signal(7) — Linux manual pages

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 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.

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 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.

Standard Signals

Linux supports the standard signals listed below. Several signal numbers 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 i386, ppc and sh, and the last one for mips. A − denotes that a signal is absent on the corresponding 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 of 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 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 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 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.

Real-time Signals

Linux supports real-time signals as originally defined in the POSIX.1b real-time extensions (and now included in POSIX.1-2001). Linux supports 32 real-time signals, numbered from 32 (SIGRTMIN) to 63 (SIGRTMAX). (Programs should always refer to real-time signals using notation SIGRTMIN+n, since the range of real-time signal numbers varies across Unix systems.)

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:

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.
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.
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.)

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.

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 program1. 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.1-2003 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(), 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().

CONFORMING TO

POSIX.1

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.