28 SIGWINCH "discard signal" "Window size change" 29 SIGINFO "discard signal" "status request from keyboard" 30 SIGUSR1 "terminate process" "User defined signal 1" 31 SIGUSR2 "terminate process" "User defined signal 2" 32 SIGTHR "terminate process" "thread 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 SIG_DFL. A SIG_DFL resets the default action. To ignore the signal func should be SIG_IGN. This will cause subsequent instances of the signal to be ignored and pending instances to be discarded. If 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. .Bf -symbolic Unlike previous signal facilities, the handler func() remains installed after a signal has been delivered. .Ef
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 sigaction 2.) The affected system calls include read(2), write(2), sendto(2), recvfrom(2), sendmsg(2) and recvmsg(2) on a communications channel or a low speed device and during a ioctl(2) or 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 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 execve(2) function; ignored signals remain ignored.
If a process explicitly specifies SIG_IGN as the action for the signal 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 wait(2) or equivalent, it will block until all of the calling processs children terminate, and then return a value of -1 with errno set to ECHILD.
See sigaction(2) for a list of functions that are considered safe for use in signal handlers.