http://www.fastcgi.com/mod_fastcgi/docs/mod_fastcgi.html

Under Unix, expect your FastCGI application to see SIGPIPE, SIGUSR1, and SIGTERM. The latest FastCGI C, C++ and Perl application library installs default handlers if none are installed by the application. If an http client aborts a request before it completes, mod_fastcgi does too - this results in a SIGPIPE to the FastCGI application. At a minimum, SIGPIPE should be ignored (applications spawned by mod_fastcgi have this setup automatically). Ideally, it should result in an early abort of the request handling within your application and a return to the top of the FastCGI accept() loop. Apache uses SIGUSR1 to request a "graceful" process restart/shutdown. It is sent to Apache's process group (which includes applications spawned by mod_fastcgi). Ideally, it should result in a FastCGI application finishing the current request, if any, and then an exit. The mod_fastcgi process manager isn't particularly patient though (there's room for improvement here) and since it has to shutdown too, sends a SIGTERM to all of the FastCGI applications it is responsible for. Apache will restart the process manager and it will restart its managed applications (as if the server was just started). SIGTERM is, well, SIGTERM - your application should exit quickly.