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Signal (IPC)
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<p>Signals are standardized messages sent to a running <a href="/facts/Computer_program/e38jH9N9">program</a> to trigger specific behavior, such as quitting or error handling. They are a limited form of <a href="/facts/Inter-process_communication/cIQYyT6y">inter-process communication</a> (IPC), typically used in <a href="/facts/Unix/0pNL2seI">Unix</a>, <a href="/facts/Unix-like/YAKrAxzd">Unix-like</a>, and other <a href="/facts/POSIX/mQkSt234">POSIX</a>-compliant operating systems. </p><p>A signal is an asynchronous notification sent to a <a href="/facts/Process_(computing)/Vcq7EnUV">process</a> or to a specific <a href="/facts/Thread_(computing)/hh8WHyPd">thread</a> within the same process to notify it of an event. Common uses of signals are to interrupt, suspend, terminate or <a href="/facts/Kill_(command)/PjqmLTUR">kill</a> a process. Signals originated in 1970s <a href="/facts/Bell_Labs/fKP5Gk0H">Bell Labs</a> Unix and were later specified in the <a href="/facts/POSIX/mQkSt234">POSIX</a> standard. </p><p>When a signal is sent, the operating system interrupts the target process's normal <a href="/facts/Control_flow/2XTDfErC">flow of execution</a> to deliver the signal. Execution can be interrupted during any <a href="/facts/Atomic_operation/u3pnNCvs">non-atomic instruction</a>. If the process has previously registered a signal handler, that routine is executed. Otherwise, the default signal handler is executed. </p><p>Embedded programs may find signals useful for inter-process communications, as signals are notable for their <a href="/facts/Algorithmic_efficiency/VutjvPTd">algorithmic efficiency</a>. </p><p>Signals are similar to <a href="/facts/Interrupt/Mm6C4rpc">interrupts</a>, the difference being that interrupts are mediated by the <a href="/facts/Central_processing_unit/Jxp1bqMV">CPU</a> and handled by the <a href="/facts/Kernel_(operating_system)/uEj4U3Rz">kernel</a> while signals are mediated by the kernel (possibly via system calls) and handled by individual <a href="/facts/Process_(computing)/Vcq7EnUV">processes</a>. The kernel may pass an interrupt as a signal to the process that caused it (typical examples are <a href="/facts/SIGSEGV/MvAqaWzT">SIGSEGV</a>, <a href="/facts/SIGBUS/6svpknPF">SIGBUS</a>, SIGILL and SIGFPE). </p>