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' 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. The kernel may pass an interrupt as a signal to the process that caused it (typical examples are SIGSEGV, SIGBUS, SIGILL and SIGFPE). History Version 1 Unix (1971) had separate system calls to catch interrupts, quits, and machine traps. appeared in Version 2 (1972). Version 4 (1973) combined all traps into one call, . Version 5 (1974) could send arbitrary signals. In Version 7 (1979) each numbered trap received a symbolic name. Plan 9 from Bell Labs (mid 80s) replaced signals with notes, which permit sending short, arbitrary strings. Sending signals The system call sends a specified signal to a specified process, if permissions allow. Similarly, the command allows a user to send signals to processes. The 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