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SIGNAL(7)                     Linux Programmer's Manual                     SIGNAL(7)

NAME         top

signal - overview of signals

DESCRIPTION         top

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 tables 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 (less
portably) signal(2).  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, the 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'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(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(2)     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 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
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'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
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.  Several signal numbers are
architecture-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 ix86, ia64, ppc, s390, arm and sh, and the last one for mips.
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 tty
SIGTTIN   21,21,26    Stop    tty input for background process
SIGTTOU   22,22,27    Stop    tty 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,-,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
SIGWINCH    28,28,20    Ign     Window resize signal (4.3BSD, Sun)
SIGUNUSED    -,31,-     Term    Unused signal (will be 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.

Real-time Signals

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 32 different real-time signals, numbered
33 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.
(Note, however, that the LinuxThreads implementation uses the first three
real-time signals.)

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(2), 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.

Async-signal-safe functions

A signal handling routine established by sigaction(2) or signal(2) 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 function, 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
implementation to guarantee that the following functions can be safely called
inside a signal handler:

_Exit()
_exit()

de0d
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()

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 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.  (A disk is
not a slow device according to this definition.)  If an I/O call on a
slow device has already transferred some data by the time it is
interrupted by a signal handler, then the call will return a success
status (normally, the number of bytes transferred).

* 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),
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 fcntl(2) F_SETLKW.

* 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).

* 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:

* Socket interfaces, when a timeout has been set on the socket using
setsockopt(2): accept(2), recv(2), recvfrom(2), and recvmsg(2), if a
receive timeout (SO_RCVTIMEO) has been set; connect(2), send(2),
sendto(2), and sendmsg(2), if a send timeout (SO_SNDTIMEO) has been set.

* 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.

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't occur on other systems.

The Linux interfaces that display this behavior are:

* Socket interfaces, when a timeout has been set on the socket using
setsockopt(2): accept(2), recv(2), recvfrom(2), and recvmsg(2), if a
receive timeout (SO_RCVTIMEO) has been set; 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).

CONFORMING TO         top

POSIX.1, except as noted.

BUGS         top

SIGIO and SIGLOST have the same value.  The latter is commented out in the
kernel source, but the build process of some software still thinks that signal
29 is SIGLOST.

SEE ALSO         top

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

COLOPHON         top

This page is part of release 3.21 of the Linux man-pages project.  A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.

Linux                                 2008-10-15                            SIGNAL(7)