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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 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:
(1.1) The signal is removed from the set of pending signals.
(1.2) 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.
(1.3) 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.)
(1.4) 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 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 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:
• 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 (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.
• 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. 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 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).
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 hardware 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 architecture, or vice versa.
For another example, using the x86 int instruction with a forbidden argument (any number
other than 3 or 128) causes delivery of SIGSEGV, even though SIGILL would make more sense,
because of how the CPU reports 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), 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),
swapcontext(3), sysv_signal(3), core(5), proc(5), nptl(7), pthreads(7), sigevent(3type)