Message 103243 - Python tracker (original) (raw)

• There seems to be no good reason to special case SIGCHLD in signal_handler. The comment about infinite recursion has no obvious interpretation to me. Fortunately, this is irrelevant on platforms with sigaction, because the handler remains installed anyway!

I think that it's possible, on certain systems, that setting a signal handler for SIGCHLD while there is un unwaited child (zombie) process triggers a new sending of SIGCHLD. See http://standards.ieee.org/reading/ieee/interp/1003-1-90_int/pasc-1003.1-132.html: "The SIGCHLD or SIGCLD were used for implementing job control. Since I had implemented job control for both BSD and System V, I pointed out to the standards group that except for SIG_DFL, these signals had different semantics.

If a signal handler was set for SIGCLD, then a signal would be generated if there were any unreaped child processes. When the signal handler was caught in System V, it was reset by default to SIG_DFL. However, if a process did not reap a child and instead reestablished the signal handler, it would go into an infinite loop since the signal would be generated again. The SIGCLD SIG_IGN behavior was that the system reaped the child when it completed so that the application didn't have to deal with it. However, I believe that a process blocked in wait() would be awakened, but I am not certain of this.

The SIGCHLD signal on the other hand was generated when a child completed if a signal handler was set at that time. No signal would be generated if a signal handler was established while there was waiting children. The SIGCHLD signal was also generated when a child process stopped. I believe that BSD treated SIGCHLD SIG_IGN the same way that it treated SIGCHLD SIG_DFL."

So, there might exist out there a couple systems that merrily mix SIGCLD and SIGCHLD, and re-raise a SIGCHLD when setting a new handler for it when unwaited children exist (which is the case in signal_handle, since child processes can't be waited for before the main thread gets to execute the handler it set up).