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 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.
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.
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 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 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).
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:
a) The signal is removed from the set of pending signals.
b) If the signal handler was installed by a call to
sigaction(2) that specified the SA_ONSTACK flag and the
thread has defined an alternate signal stack (using
sigaltstack(2)), then that stack is installed.
c) Various pieces of signal-related context are saved into a
special frame that is created on the stack. The saved
information includes:
+ the program counter register (i.e., the address of the
next instruction in the main program that should be
executed when the signal handler returns);
+ architecture-specific register state required for
resuming 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.)
d) Any signals specified in act->sa_mask when registering the
handler with sigprocmask(2) are added to the thread's
signal mask. The signal being delivered is also added to
the signal mask, unless SA_NODEFER was specified when
registering 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 sigreturn(2)).
3. The kernel passes control back to user-space, where execution
commences 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 restored 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 interrupted 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 executes a new program with execve(2)), then the final
step is not performed. In particular, in such scenarios it is
the programmer's responsibility to restore the state of the
signal mask (using sigprocmask(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 specified 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 information 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,
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.
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 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.
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 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.
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.
In certain circumstances, the seccomp(2) user-space notification
feature 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 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).
