signal(7) Miscellaneous Information Manual signal(7)
NAME
signal - overview of signals
DESCRIPTION
Linux supports both POSIX reliable signals (hereinafter "standard sig-
nals") 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 de-
fault 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's signal dis-
positions. 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 spec-
ified process group, or to all processes on the system.
pidfd_send_signal(2)
Sends a signal to a process identified by a PID file descriptor.
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 un-
til 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 ex-
ecution 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 exe-
cution 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 in-
formation 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 de-
scribing the signal.
Signal mask and pending signals
A signal may be blocked, which means that it will not be delivered un-
til 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 indi-
cates 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 process-directed or thread-directed. A process-di-
rected 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 ex-
ception (e.g., SIGSEGV for an invalid memory access, or SIGFPE for a
math error), or because it was targeted at a specific thread using in-
terfaces 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 arbi-
trary 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).
Execution of signal handlers
Whenever there is a transition from kernel-mode to user-mode execution
(e.g., on return from a system call or scheduling of a thread onto the
CPU), the kernel checks whether there is a pending unblocked signal for
which the process has established a signal handler. If there is such a
pending signal, the following steps occur:
(1) The kernel performs the necessary preparatory steps for execution
of the signal handler:
(1.1) The signal is removed from the set of pending signals.
(1.2) If the signal handler was installed by a call to sigac-
tion(2) that specified the SA_ONSTACK flag and the thread
has defined an alternate signal stack (using sigalt-
stack(2)), then that stack is installed.
(1.3) Various pieces of signal-related context are saved into a
special frame that is created on the stack. The saved in-
formation includes:
• the program counter register (i.e., the address of the
next instruction in the main program that should be exe-
cuted when the signal handler returns);
• architecture-specific register state required for resum-
ing the interrupted program;
• the thread's current signal mask;
• the thread's alternate signal stack settings.
(If the signal handler was installed using the sigaction(2)
SA_SIGINFO flag, then the above information is accessible
via the ucontext_t object that is pointed to by the third
argument of the signal handler.)
(1.4) Any signals specified in act->sa_mask when registering the
handler with sigprocmask(2) are added to the thread's sig-
nal mask. The signal being delivered is also added to the
signal mask, unless SA_NODEFER was specified when register-
ing the handler. These signals are thus blocked while the
handler executes.
(2) The kernel constructs a frame for the signal handler on the stack.
The kernel sets the program counter for the thread to point to the
first instruction of the signal handler function, and configures
the return address for that function to point to a piece of user-
space code known as the signal trampoline (described in sigre-
turn(2)).
(3) The kernel passes control back to user-space, where execution com-
mences at the start of the signal handler function.
(4) When the signal handler returns, control passes to the signal
trampoline code.
(5) The signal trampoline calls sigreturn(2), a system call that uses
the information in the stack frame created in step 1 to restore
the thread to its state before the signal handler was called. The
thread's signal mask and alternate signal stack settings are re-
stored as part of this procedure. Upon completion of the call to
sigreturn(2), the kernel transfers control back to user space, and
the thread recommences execution at the point where it was inter-
rupted by the signal handler.
Note that if the signal handler does not return (e.g., control is
transferred out of the handler using siglongjmp(3), or the handler exe-
cutes a new program with execve(2)), then the final step is not per-
formed. In particular, in such scenarios it is the programmer's re-
sponsibility to restore the state of the signal mask (using sigproc-
mask(2)), if it is desired to unblock the signals that were blocked on
entry to the signal handler. (Note that siglongjmp(3) may or may not
restore the signal mask, depending on the savesigs value that was spec-
ified in the corresponding call to sigsetjmp(3).)
From the kernel's point of view, execution of the signal handler code
is exactly the same as the execution of any other user-space code.
That is to say, the kernel does not record any special state informa-
tion indicating that the thread is currently executing inside a signal
handler. All necessary state information is maintained in user-space
registers and the user-space stack. The depth to which nested signal
handlers may be invoked is thus limited only by the user-space stack
(and sensible software design!).
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, SIGX-
CPU, 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 ter-
minate 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 in-
stance of the signal is marked as pending (and the signal will be de-
livered 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 in-
formation 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 dif-
ferent 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 corre-
sponding 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 Linux 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 imple-
mentation 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 in-
ternally uses two (for NPTL) or three (for LinuxThreads) real-time sig-
nals (see pthreads(7)), and adjusts the value of SIGRTMIN suitably (to
34 or 35). Because the range of available real-time signals varies ac-
cording to the glibc threading implementation (and this variation can
occur at run time according to the available kernel and glibc), and in-
deed the range of real-time signals varies across UNIX systems, pro-
grams 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:
• 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.
• If the signal is sent using sigqueue(3), an accompanying value (ei-
ther 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.
• 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 multi-
ple 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. Up to and including Linux
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 inter-
faces 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 re-
turns; 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 re-
turn a success status (normally, the number of bytes transferred).
Note that a (local) disk is not a slow device according to this def-
inition; 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).
• read(2) from an inotify(7) file descriptor (since Linux 3.8; before-
hand, 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), sig-
timedwait(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 sem-
timedop(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 han-
dler, but gives a success return: the number of seconds remaining to
sleep.
In certain circumstances, the seccomp(2) user-space notification fea-
ture can lead to restarting of system calls that would otherwise never
be restarted by SA_RESTART; for details, see seccomp_unotify(2).
Interruption of system calls and library functions by stop signals
On Linux, even in the absence of signal handlers, certain blocking in-
terfaces 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).
STANDARDS
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.
BUGS
There are six signals that can be delivered as a consequence of a hard-
ware exception: SIGBUS, SIGEMT, SIGFPE, SIGILL, SIGSEGV, and SIGTRAP.
Which of these signals is delivered, for any given hardware exception,
is not documented and does not always make sense.
For example, an invalid memory access that causes delivery of SIGSEGV
on one CPU architecture may cause delivery of SIGBUS on another archi-
tecture, or vice versa.
For another example, using the x86 int instruction with a forbidden ar-
gument (any number other than 3 or 128) causes delivery of SIGSEGV,
even though SIGILL would make more sense, because of how the CPU re-
ports the forbidden operation to the kernel.
SEE ALSO
kill(1), clone(2), getrlimit(2), kill(2), pidfd_send_signal(2),
restart_syscall(2), rt_sigqueueinfo(2), setitimer(2), setrlimit(2),
sgetmask(2), sigaction(2), sigaltstack(2), signal(2), signalfd(2), sig-
pending(2), sigprocmask(2), sigreturn(2), sigsuspend(2), sigwait-
info(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), swapcontext(3), sysv_signal(3),
core(5), proc(5), nptl(7), pthreads(7), sigevent(7)
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