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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 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 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, 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(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 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 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 architecture.) 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.
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 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)
Async-signal-safe functions
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 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()
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()
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. 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). Note
that a (local) disk is not a slow device according to this definition; I/O
operations on disk devices are not interrupted by signals.
* 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.
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:
* "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).
CONFORMING TO
POSIX.1, except as noted.
SEE ALSO
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)
COLOPHON
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