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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' 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.

• Typing certain 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 (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.
• 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 processses. The raise system call 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 cause a core dump by default).
• 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.

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.

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.

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. With some exceptions, such as a page fault, the kernel has sufficient information to fully handle the event and resume the process's execution. In other exceptions, however, the the kernel cannot proceed intelligently and 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.

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

SIGABRT - process aborted

SIGALRM - signal raised by alarm

SIGBUS - bus error: "access to undefined portion of memory object"

SIGCHLD - child process terminated, stopped (*or continued)

SIGCONT - continue if stopped

SIGFPE - floating point exception: "erroneous arithmetic operation"

SIGHUP - hangup

SIGILL - illegal instruction

SIGINT - interrupt

SIGKILL - kill

SIGPIPE - write to pipe with no one reading

SIGQUIT - quit

SIGSEGV - segmentation violation

SIGSTOP - stop executing temporarily

SIGTERM - termination

SIGTSTP - terminal stop signal

SIGTTIN - background process attempting to read ("in")

SIGTTOU - background process attempting to write ("out")

SIGUSR1 - user defined 1

SIGUSR2 - user defined 2

*SIGPOLL - pollable event

*SIGPROF - profiling timer expired

*SIGSYS - bad syscall

*SIGTRAP - trace/breakpoint trap

SIGURG - urgent data available on socket

*SIGVTALRM - signal raised by timer counting virtual time: "virtual timer expired"

*SIGXCPU - CPU time limit exceeded

*SIGXFSZ - file size limit exceeded

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].

• <signal.h>

• Introduction to Unix Signals Programming
• UNIX and Reliable POSIX Signals by Baris Simsek