The entries in the "Action" column of the table 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, 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.
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 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.
|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|
|SIGHUP||P1990||Term||Hangup detected on controlling terminal|
|or death of controlling process|
|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|
|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)|
|SIGTSTP||P1990||Stop||Stop typed at terminal|
|SIGSYS||P2001||Core||Bad system call (SVr4);|
|see also seccomp(2)|
|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);|
|SIGXFSZ||P2001||Core||File size limit exceeded (4.2BSD);|
|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.
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.
|SIGPOLL||Same as SIGIO|
Note the following:
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:
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.
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|
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:
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:
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.
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.