CHAPTER 16 – Parsing Command-Line Options – Signals

In UNIX, signals are a basic mechanism to pass messages between processes. They enable processes to tell each other that some type of event has just occurred. This type of event is the only information passed to basic UNIX signal handlers. There is another signal-handling mechanism called "sigaction" in which signal handlers receive more information, but PHP signals are based on the former, basic form. For example, if the user presses Ctrl-c to stop a command-line program, the program receives an interrupt sig- nal, called SIGINT. In PHP, you can set up a function to handle one or more signals with the pcntl_signal() function, like this: <?php function sigint_handler($signal) { print "Interrupt!n"; exit; } pcntl_signal(SIGINT, "sigint_handler"); declare (ticks = 1) { while (sleep(1)); } ?> This script sleeps until you terminate it. If you do press Ctrl-c, it prints Interrupt! and exits. You could change this example to ignore Ctrl-c com- pletely by changing the signal-handler function to the predefined SIG_IGN: pcntl_signal(SIGINT, SIG_IGN); You may change a signal handler anytime, including inside a signal- handling function. To revert to the default signal handler, use SIG_DFL: pcntl_signal(SIGINT, SIG_DFL); PHP probably supports all the signals your system supports. Try typing kill ­l in your shell to see some. Table 16.3 lists of signals that may be useful from PHP, either catching and handling them, or sending them to (killing) other processes.

Table 16.3 Signal Description SIGHUP Hangup. Used to notify when terminal connection is lost. SIGINT Interrupt. Send when user hits the interrupt (Ctrl-c) key. SIGABRT Sent by the abort() C function; used by assert(). SIGKILL Non-graceful termination of the process; cannot be caught. SIGUSR1 User-defined signal 1. SIGSEGV Segmentation fault; in some operating systems, it's known as General Protec- tion Failure. SIGUSR2 User-defined signal 2. SIGPIPE Sent when a pipe the process is reading closes unexpectedly. SIGALRM Sent when an alarm times out. SIGTERM Terminate process normally. SIGCHLD A child process just died or changed status. SIGCONT Continue after stopping with SIGSTOP. SIGSTOP Halt process; cannot be caught. SIGTSTP Halt process; may be caught. SIGTTIN Process stopped due to tty input. SIGTTOU Process stopped due to tty output. SIGCXPU CPU time limit exceeded. SIGXFSZ File size limit exceeded. SIGBABY Passed when a baby is ready to change diapers, hungry, about to climb some- thing dangerous or doing anything else that requires immediate attention from a parent PHP programmer.