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 tables below specify the default disposi‐

tion 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 behav‐

iors to occur on delivery of the signal: perform the default action; ignore the sig‐

nal; or catch the signal with a signal handler, a programmer-defined function that

is automatically invoked when the signal is delivered. (By default, the signal han‐

dler 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’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.

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(2) 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.

The following system calls suspend execution of the calling process or 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 execu‐

tion until one of the unmasked signals is caught.

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.

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, sig‐

procmask(2) can be used to manipulate the signal mask.

A child created via fork(2) inherits a copy of its parent’s signal mask; the signal

mask is preserved across execve(2).

A signal may be generated (and thus pending) for a process as a whole (e.g., when

sent using kill(2)) or for a specific thread (e.g., certain signals, such as SIGSEGV

and SIGFPE, generated as a consequence of executing a specific machine-language

instruction are thread directed, as are signals targeted at a specific thread using

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 sigpend‐

ing(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).

Linux supports the standard signals listed below. Several signal numbers are archi‐

tecture-dependent, as indicated in the “Value” column. (Where three values are

given, the first one is usually valid for alpha and sparc, the middle one for x86,

arm, and most other architectures, and the last one for mips. (Values for parisc

are not shown; see the Linux kernel source for signal numbering on that architec‐

ture.) A – denotes that a signal is absent on the corresponding architecture.)

First the signals described in the original POSIX.1-1990 standard.

Signal Value Action Comment

──────────────────────────────────────────────────────────────────────

SIGHUP 1 Term Hangup detected on controlling terminal or death of controlling process

SIGINT 2 Term Interrupt from keyboard

SIGQUIT 3 Core Quit from keyboard

SIGILL 4 Core Illegal Instruction

SIGABRT 6 Core Abort signal from abort(3)

SIGFPE 8 Core Floating point exception

SIGKILL 9 Term Kill signal

SIGSEGV 11 Core Invalid memory reference

SIGPIPE 13 Term Broken pipe: write to pipe with no readers

SIGALRM 14 Term Timer signal from alarm(2)

SIGTERM 15 Term Termination signal

SIGUSR1 30,10,16 Term User-defined signal 1

SIGUSR2 31,12,17 Term User-defined signal 2

SIGCHLD 20,17,18 Ign Child stopped or terminated

SIGCONT 19,18,25 Cont Continue if stopped

SIGSTOP 17,19,23 Stop Stop process

SIGTSTP 18,20,24 Stop Stop typed at terminal

SIGTTIN 21,21,26 Stop Terminal input for background process

SIGTTOU 22,22,27 Stop Terminal output for background process

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

Next the signals not in the POSIX.1-1990 standard but described in SUSv2 and

POSIX.1-2001.

Signal Value Action Comment

────────────────────────────────────────────────────────────────────

SIGBUS 10,7,10 Core Bus error (bad memory access)

SIGPOLL Term Pollable event (Sys V). Synonym for SIGIO

SIGPROF 27,27,29 Term Profiling timer expired

SIGSYS 12,31,12 Core Bad argument to routine (SVr4)

SIGTRAP 5 Core Trace/breakpoint trap

SIGURG 16,23,21 Ign Urgent condition on socket (4.2BSD)

SIGVTALRM 26,26,28 Term Virtual alarm clock (4.2BSD)

SIGXCPU 24,24,30 Core CPU time limit exceeded (4.2BSD)

SIGXFSZ 25,25,31 Core File size limit exceeded (4.2BSD)

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.

Next various other signals.

Signal Value Action Comment

────────────────────────────────────────────────────────────────────

SIGIOT 6 Core IOT trap. A synonym for SIGABRT

SIGEMT 7,-,7 Term

SIGSTKFLT -,16,- Term Stack fault on coprocessor (unused)

SIGIO 23,29,22 Term I/O now possible (4.2BSD)

SIGCLD -,-,18 Ign A synonym for SIGCHLD

SIGPWR 29,30,19 Term Power failure (System V)

SIGINFO 29,-,- A synonym for SIGPWR

SIGLOST -,-,- Term File lock lost (unused)

SIGWINCH 28,28,20 Ign Window resize signal (4.3BSD, Sun)

SIGUNUSED -,31,- Core Synonymous with SIGSYS

(Signal 29 is SIGINFO / SIGPWR on an alpha but SIGLOST on a sparc.)

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.

Where defined, SIGUNUSED is synonymous with SIGSYS on most architectures.

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 inte‐

ger or a pointer) can be sent with the signal. If the receiving process estab‐

lishes 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 sig‐

nals 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 dif‐

ferently. 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 or 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:

Linux 2.0 and earlier Linux 2.2 and later

sigaction(2) rt_sigaction(2)

sigpending(2) rt_sigpending(2)

sigprocmask(2) rt_sigprocmask(2)

sigreturn(2) rt_sigreturn(2)

sigsuspend(2) rt_sigsuspend(2)

sigtimedwait(2) rt_sigtimedwait(2)

A signal handler function must be very careful, since processing elsewhere may be

interrupted at some arbitrary point in the execution of the program. POSIX has the

concept of “safe function”. If a signal interrupts the execution of an unsafe func‐

tion, and handler calls an unsafe function, then the behavior of the program is

undefined.

POSIX.1-2004 (also known as POSIX.1-2001 Technical Corrigendum 2) requires an imple‐

mentation to guarantee that the following functions can be safely called inside a

signal handler:

_Exit()

_exit()

abort()

accept()

access()

aio_error()

aio_return()

aio_suspend()

alarm()

bind()

cfgetispeed()

cfgetospeed()

cfsetispeed()

cfsetospeed()

chdir()

chmod()

chown()

clock_gettime()

close()

connect()

creat()

dup()

dup2()

execle()

execve()

fchmod()

fchown()

fcntl()

fdatasync()

fork()

fpathconf()

fstat()

fsync()

ftruncate()

getegid()

geteuid()

getgid()

getgroups()

getpeername()

getpgrp()

getpid()

getppid()

getsockname()

getsockopt()

getuid()

kill()

link()

listen()

lseek()

lstat()

mkdir()

mkfifo()

open()

pathconf()

pause()

pipe()

poll()

posix_trace_event()

pselect()

raise()

read()

readlink()

recv()

recvfrom()

recvmsg()

rename()

rmdir()

select()

sem_post()

send()

sendmsg()

sendto()

setgid()

setpgid()

setsid()

setsockopt()

setuid()

shutdown()

sigaction()

sigaddset()

sigdelset()

sigemptyset()

sigfillset()

sigismember()

signal()

sigpause()

sigpending()

sigprocmask()

sigqueue()

sigset()

sigsuspend()

sleep()

sockatmark()

socket()

socketpair()

stat()

symlink()

sysconf()

tcdrain()

tcflow()

tcflush()

tcgetattr()

tcgetpgrp()

tcsendbreak()

tcsetattr()

tcsetpgrp()

time()

timer_getoverrun()

timer_gettime()

timer_settime()

times()

umask()

uname()

unlink()

utime()

wait()

waitpid()

write()

POSIX.1-2008 removes fpathconf(), pathconf(), and sysconf() from the above list, and

adds the following functions:

execl()

execv()

faccessat()

fchmodat()

fchownat()

fexecve()

fstatat()

futimens()

linkat()

mkdirat()

mkfifoat()

mknod()

mknodat()

openat()

readlinkat()

renameat()

symlinkat()

unlinkat()

utimensat()

utimes()

POSIX.1-2008 Technical Corrigendum 1 (2013) adds the following functions:

fchdir()

pthread_kill()

pthread_self()

pthread_sigmask()

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 han‐

dler, then the call will be automatically restarted after the signal handler returns

if the SA_RESTART flag was used; otherwise the call will fail with the error EINTR:

* read(2), readv(2), write(2), writev(2), and ioctl(2) calls on “slow” devices.

A “slow” device is one where the I/O call may block for an indefinite time,

for example, a terminal, pipe, or socket. If an I/O call on a slow device has

already transferred some data by the time it is interrupted by a signal han‐

dler, then the call will return a success status (normally, the number of

bytes transferred). Note that a (local) disk is not a slow device according

to this definition; I/O operations on disk devices are not interrupted by sig‐

nals.

* open(2), if it can block (e.g., when opening a FIFO; see fifo(7)).

* wait(2), wait3(2), wait4(2), waitid(2), and waitpid(2).

* Socket interfaces: accept(2), connect(2), recv(2), recvfrom(2), recvmmsg(2),

recvmsg(2), send(2), sendto(2), and sendmsg(2), unless a timeout has been set

on the socket (see below).

* File locking interfaces: flock(2) and the F_SETLKW and F_OFD_SETLKW operations

of fcntl(2)

* POSIX message queue interfaces: mq_receive(3), mq_timedreceive(3), mq_send(3),

and mq_timedsend(3).

* futex(2) FUTEX_WAIT (since Linux 2.6.22; beforehand, always failed with

EINTR).

* getrandom(2).

* pthread_mutex_lock(3), pthread_cond_wait(3), and related APIs.

* futex(2) FUTEX_WAIT_BITSET.

* POSIX semaphore interfaces: sem_wait(3) and sem_timedwait(3) (since Linux

2.6.22; beforehand, always failed with 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:

* “Input” socket interfaces, when a timeout (SO_RCVTIMEO) has been set on the

socket using setsockopt(2): accept(2), recv(2), recvfrom(2), recvmmsg(2) (also

with a non-NULL timeout argument), and recvmsg(2).

* “Output” socket interfaces, when a timeout (SO_RCVTIMEO) has been set on the

socket using setsockopt(2): connect(2), send(2), sendto(2), and sendmsg(2).

* Interfaces used to wait for signals: pause(2), sigsuspend(2), sigtimedwait(2),

and sigwaitinfo(2).

* File descriptor multiplexing interfaces: epoll_wait(2), epoll_pwait(2),

poll(2), ppoll(2), select(2), and pselect(2).

* System V IPC interfaces: msgrcv(2), msgsnd(2), semop(2), and semtimedop(2).

* Sleep interfaces: clock_nanosleep(2), nanosleep(2), and usleep(3).

* read(2) from an inotify(7) file descriptor.

* io_getevents(2).

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.

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’t occur on other systems.

The Linux interfaces that display this behavior are:

* “Input” socket interfaces, when a timeout (SO_RCVTIMEO) has been set on the

socket using setsockopt(2): accept(2), recv(2), recvfrom(2), recvmmsg(2) (also

with a non-NULL timeout argument), and recvmsg(2).

* “Output” socket interfaces, when a timeout (SO_RCVTIMEO) has been set on the

socket using setsockopt(2): connect(2), send(2), sendto(2), and sendmsg(2), if

a send timeout (SO_SNDTIMEO) has been set.

* epoll_wait(2), epoll_pwait(2).

* semop(2), semtimedop(2).

* sigtimedwait(2), sigwaitinfo(2).

* read(2) from an inotify(7) file descriptor.

* Linux 2.6.21 and earlier: futex(2) FUTEX_WAIT, sem_timedwait(3), sem_wait(3).

* Linux 2.6.8 and earlier: msgrcv(2), msgsnd(2).

* Linux 2.4 and earlier: nanosleep(2).

kill(1), getrlimit(2), kill(2), killpg(2), restart_syscall(2), rt_sigqueueinfo(2),

setitimer(2), setrlimit(2), sgetmask(2), sigaction(2), sigaltstack(2), signal(2),

signalfd(2), sigpending(2), sigprocmask(2), sigsuspend(2), sigwaitinfo(2), abort(3),

bsd_signal(3), longjmp(3), raise(3), pthread_sigqueue(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)