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SIGNAL(7) Linux Programmer's Manual SIGNAL(7)
NAME
signal - list of available 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 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 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 (less portably)
signal(2). 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 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 threads.
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 sig-
nals 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 executing 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 cur-
rently 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.
Standard Signals
Linux supports the standard signals listed below. Several signal numbers are architec-
ture-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 corre-
sponding architecture.)
First the signals described in the original POSIX.1-1990 standard.
Signal Value Action Comment
----------------------------------------------------------------------
SIGHUP 1 Term Hangup detected on controlling terminal
or death of controlling process
SIGINT 2 Term Interrupt from keyboard
SIGQUIT 3 Core Quit from keyboard
SIGILL 4 Core Illegal Instruction
SIGABRT 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 Termination 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
SIGCONT 19,18,25 Cont Continue if stopped
SIGSTOP 17,19,23 Stop Stop process
SIGTSTP 18,20,24 Stop Stop typed at tty
SIGTTIN 21,21,26 Stop tty input for background process
SIGTTOU 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.
Signal Value Action Comment
--------------------------------------------------------------------
SIGBUS 10,7,10 Core Bus error (bad memory access)
SIGPOLL 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)
SIGTRAP 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, 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.
Next various other signals.
Signal Value Action Comment
--------------------------------------------------------------------
SIGIOT 6 Core IOT trap. A synonym for SIGABRT
SIGEMT 7,-,7 Term
SIGSTKFLT -,16,- Term Stack fault on coprocessor (unused)
SIGIO 23,29,22 Term I/O now possible (4.2BSD)
SIGCLD -,-,18 Ign A synonym for SIGCHLD
SIGPWR 29,30,19 Term Power failure (System V)
SIGINFO 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 sys-
tems, 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.
Real-time Signals
Linux supports real-time signals as originally defined in the POSIX.1b real-time exten-
sions (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, programs should never refer to real-time signals using hard-coded
numbers, but instead should always refer to real-time signals using the notation SIGRT-
MIN+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 pro-
cess.
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(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 sig-
nals are sent to a process, they are delivered starting with the lowest-numbered sig-
nal. (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 unspeci-
fied 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.
Async-signal-safe functions
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 execu-
tion 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 implementa-
tion to guarantee that the following functions can be safely called inside a signal han-
dler:
_Exit()
_exit()
abort()
accept()
access()
aio_error()
aio_return()
aio_suspend()
alarm()
bind()
cfgetispeed()
cfgetospeed()
cfsetispeed()
cfsetospeed()
chdir()
chmod()
chown()
clock_gettime()
close()
connect()
creat()
dup()
dup2()
execle()
execve()
fchmod()
fchown()
fcntl()
fdatasync()
fork()
fpathconf()
fstat()
fsync()
ftruncate()
getegid()
geteuid()
getgid()
getgroups()
getpeername()
getpgrp()
getpid()
getppid()
getsockname()
getsockopt()
getuid()
kill()
link()
listen()
lseek()
lstat()
mkdir()
mkfifo()
open()
pathconf()
pause()
pipe()
poll()
posix_trace_event()
pselect()
raise()
read()
readlink()
recv()
recvfrom()
recvmsg()
rename()
rmdir()
select()
sem_post()
send()
sendmsg()
sendto()
setgid()
setpgid()
setsid()
setsockopt()
setuid()
shutdown()
sigaction()
sigaddset()
sigdelset()
sigemptyset()
sigfillset()
sigismember()
signal()
sigpause()
sigpending()
sigprocmask()
sigqueue()
sigset()
sigsuspend()
sleep()
sockatmark()
socket()
socketpair()
stat()
symlink()
sysconf()
tcdrain()
tcflow()
tcflush()
tcgetattr()
tcgetpgrp()
tcsendbreak()
tcsetattr()
tcsetpgrp()
time()
timer_getoverrun()
timer_gettime()
timer_settime()
times()
umask()
uname()
unlink()
utime()
wait()
waitpid()
write()
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 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 exam-
ple, a terminal, pipe, or socket. (A disk is not a slow device according to this
definition.) 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).
* 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), 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_timedreceive(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 inter-
rupted 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 semtimedop(2).
* Sleep interfaces: clock_nanosleep(2), nanosleep(2), and usleep(3).
* read(2) from an inotify(7) file descriptor.
* 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.
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), sem_wait(3).
* Linux 2.6.8 and earlier: msgrcv(2), msgsnd(2).
* Linux 2.4 and earlier: nanosleep(2).
CONFORMING TO
POSIX.1, except as noted.
BUGS
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.
SEE ALSO
kill(1), getrlimit(2), kill(2), killpg(2), setitimer(2), setrlimit(2), sgetmask(2), sigac-
tion(2), sigaltstack(2), signal(2), signalfd(2), sigpending(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), pthreads(7)
COLOPHON
This page is part of release 3.05 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at http://www.ker-
nel.org/doc/man-pages/.
Linux 2008-07-07 SIGNAL(7)
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