Signal

This article is about POSIX-style signals. For signals in event-driven processing, see Signal programming.

A signal is a limited form of inter-process communication used in Unix, Unix-like, and other POSIX-compliant operating systems. Essentially it is an asynchronous notification sent to a process in order to notify it of an event that occurred. When a signal is sent to a process, the operating system interrupts the process's normal flow of execution. 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.

Sending signals

• Typing certain key combinations at the controlling terminal of a running process causes the system to send it certain signals:
  • Ctrl-C (in older Unices, DEL) sends an INT signal (SIGINT); by default, this causes the process to terminate.
  • Ctrl-Z sends a TSTP signal (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.
  • (Those default key combinations can be changed with the stty command.)
• The kill(2) system call will send the specified signal to the 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 and SIGSEGV respectively, which both by default cause a core dump and a program exit).
• The kernel can generate a signal to notify the process of an event. 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.

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. Because 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.

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.

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 to 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.

List of signals

The Single Unix Specification specifies the following signals which are defined in <signal.h>:

Signal	Description	Signal number on Linux x86[1]
SIGABRT	Process aborted	6
SIGALRM	Signal raised by alarm	14
SIGBUS	Bus error: "access to undefined portion of memory object"	7
SIGCHLD	Child process terminated, stopped (or continued*)	17
SIGCONT	Continue if stopped	18
SIGFPE	Floating point exception: "erroneous arithmetic operation"	8
SIGHUP	Hangup	1
SIGILL	Illegal instruction	4
SIGINT	Interrupt	2
SIGKILL	Kill (terminate immediately)	9
SIGPIPE	Write to pipe with no one reading	13
SIGQUIT	Quit and dump core	3
SIGSEGV	Segmentation violation	11
SIGSTOP	Stop executing temporarily	19
SIGTERM	Termination (request to terminate)	15
SIGTSTP	Terminal stop signal	20
SIGTTIN	Background process attempting to read from tty ("in")	21
SIGTTOU	Background process attempting to write to tty ("out")	22
SIGUSR1	User-defined 1	10
SIGUSR2	User-defined 2	12
SIGPOLL	Pollable event	29
SIGPROF	Profiling timer expired	27
SIGSYS	Bad syscall	31
SIGTRAP	Trace/breakpoint trap	5
SIGURG	Urgent data available on socket	23
SIGVTALRM	Signal raised by timer counting virtual time: "virtual timer expired"	26
SIGXCPU	CPU time limit exceeded	24
SIGXFSZ	File size limit exceeded	25

Note: Where a section is marked by an asterisk, this denotes an X/Open System Interfaces (XSI) extension. Wording in quotes appended with (SUS) denotes the wording from the SUS[1].

In addition to signals listed above, a process can send the pseudo-signal 0. This checks for errors as if a signal were sent, without actually sending any, which is useful for e.g. checking if a process exists.

See also

• <signal.h>

References

External links

• Introduction to Unix Signals Programming
• Another Introduction to Unix Signals Programming
• UNIX and Reliable POSIX Signals by Baris Simsek
• Signal Handlers by Henning Brauer