signal - overview of signals
Linux supports both POSIX reliable signals (hereinafter "standard
signals") and POSIX real-time signals.
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
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, 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
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(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.
The following system calls suspend execution of the calling 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.
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,
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-directed 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 exception (e.g.,
SIGSEGV for an
invalid memory access, or
SIGFPE for a math error), or because it was
targeted at a specific thread using interfaces 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 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).
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,
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.
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.
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 instance of the signal
is marked as pending (and the signal will be delivered 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 information associated with the
first instance of the signal.
The numeric value for each signal is given in the table below. As shown in the
table, many signals have different numeric values on different 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 corresponding 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.
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) |
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.
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).
POSIX.1, except as noted.
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.
kill(1),
clone(2),
getrlimit(2),
kill(2),
restart_syscall(2),
rt_sigqueueinfo(2),
pidfd_send_signal(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)