This signal() facility is a simplified interface to the more general Syscall.sigaction(2) facility.

Signals allow the manipulation of a process from outside its domain as well as allowing the process to manipulate itself or copies of itself (children). There are two general types of signals: those that cause termination of a process and those that do not. Signals which cause termination of a program might result from an irrecoverable error or might be the result of a user at a terminal typing the 'interrupt' character. Signals are used when a process is stopped because it wishes to access its control terminal while in the background (see tty(4)). Signals are optionally generated when a process resumes after being stopped, when the status of child processes changes, or when input is ready at the control terminal. Most signals result in the termination of the process receiving them if no action is taken; some signals instead cause the process receiving them to be stopped, or are simply discarded if the process has not requested otherwise. Except for the Signum.SIGKILL and Signum.SIGSTOP signals, the signal() function allows for a signal to be caught, to be ignored, or to generate an interrupt.

The sig argument specifies which signal was received. The func procedure allows a user to choose the action upon receipt of a signal. To set the default action of the signal to occur as listed above, func should be Stdlib.SIG_DFL. A Stdlib.SIG_DFL resets the default action. To ignore the signal func should be Stdlib.SIG_IGN. This will cause subsequent instances of the signal to be ignored and pending instances to be discarded. If Stdlib.SIG_IGN is not used, further occurrences of the signal are automatically blocked and func is called.

The handled signal is unblocked when the function returns and the process continues from where it left off when the signal occurred.

For some system calls, if a signal is caught while the call is executing and the call is prematurely terminated, the call is automatically restarted. (The handler is installed using the SA_RESTART flag with Syscall.sigaction(2).) The affected system calls include Syscall.read(2), Syscall.write(2), Syscall.sendto(2), Syscall.recvfrom(2), Syscall.sendmsg(2) and Syscall.recvmsg(2) on a communications channel or a low speed device and during a Syscall.ioctl(2) or Syscall.wait(2). However, calls that have already committed are not restarted, but instead return a partial success (for example, a short read count). These semantics could be changed with Syscall.siginterrupt(3)

When a process which has installed signal handlers forks, the child process inherits the signals. All caught signals may be reset to their default action by a call to the Syscall.execve(2) function; ignored signals remain ignored.

If a process explicitly specifies Stdlib.SIG_IGN as the action for the signal Stdlib.SIGCHLD, the system will not create zombie processes when children of the calling process exit. As a consequence, the system will discard the exit status from the child processes. If the calling process subsequently issues a call to Syscall.wait(2) or equivalent, it will block until all of the calling process's children terminate, and then return a value of -1 with Stdlib.GetLastError returning Errno.ECHILD.