supports real-time signals as originally defined in the POSIX.1b real-time extensions (and now included in POSIX.1-2001). The range of supported real-time signals is defined by the macros SIGRTMIN and SIGRTMAX. POSIX.1-2001 requires that an implementation support at least _POSIX_RTSIG_MAX (8) real-time signals.

The Linux kernel supports a range of 33 different real-time signals, numbered 32 to 64. However, the glibc POSIX threads implementation internally uses two (for NPTL) or three (for LinuxThreads) real-time signals (see pthreads(7)), and adjusts the value of SIGRTMIN suitably (to 34 or 35). Because the range of available real-time signals varies according to the glibc threading implementation (and this variation can occur at run time according to the available kernel and glibc), and indeed the range of real-time signals varies across UNIX systems, programs should never refer to real-time signals using hard-coded numbers, but instead should always refer to real-time signals using the notation SIGRTMIN+n, and include suitable (run-time) checks that SIGRTMIN+n does not exceed SIGRTMAX.

Unlike standard signals, real-time signals have no predefined meanings: the entire set of real-time signals can be used for application-defined purposes.

The default action for an unhandled real-time signal is to terminate the receiving process.

Real-time signals are distinguished by the following:

  1. Multiple instances of real-time signals can be queued. By contrast, if multiple instances of a standard signal are delivered while that signal is currently blocked, then only one instance is queued.

  2. If the signal is sent using sigqueue(3), an accompanying value (either an integer or a pointer) can be sent with the signal. If the receiving process establishes a handler for this signal using the SA_SIGINFO flag to sigaction(2), then it can obtain this data via the si_value field of the siginfo_t structure passed as the second argument to the handler. Furthermore, the si_pid and si_uid fields of this structure can be used to obtain the PID and real user ID of the process sending the signal.

  3. Real-time signals are delivered in a guaranteed order. Multiple real-time signals of the same type are delivered in the order they were sent. If different real-time signals are sent to a process, they are delivered starting with the lowest-numbered signal. (I.e., low-numbered signals have highest priority.) By contrast, if multiple standard signals are pending for a process, the order in which they are delivered is unspecified.

If both standard and real-time signals are pending for a process, POSIX leaves it unspecified which is delivered first. Linux, like many other implementations, gives priority to standard signals in this case.

According to POSIX, an implementation should permit at least _POSIX_SIGQUEUE_MAX (32) real-time signals to be queued to a process. However, Linux does things differently. In kernels up to and including 2.6.7, Linux imposes a system-wide limit on the number of queued real-time signals for all processes. This limit can be viewed and (with privilege) changed via the /proc/sys/kernel/rtsig-max file. A related file, /proc/sys/kernel/rtsig-nr, can be used to find out how many real-time signals are currently queued. In Linux 2.6.8, these /proc interfaces were replaced by the RLIMIT_SIGPENDING resource limit, which specifies a per-user limit for queued signals; see setrlimit(2) for further details.

The addition of real-time signals required the widening of the signal set structure (sigset_t) from 32 to 64 bits. Consequently, various system calls were superseded by new system calls that supported the larger signal sets. The old and new system calls are as follows:

Interruption of system calls and library functions by signal handlers

If a signal handler is invoked while a system call or library function call is blocked, then either:

  • the call is automatically restarted after the signal handler returns; or

  • the call fails with the error EINTR.

Which of these two behaviors occurs depends on the interface and whether or not the signal handler was established using the SA_RESTART flag (see sigaction(2)). The details vary across UNIX systems; below, the details for Linux.

If a blocked call to one of the following interfaces is interrupted by a signal handler, then the call is automatically restarted after the signal handler returns if the SA_RESTART flag was used; otherwise the call fails with the error EINTR:

The following interfaces are never restarted after being interrupted by a signal handler, regardless of the use of SA_RESTART; they always fail with the error EINTR when interrupted by a signal handler:

The sleep(3) function is also never restarted if interrupted by a handler, but gives a success return: the number of seconds remaining to sleep.

Interruption of system calls and library functions by stop signals

On Linux, even in the absence of signal handlers, certain blocking interfaces can fail with the error EINTR after the process is stopped by one of the stop signals and then resumed via SIGCONT. This behavior is not sanctioned by POSIX.1, and doesn_zsingle_quotesz_t occur on other systems.

The Linux interfaces that display this behavior are:

CONFORMING TO

POSIX.1, except as noted.

NOTES

For a discussion of async-signal-safe functions, see signal-safety(7).

The /proc/[pid]/task/[tid]/status file contains various fields that show the signals that a thread is blocking (SigBlk), catching (SigCgt), or ignoring (SigIgn). (The set of signals that are caught or ignored will be the same across all threads in a process.) Other fields show the set of pending signals that are directed to the thread (SigPnd) as well as the set of pending signals that are directed to the process as a whole (ShdPnd). The corresponding fields in /proc/[pid]/status show the information for the main thread. See proc(5) for further details.

SEE ALSO

kill(1), clone(2), getrlimit(2), kill(2), restart_syscall(2), rt_sigqueueinfo(2), pidfd_send_signal(2), setitimer(2), setrlimit(2), sgetmask(2), sigaction(2), sigaltstack(2), signal(2), signalfd(2), sigpending(2), sigprocmask(2), sigreturn(2), sigsuspend(2), sigwaitinfo(2), abort(3), bsd_signal(3), killpg(3), longjmp(3), pthread_sigqueue(3), raise(3), sigqueue(3), sigset(3), sigsetops(3), sigvec(3), sigwait(3), strsignal(3), sysv_signal(3), core(5), proc(5), nptl(7), pthreads(7), sigevent(7)

COLOPHON

This page is part of release 5.04 of the Linux 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/.


  t
Copyright (c) 1993 by Thomas Koenig (ig25rz.uni-karlsruhe.de)
and Copyright (c) 2002, 2006 by Michael Kerrisk <mtk.manpagesgmail.com>
and Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
    <mtk.manpagesgmail.com>

%%%LICENSE_START(VERBATIM)
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
%%%LICENSE_END

Modified Sat Jul 24 17:34:08 1993 by Rik Faith (faithcs.unc.edu)
Modified Sun Jan  7 01:41:27 1996 by Andries Brouwer (aebcwi.nl)
Modified Sun Apr 14 12:02:29 1996 by Andries Brouwer (aebcwi.nl)
Modified Sat Nov 13 16:28:23 1999 by Andries Brouwer (aebcwi.nl)
Modified 10 Apr 2002, by Michael Kerrisk <mtk.manpagesgmail.com>
Modified  7 Jun 2002, by Michael Kerrisk <mtk.manpagesgmail.com>
Added information on real-time signals
Modified 13 Jun 2002, by Michael Kerrisk <mtk.manpagesgmail.com>
Noted that SIGSTKFLT is in fact unused
2004-12-03, Modified mtk, added notes on RLIMIT_SIGPENDING
2006-04-24, mtk, Added text on changing signal dispositions,
signal mask, and pending signals.
2008-07-04, mtk:
    Added section on system call restarting (SA_RESTART)
    Added section on stop/cont signals interrupting syscalls.
2008-10-05, mtk: various additions