SIGNAL(7) Linux Programmer's Manual SIGNAL(7)
signal - list of available signals
Linux supports both POSIX reliable signals (hereinafter "standard sig-
nals") 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 tables below specify the
default disposition for each signal, as follows:
Term Default action is to terminate the process.
Ign Default action is to ignore the signal.
Core Default action is to terminate the process and dump core (see
Stop Default action is to stop the process.
Cont Default action is to continue the process if it is currently
A process can change the disposition of a signal using sigaction(2) or
(less portably) signal(2). Using these system calls, a process can
elect one of the following behaviors to occur on delivery of the sig-
nal: perform the default action; ignore the signal; or catch the signal
with a signal handler, a programmer-defined function that is automati-
cally invoked when the signal is delivered.
The signal disposition is a per-process attribute: in a multithreaded
application, the disposition of a particular signal is the same for all
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 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 signal may be generated (and thus pending) for a process as a whole
(e.g., when sent using kill(2)) or for a specific thread (e.g., certain
signals, such as SIGSEGV and SIGFPE, generated as a consequence of exe-
cuting a specific machine-language instruction are thread directed, as
are signals targeted at a specific thread using pthread_kill(3)). A
process-directed signal may be delivered to any one of the threads that
does not currently have the signal blocked. If more than one of the
threads has the signal unblocked, then the kernel chooses an arbitrary
thread to which to deliver the signal.
A thread can obtain the set of signals that it currently has pending
using sigpending(2). This set will consist of the union of the set of
pending process-directed signals and the set of signals pending for the
Linux supports the standard signals listed below. Several signal num-
bers are architecture-dependent, as indicated in the "Value" column.
(Where three values are given, the first one is usually valid for alpha
and sparc, the middle one for ix86, ia64, ppc, s390, arm and sh, and
the last one for mips. A - denotes that a signal is absent on the cor-
First the signals described in the original POSIX.1-1990 standard.
l c c l ____ lB c c l. Signal Value Action Comment
SIGHUP 1 Term Hangup detected on controlling terminal
or death of controlling process SIG-
INT 2 Term Interrupt from keyboard SIGQUIT 3 Core Quit from
keyboard SIGILL 4 Core Illegal Instruction SIGA-
BRT 6 Core Abort signal from abort(3) SIGFPE 8 Core Floating
point exception SIGKILL 9 Term Kill signal
SIGSEGV 11 Core Invalid memory reference SIGPIPE 13 Term Broken
pipe: write to pipe with no readers
SIGALRM 14 Term Timer signal from alarm(2) SIGTERM 15 Term Ter-
mination signal SIGUSR1 30,10,16 Term User-defined signal 1
SIGUSR2 31,12,17 Term User-defined signal 2
SIGCHLD 20,17,18 Ign Child stopped or terminated SIG-
CONT 19,18,25 Cont Continue if stopped SIGSTOP 17,19,23 Stop Stop
process SIGTSTP 18,20,24 Stop Stop typed at tty SIGT-
TIN 21,21,26 Stop tty input for background process SIGT-
TOU 22,22,27 Stop tty output for background process
The signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored.
Next the signals not in the POSIX.1-1990 standard but described in
SUSv2 and POSIX.1-2001.
l c c l ____ lB c c l. Signal Value Action Comment SIG-
BUS 10,7,10 Core Bus error (bad memory access) SIG-
POLL Term Pollable event (Sys V). Synonym for
SIGIO SIGPROF 27,27,29 Term Profiling timer expired
SIGSYS 12,-,12 Core Bad argument to routine (SVr4) SIG-
TRAP 5 Core Trace/breakpoint trap SIGURG 16,23,21 Ign Urgent
condition on socket (4.2BSD) SIGVTALRM 26,26,28 Term Virtual alarm
clock (4.2BSD) SIGXCPU 24,24,30 Core CPU time limit exceeded
(4.2BSD) SIGXFSZ 25,25,31 Core File size limit exceeded (4.2BSD)
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.
Next various other signals.
l c c l ____ lB c c l. Signal Value Action Comment
SIGIOT 6 Core IOT trap. A synonym for SIGABRT
SIGEMT 7,-,7 Term SIGSTKFLT -,16,- Term Stack fault on copro-
cessor (unused) SIGIO 23,29,22 Term I/O now possible (4.2BSD) SIG-
CLD -,-,18 Ign A synonym for SIGCHLD SIG-
PWR 29,30,19 Term Power failure (System V) SIG-
INFO 29,-,- A synonym for SIGPWR
SIGLOST -,-,- Term File lock lost SIGWINCH 28,28,20 Ign Window
resize signal (4.3BSD, Sun) SIGUNUSED -,31,- Term Unused signal
(will be SIGSYS)
(Signal 29 is SIGINFO / SIGPWR on an alpha but SIGLOST on a sparc.)
SIGEMT is not specified in POSIX.1-2001, but nevertheless appears on
most other Unix systems, where its default action is typically to 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.
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 32 different real-time signals,
numbered 33 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, 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. (Note, however, that the LinuxThreads implementation uses
the first three real-time signals.)
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 con-
trast, if multiple instances of a standard signal are delivered
while that signal is currently blocked, then only one instance is
2. If the signal is sent using sigqueue(2), 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 inter-
faces were replaced by the RLIMIT_SIGPENDING resource limit, which
specifies a per-user limit for queued signals; see setrlimit(2) for
A signal handling routine established by sigaction(2) or signal(2) must
be very careful, since processing elsewhere may be interrupted at some
arbitrary point in the execution of the program. POSIX has the concept
of "safe function". If a signal interrupts the execution of an unsafe
function, and handler calls an unsafe function, then the behavior of
the program is undefined.
POSIX.1-2004 (also known as POSIX.1-2001 Technical Corrigendum 2)
requires an implementation to guarantee that the following functions
can be safely called inside a signal handler:
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;
* 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 will be automatically restarted after
the signal handler returns if the SA_RESTART flag was used; otherwise
the call will fail 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. (A disk is not a slow device according to this defini-
tion.) 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
* open(2), if it can block (e.g., when opening a FIFO; see
* wait(2), wait3(2), wait4(2), waitid(2), and waitpid(2).
* Socket interfaces: accept(2), connect(2), recv(2), recvfrom(2),
recvmsg(2), send(2), sendto(2), and sendmsg(2).
* File locking interfaces: flock(2) and fcntl(2) F_SETLKW.
* POSIX message queue interfaces: mq_receive(3), mq_time-
dreceive(3), mq_send(3), and mq_timedsend(3).
* futex(2) FUTEX_WAIT (since Linux 2.6.22; beforehand, always
failed with EINTR).
* POSIX semaphore interfaces: sem_wait(3) and sem_timedwait(3)
(since Linux 2.6.22; 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:
* 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 sem-
* Sleep interfaces: clock_nanosleep(2), nanosleep(2), and
* read(2) from an inotify(7) file descriptor.
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
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:
* epoll_wait(2), epoll_pwait(2).
* semop(2), semtimedop(2).
* sigtimedwait(2), sigwaitinfo(2).
* read(2) from an inotify(7) file descriptor.
* Linux 2.6.21 and earlier: futex(2) FUTEX_WAIT, sem_timedwait(3),
* Linux 2.6.8 and earlier: msgrcv(2), msgsnd(2).
* Linux 2.4 and earlier: nanosleep(2).
POSIX.1, except as noted.
SIGIO and SIGLOST have the same value. The latter is commented out in
the kernel source, but the build process of some software still thinks
that signal 29 is SIGLOST.
kill(1), getrlimit(2), kill(2), killpg(2), setitimer(2), setrlimit(2),
sgetmask(2), sigaction(2), sigaltstack(2), signal(2), signalfd(2), sig-
pending(2), sigprocmask(2), sigqueue(2), sigsuspend(2), sigwaitinfo(2),
abort(3), bsd_signal(3), longjmp(3), raise(3), sigset(3), sigsetops(3),
sigvec(3), sigwait(3), strsignal(3), sysv_signal(3), core(5), proc(5),
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Linux 2008-07-07 SIGNAL(7)