Unix signal

Signals are a limited form of inter-process communication 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 in order to notify it of an event that occurred. Signals originated in 1970s Bell Labs Unix and have been more recently specified in the POSIX standard.

When a signal is sent, the operating system interrupts the target process' 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 interprocess communications, as the computational and memory footprint for signals is small.

History

Version 1 Unix had separate system calls to catch interrupts, quits, and machine traps. Version 4 combined all traps into one call, signal, and each numbered trap received a symbolic name in Version 7. kill appeared in Version 2, and in Version 5 could send arbitrary signals.^[1] Plan 9 from Bell Labs replaced signals with notes, which permit sending short, arbitrary strings.

Sending signals

The kill(2) system call sends a specified signal to a specified process, if permissions allow. Similarly, the kill(1) command allows a user to send signals to processes. The raise(3) library function sends the specified signal to the current process.

Exceptions such as division by zero or a segmentation violation will generate signals (here, SIGFPE "floating point exception" and SIGSEGV "segmentation violation" respectively, which both by default cause a core dump and a program exit).

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:

• 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.
• 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.

These default key combinations with modern operating systems can be changed with the stty command.

Handling signals

Signal handlers can be installed with the signal() 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.

Risks

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 sigprocmask() 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 doesn't result in any unwanted side-effects, e.g. errno alteration, signal mask alteration, signal disposition change, and other global process attribute changes. Use of non-reentrant functions, e.g., malloc or printf, inside signal handlers is also unsafe. In particular, the POSIX specification and the Linux man page signal(7) requires that all system functions directly or indirectly called from a signal function are async-signal safe and gives a list of such async-signal safe system functions (practically the system calls), otherwise it is an undefined behavior. It is suggested to simply set some volatile sig_atomic_t variable in a signal handler, and to test it elsewhere.

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.

Relationship with hardware exceptions

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 TLB miss.

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 signal. The exact mapping between signal names and exceptions is obviously dependent upon the CPU, since exception types differ between architectures.

POSIX signals

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

SIGABRT

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.

SIGALRM, SIGVTALRM and SIGPROF

The SIGALRM, SIGVTALRM and SIGPROF signal is sent to a process when the time limit specified in a call to a preceding alarm setting function (such as setitimer) elapses. SIGALRM is sent when real or clock time elapses. SIGVTALRM is sent when CPU time used by the process elapses. SIGPROF is sent when CPU time used by the process and by the system on behalf of the process elapses.

SIGBUS

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.

SIGCHLD

The SIGCHLD signal is sent to a process when a child process terminates, is interrupted, or resumes after being interrupted. 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.

SIGCONT

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.

SIGFPE

The SIGFPE signal is sent to a process when it executes an erroneous arithmetic operation, such as division by zero (the name "FPE", standing for floating-point exception, is a misnomer as the signal covers integer-arithmetic errors as well).^[2]

SIGHUP

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.^[3] Many daemons will reload their configuration files and reopen their logfiles instead of exiting when receiving this signal.^[4] nohup is a command to make a command ignore the signal.

SIGILL

The SIGILL signal is sent to a process when it attempts to execute an illegal, malformed, unknown, or privileged instruction.

SIGINT

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 Ctrl-C, but on some systems, the "delete" character or "break" key can be used.^[5]

SIGKILL

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.

SIGPIPE

The SIGPIPE signal is sent to a process when it attempts to write to a pipe without a process connected to the other end.

SIGPOLL

The SIGPOLL signal is sent when an event occurred on an explicitly watched file descriptor.^[6] 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.

SIGRTMIN to SIGRTMAX

The SIGRTMIN to SIGRTMAX signals are intended to be used for user-defined purposes. They are real-time signals.

SIGQUIT

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.

SIGSEGV

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.^[7]

SIGSTOP

The SIGSTOP signal instructs the operating system to stop a process for later resumption.

SIGSYS

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.

SIGTERM

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.

SIGTSTP

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. Unlike SIGSTOP, the process can register a signal handler for or ignore the signal.

SIGTTIN and SIGTTOU

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.

SIGTRAP

The SIGTRAP signal is sent to a process when an exception (or trap) occurs: a condition that a debugger has requested to be informed of — for example, when a particular function is executed, or when a particular variable changes value.

SIGURG

The SIGURG signal is sent to a process when a socket has urgent or out-of-band data available to read.

SIGUSR1 and SIGUSR2

The SIGUSR1 and SIGUSR2 signals are sent to a process to indicate user-defined conditions.

SIGXCPU

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.^[8] 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.

SIGXFSZ

The SIGXFSZ signal is sent to a process when it grows a file larger than the maximum allowed size.

Portable number:

For most signals the corresponding signal number is implementation-defined. This column lists the numbers specified in the POSIX standard.^[9]

Actions explained:

Terminate — 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) — Abnormal termination of the process. Additionally, implementation-defined abnormal termination actions, such as creation of a core file, may occur.

Ignore — Ignore the signal.

Stop — Stop (not terminate) the process.

Continue — Continue the process, if it is stopped; otherwise, ignore the signal.

Miscellaneous signals

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

SIGEMT

The SIGEMT signal is sent to a process when an emulator trap occurs.

SIGINFO

The SIGINFO signal is sent to a process when a status (info) request is received from the controlling terminal.

SIGPWR

The SIGPWR signal is sent to a process when the system experiences a power failure.

SIGLOST

The SIGLOST signal is sent to a process when a file lock is lost.

SIGWINCH

The SIGWINCH signal is sent to a process when its controlling terminal changes its size (a window change).

See also

• C signal handling

References

• Stevens, W. Richard (1992). Advanced Programming in the UNIX® Environment. Reading, Massachusetts: Addison Wesley. ISBN 0-201-56317-7. 
• "The Open Group Base Specifications Issue 7, 2013 Edition". The Open Group. Retrieved 19 June 2015. 

External links

• Unix Signals Table, Ali Alanjawi, University of Pittsburgh
• Man7.org Signal Man Page
• Introduction To Unix Signals Programming Introduction To Unix Signals Programming at the Wayback Machine (archived 26 September 2013)
• Another Introduction to Unix Signals Programming
• UNIX and Reliable POSIX Signals by Baris Simsek
• Signal Handlers by Henning Brauer