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 (less portably) signal(2). Using these system calls, a process can elect one of the following behaviours 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.
The signal disposition is a per-process attribute: in a multithreaded application, the disposition of a particular signal is the same for all threads.
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(2)). 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 readers|
|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 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.
|SIGPOLL||Term||Pollable event (Sys V). Synonym of SIGIO|
|SIGPROF||27,27,29||Term||Profiling timer expired|
|SIGSYS||12,-,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 behaviour 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 Unices 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|
|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 Unices, 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 Unices where it appears.
SIGIO (which is not specified in POSIX.1-2001) is ignored by default on several other Unices.
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:
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.