sent to a process when it makes an invalid virtual memory reference, or segmentation fault, i.e. when it performs a segmentation violation. "Signal stop" The SIGSTOP signal instructs the operating system to stop a process for later resumption. "Signal system call" 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. SIGSYS can be received by applications violating the Linux Seccomp security rules configured to restrict them. SIGSYS can also be used to emulate foreign system calls, e.g. emulate Windows system calls on Linux. "Signal terminate" 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. "Signal terminal stop" 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 . Unlike SIGSTOP, the process can register a signal handler for, or ignore, the signal. and 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. "Signal trap" 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. "Signal urgent" The SIGURG signal is sent to a process when a socket has urgent or out-of-band data available to read. and "Signal user 1", "signal user 2"" The SIGUSR1 and SIGUSR2 signals are sent to a process to indicate user-defined conditions. "Signal exceeded CPU" 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. 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