The entries in the "Action" column of the tables below specify the default disposition for each signal, as follows:
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, the 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.
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 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 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.
First the signals described in the original POSIX.1-1990 standard.
|or death of controlling process|
|SIGINT||2||Term||Interrupt from keyboard|
|SIGQUIT||3||Core||Quit from keyboard|
|SIGABRT||6||Core||Abort signal from abort(3)|
|SIGFPE||8||Core||Floating point exception|
|SIGSEGV||11||Core||Invalid memory reference|
|SIGPIPE||13||Term||Broken pipe: write to pipe with no|
|SIGALRM||14||Term||Timer signal from alarm(2)|
|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|
|SIGTSTP||18,20,24||Stop||Stop typed at terminal|
|SIGTTIN||21,21,26||Stop||Terminal input for background process|
|SIGTTOU||22,22,27||Stop||Terminal 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.
|SIGPOLL||Term||Pollable event (Sys V).|
|Synonym for SIGIO|
|SIGPROF||27,27,29||Term||Profiling timer expired|
|SIGSYS||12,31,12||Core||Bad argument to routine (SVr4)|
|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.
|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 (unused)|
|SIGWINCH||28,28,20||Ign||Window resize signal (4.3BSD, Sun)|
|SIGUNUSED||-,31,-||Core||Synonymous with 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 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.
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 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:
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.
A signal handler function 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:
_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()
POSIX.1-2008 removes fpathconf(), pathconf(), and sysconf() from the above list, and adds the following functions:
execl() execv() faccessat() fchmodat() fchownat() fexecve() fstatat() futimens() linkat() mkdirat() mkfifoat() mknod() mknodat() openat() readlinkat() renameat() symlinkat() unlinkat() utimensat() utimes()
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:
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:
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.
The Linux interfaces that display this behavior are: