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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(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 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 dash
(-) 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; see pipe(7)
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 system call (SVr4);
see also seccomp(2)
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);
see setrlimit(2)
SIGXFSZ 25,25,31 Core File size limit exceeded (4.2BSD);
see setrlimit(2)
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 Emulator trap
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. Since glibc 2.26, SIGUNUSED is
no longer defined on any architecture.
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:
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)
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:
* 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).
* read(2) from an inotify(7) file descriptor (since Linux 3.8; 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).
* 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).
* Linux 3.7 and earlier: 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.
NOTES
For a discussion of async-signal-safe functions, see signal-safety(7).
SEE ALSO
kill(1), getrlimit(2), kill(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), 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
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