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Signals are standardized messages sent to a running program to trigger specific behavior, such as quitting or error handling. They are a limited form of inter-process communication (IPC), typically used in Unix, Unix-like, and other POSIX-compliant operating systems.

A signal is an asynchronous notification sent to a process or to a specific thread within the same process to notify it of an event. Common uses of signals are to interrupt, suspend, terminate or kill a process. Signals originated in 1970s Bell Labs Unix and were later specified in the POSIX standard.

When a signal is sent, the operating system interrupts the target process's normal flow of execution to deliver the signal. Execution can be interrupted during any non-atomic instruction. If the process has previously registered a signal handler, that routine is executed. Otherwise, the default signal handler is executed.

Embedded programs may find signals useful for inter-process communications, as signals are notable for their algorithmic efficiency.

Signals are similar to interrupts, the difference being that interrupts are mediated by the CPU and handled by the kernel while signals are mediated by the kernel (possibly via system calls) and handled by individual processes.Template:Cn The kernel may pass an interrupt as a signal to the process that caused it (typical examples are SIGSEGV, SIGBUS, SIGILL and SIGFPE).

HistoryEdit

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Sending signalsEdit

The Template:Tt system call sends a specified signal to a specified process, if permissions allow. Similarly, the Template:Tt command allows a user to send signals to processes. The Template:Tt library function sends the specified signal to the current process.

Exceptions such as division by zero, segmentation violation (SIGSEGV), and floating point exception (SIGFPE) will cause a core dump and terminate the program.

The kernel can generate signals to notify processes of events. For example, SIGPIPE will be generated when a process writes to a pipe which has been closed by the reader; by default, this causes the process to terminate, which is convenient when constructing shell pipelines.

Typing certain key combinations at the controlling terminal of a running process causes the system to send it certain signals:<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

  • Ctrl-C (in older Unixes, DEL) sends an INT signal ("interrupt", SIGINT); by default, this causes the process to terminate.
  • Ctrl-Z sends a TSTP signal ("terminal stop", SIGTSTP); by default, this causes the process to suspend execution.<ref>{{#invoke:citation/CS1|citation

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  • Ctrl-\ sends a QUIT signal (SIGQUIT); by default, this causes the process to terminate and dump core.
  • Ctrl-T (not supported on all UNIXes) sends an INFO signal (SIGINFO); by default, and if supported by the command, this causes the operating system to show information about the running command.<ref>{{#invoke:citation/CS1|citation

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These default key combinations with modern operating systems can be changed with the Template:Tt command.

Handling signalsEdit

Signal handlers can be installed with the [[sigaction|Template:Tt or Template:Tt]] system call. If a signal handler is not installed for a particular signal, the default handler is used. Otherwise the signal is intercepted and the signal handler is invoked. The process can also specify two default behaviors, without creating a handler: ignore the signal (SIG_IGN) and use the default signal handler (SIG_DFL). There are two signals which cannot be intercepted and handled: SIGKILL and SIGSTOP.

RisksEdit

Signal handling is vulnerable to race conditions. As signals are asynchronous, another signal (even of the same type) can be delivered to the process during execution of the signal handling routine.

The Template:Tt call can be used to block and unblock delivery of signals. Blocked signals are not delivered to the process until unblocked. Signals that cannot be ignored (SIGKILL and SIGSTOP) cannot be blocked.

Signals can cause the interruption of a system call in progress, leaving it to the application to manage a non-transparent restart.

Signal handlers should be written in a way that does not result in any unwanted side-effects, e.g. Template:Tt alteration, signal mask alteration, signal disposition change, and other global process attribute changes. Use of non-reentrant functions, e.g., Template:Tt or Template:Tt, inside signal handlers is also unsafe. In particular, the POSIX specification and the Linux man page Template:Tt require that all system functions directly or indirectly called from a signal function are async-signal safe.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The Template:Tt man page gives a list of such async-signal safe system functions (practically the system calls), otherwise it is an undefined behavior.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It is suggested to simply set some <syntaxhighlight lang="text" class="" style="" inline="1">volatile sig_atomic_t</syntaxhighlight> variable in a signal handler, and to test it elsewhere.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Signal handlers can instead put the signal into a queue and immediately return. The main thread will then continue "uninterrupted" until signals are taken from the queue, such as in an event loop. "Uninterrupted" here means that operations that block may return prematurely and must be resumed, as mentioned above. Signals should be processed from the queue on the main thread and not by worker pools, as that reintroduces the problem of asynchronicity. However, managing a queue is not possible in an async-signal safe way with only Template:Tt, as only single reads and writes to such variables are guaranteed to be atomic, not increments or (fetch-and)-decrements, as would be required for a queue. Thus, effectively, only one signal per handler can be queued safely with Template:Tt until it has been processed.

Relationship with hardware exceptionsEdit

A process's execution may result in the generation of a hardware exception, for instance, if the process attempts to divide by zero or incurs a page fault.

In Unix-like operating systems, this event automatically changes the processor context to start executing a kernel exception handler. In case of some exceptions, such as a page fault, the kernel has sufficient information to fully handle the event itself and resume the process's execution.

Other exceptions, however, the kernel cannot process intelligently and it must instead defer the exception handling operation to the faulting process. This deferral is achieved via the signal mechanism, wherein the kernel sends to the process a signal corresponding to the current exception. For example, if a process attempted integer divide by zero on an x86 CPU, a divide error exception would be generated and cause the kernel to send the SIGFPE signal to the process.

Similarly, if the process attempted to access a memory address outside of its virtual address space, the kernel would notify the process of this violation via a SIGSEGV (segmentation violation signal). The exact mapping between signal names and exceptions is obviously dependent upon the CPU, since exception types differ between architectures.

POSIX signalsEdit

The list below documents the signals specified in the Single Unix Specification Version 5. All signals are defined as macro constants in the <signal.h> header file. The name of the macro constant consists of a "SIG" prefix followed by a mnemonic name for the signal.

A process can define how to handle incoming POSIX signals. If a process does not define a behaviour for a signal, then the default handler for that signal is being used. The table below lists some default actions for POSIX-compliant UNIX systems, such as FreeBSD, OpenBSD and Linux.

Signal Portable
number
Default action Description
SIGABRT 6 Template:Terminated (core dump) Process abort signal
SIGALRM 14 Template:Terminated Alarm clock
SIGBUS Template:N/a Template:Terminated (core dump) Access to an undefined portion of a memory object
SIGCHLD Template:N/a Ignore Child process terminated, stopped, or continued
SIGCONT Template:N/a Continue Continue executing, if stopped
SIGFPE 8 Template:Terminated (core dump) Erroneous arithmetic operation
SIGHUP 1 Template:Terminated Hangup
SIGILL 4 Template:Terminated (core dump) Illegal instruction
SIGINT 2 Template:Terminated Terminal interrupt signal
SIGKILL 9 Template:Terminated Kill (cannot be caught or ignored)
SIGPIPE 13 Template:Terminated Write on a pipe with no one to read it
SIGQUIT 3 Template:Terminated (core dump) Terminal quit signal
SIGSEGV 11 Template:Terminated (core dump) Invalid memory reference
SIGSTOP Template:N/a Template:Dropped Stop executing (cannot be caught or ignored)
SIGSYS Template:N/a Template:Terminated (core dump) Bad system call
SIGTERM 15 Template:Terminated Termination signal
SIGTRAP 5 Template:Terminated (core dump) Trace/breakpoint trap
SIGTSTP Template:N/a Template:Dropped Terminal stop signal
SIGTTIN Template:N/a Template:Dropped Background process attempting read
SIGTTOU Template:N/a Template:Dropped Background process attempting write
SIGUSR1 Template:N/a Template:Terminated User-defined signal 1
SIGUSR2 Template:N/a Template:Terminated User-defined signal 2
SIGURG Template:N/a Ignore Out-of-band data is available at a socket
SIGVTALRM Template:N/A Template:Terminated Virtual timer expired
SIGXCPU Template:N/a Template:Terminated (core dump) CPU time limit exceeded
SIGXFSZ Template:N/a Template:Terminated (core dump) File size limit exceeded
SIGWINCH Template:N/a Ignore Terminal window size changed
Portable number:
For most signals the corresponding signal number is implementation-defined. This column lists the numbers specified in the POSIX standard.<ref>{{#invoke:citation/CS1|citation

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Actions explained:
TerminateTemplate:Snd Abnormal termination of the process. The process is terminated with all the consequences of _exit() except that the status made available to wait() and waitpid() indicates abnormal termination by the specified signal.
Terminate (core dump)Template:Snd Abnormal termination of the process. Additionally, implementation-defined abnormal termination actions, such as creation of a core file, may occur.
IgnoreTemplate:Snd Ignore the signal.
StopTemplate:Snd Stop (or suspend) the process.
ContinueTemplate:Snd Continue the process, if it is stopped; otherwise, ignore the signal.
Template:Mono and Template:Mono
The SIGABRT signal is sent to a process to tell it to abort, i.e. to terminate. The signal is usually initiated by the process itself when it calls abort() function of the C Standard Library, but it can be sent to the process from outside like any other signal.
SIGIOT indicates that the CPU has executed an explicit "trap" instruction (without a defined function), or an unimplemented instruction (when emulation is unavailable).
Note: "input/output trap" is a misnomer for any CPU "trap" instruction. The term reflects early usage of such instructions, predominantly to implement I/O functions, but they are not inherently tied to device I/O and may be used for other purposes such as communication between virtual & real hosts.
SIGIOT and SIGABRT are typically the same signal, and receipt of that signal may indicate any of the conditions above.
Template:Mono, Template:Mono and Template:Mono
The SIGALRM, SIGVTALRM and SIGPROF signals are sent to a process when the corresponding time limit is reached. The process sets these time limits by calling alarm or setitimer. The time limit for SIGALRM is based on real or clock time; SIGVTALRM is based on CPU time used by the process; and SIGPROF is based on CPU time used by the process and by the system on its behalf (known as a profiling timer). On some systems SIGALRM may be used internally by the implementation of the sleep function.
Template:Mono
The SIGBUS signal is sent to a process when it causes a bus error. The conditions that lead to the signal being sent are, for example, incorrect memory access alignment or non-existent physical address.
Template:Mono
The SIGCHLD signal is sent to a process when a child process terminates, is stopped, or resumes after being stopped. One common usage of the signal is to instruct the operating system to clean up the resources used by a child process after its termination without an explicit call to the wait system call.
Template:Mono
The SIGCONT signal instructs the operating system to continue (restart) a process previously paused by the SIGSTOP or SIGTSTP signal. One important use of this signal is in job control in the Unix shell.
Template:Mono
The SIGFPE signal is sent to a process when an exceptional (but not necessarily erroneous) condition has been detected in the floating-point or integer arithmetic hardware. This may include division by zero, floating-point underflow or overflow, integer overflow, an invalid operation or an inexact computation. Behaviour may differ depending on hardware.
Template:Mono
The SIGHUP signal is sent to a process when its controlling terminal is closed. It was originally designed to notify the process of a serial line drop (a hangup). In modern systems, this signal usually means that the controlling pseudo or virtual terminal has been closed.<ref name="linux-signal7">{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> Many daemons (who have no controlling terminal) interpret receipt of this signal as a request to reload their configuration files and flush/reopen their logfiles instead of exiting.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> nohup is a command to make a command ignore the signal.

Template:Mono
The SIGILL signal is sent to a process when it attempts to execute an illegal, malformed, unknown, or privileged instruction.
Template:Mono
The SIGINT signal is sent to a process by its controlling terminal when a user wishes to interrupt the process. This is typically initiated by pressing [[Control-C|Template:Keypress]], but on some systems, the "delete" character or "break" key can be used.<ref>{{#invoke:citation/CS1|citation

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Template:Mono
The SIGKILL signal is sent to a process to cause it to terminate immediately (kill). In contrast to SIGTERM and SIGINT, this signal cannot be caught or ignored, and the receiving process cannot perform any clean-up upon receiving this signal. The following exceptions apply:

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  • An uninterruptibly sleeping process may not terminate (and free its resources) even when sent SIGKILL. This is one of the few cases in which a UNIX system may have to be rebooted to solve a temporary software problem.
SIGKILL is used as a last resort when terminating processes in most system shutdown procedures if it does not voluntarily exit in response to SIGTERM. To speed the computer shutdown procedure, Mac OS X 10.6, aka Snow Leopard, will send SIGKILL to applications that have marked themselves "clean" resulting in faster shutdown times with, presumably, no ill effects.<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> The command <syntaxhighlight lang="text" class="" style="" inline="1">killall -9</syntaxhighlight> has a similar, while dangerous effect, when executed e.g. in Linux; it does not let programs save unsaved data. It has other options, and with none, uses the safer SIGTERM signal.

Template:Mono
The SIGPIPE signal is sent to a process when it attempts to write to a pipe without a process connected to the other end.
Template:Mono
The SIGPOLL signal is sent when an event occurred on an explicitly watched file descriptor.<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> Using it effectively leads to making asynchronous I/O requests since the kernel will poll the descriptor in place of the caller. It provides an alternative to active polling.

Template:Mono to Template:Mono
The SIGRTMIN to SIGRTMAX signals are intended to be used for user-defined purposes. They are real-time signals.
Template:Mono
The SIGQUIT signal is sent to a process by its controlling terminal when the user requests that the process quit and perform a core dump.
Template:Mono
The SIGSEGV signal is sent to a process when it makes an invalid virtual memory reference, or segmentation fault, i.e. when it performs a segmentation violation.<ref>{{#invoke:citation/CS1|citation

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Template:Mono
The SIGSTOP signal instructs the operating system to stop a process for later resumption.
Template:Mono
The SIGSYS signal is sent to a process when it passes a bad argument to a system call. In practice, this kind of signal is rarely encountered since applications rely on libraries (e.g. libc) to make the call for them. SIGSYS can be received by applications violating the Linux Seccomp security rules configured to restrict them. SIGSYS can also be used to emulate foreign system calls, e.g. emulate Windows system calls on Linux.<ref>{{#invoke:citation/CS1|citation

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Template:Mono
The SIGTERM signal is sent to a process to request its termination. Unlike the SIGKILL signal, it can be caught and interpreted or ignored by the process. This allows the process to perform nice termination releasing resources and saving state if appropriate. SIGINT is nearly identical to SIGTERM.
Template:Mono
The SIGTSTP signal is sent to a process by its controlling terminal to request it to stop (terminal stop). It is commonly initiated by the user pressing [[Ctrl-Z|Template:Keypress]]. Unlike SIGSTOP, the process can register a signal handler for, or ignore, the signal.
Template:Mono and Template:Mono
The SIGTTIN and SIGTTOU signals are sent to a process when it attempts to read in or write out respectively from the tty while in the background. Typically, these signals are received only by processes under job control; daemons do not have controlling terminals and, therefore, should never receive these signals.
Template:Mono
The SIGTRAP signal is sent to a process when an exception (or trap) occurs: a condition that a debugger has requested to be informed ofTemplate:Snd for example, when a particular function is executed, or when a particular variable changes value.
Template:Mono
The SIGURG signal is sent to a process when a socket has urgent or out-of-band data available to read.
Template:Mono and Template:Mono
The SIGUSR1 and SIGUSR2 signals are sent to a process to indicate user-defined conditions.
Template:Mono
The SIGXCPU signal is sent to a process when it has used up the CPU for a duration that exceeds a certain predetermined user-settable value.<ref name="setrlimit_posix_spec">{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> The arrival of a SIGXCPU signal provides the receiving process a chance to quickly save any intermediate results and to exit gracefully, before it is terminated by the operating system using the SIGKILL signal.

Template:Mono
The SIGXFSZ signal is sent to a process when it grows a file that exceeds the maximum allowed size.
Template:Mono
The SIGWINCH signal is sent to a process when its controlling terminal changes its size (a window change).<ref>{{#invoke:citation/CS1|citation

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Miscellaneous signalsEdit

The following signals are not specified in the POSIX specification. They are, however, sometimes used on various systems.

Template:Mono
The SIGEMT signal is sent to a process when an emulator trap occurs. While an emulator usually means software that executes other programs, in this case it means a program executed a supervisor call instruction (EMT was the instruction for this purpose on the DEC PDP-11 series of computers.)
Template:Mono
The SIGINFO signal is sent to a process when a status (info) request is received from the controlling terminal.
Template:Mono
The SIGPWR signal is sent to a process when the system experiences a power failure.
Template:Mono
The SIGLOST signal is sent to a process when a file lock is lost.
Template:Mono
The SIGSTKFLT signal is sent to a process when the coprocessor experiences a stack fault (i.e. popping when the stack is empty or pushing when it is full).<ref name="courier-mta.org">{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref> It is defined by, but not used on Linux, where a x87 coprocessor stack fault will generate SIGFPE instead.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Template:Mono
The SIGUNUSED signal is sent to a process when a system call with an unused system call number is made. It is synonymous with SIGSYS on most architectures.<ref name="courier-mta.org"/>
Template:Mono
The SIGCLD signal is synonymous with SIGCHLD.<ref name="courier-mta.org"/>

See alsoEdit

ReferencesEdit

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External linksEdit

Template:Inter-process communication