06-30-2005
I would like to add that signals in unix are generally not queued despite some documentation to the contrary. So if 3 children exit while the parent is not running, it is not probable that the parent will receive 3 SIGCHLD signals. So the handler should be willing to loop to reap zero or more children.
I see that Posix seems to be blessing most of the System V SIGCHLD semantics:
Quote:
SA_NOCLDWAIT --If set, and sig equals SIGCHLD, child processes of the calling processes shall not be transformed into zombie processes when they terminate. If the calling process subsequently waits for its children, and the process has no unwaited-for children that were transformed into zombie processes, it shall block until all of its children terminate, and wait(), waitid(), and waitpid() shall fail and set errno to [ECHILD]. Otherwise, terminating child processes shall be transformed into zombie processes, unless SIGCHLD is set to SIG_IGN.
That's from the Posix sigaction man page. So properly ignoring the signal may be an option as well.
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WAIT(2) System Calls Manual WAIT(2)
NAME
wait - wait for process to terminate
SYNOPSIS
wait(status)
int *status;
wait(0)
DESCRIPTION
Wait causes its caller to delay until a signal is received or one of its child processes terminates. If any child has died since the last
wait, return is immediate; if there are no children, return is immediate with the error bit set (resp. with a value of -1 returned). The
normal return yields the process ID of the terminated child. In the case of several children several wait calls are needed to learn of all
the deaths.
If (int)status is nonzero, the high byte of the word pointed to receives the low byte of the argument of exit when the child terminated.
The low byte receives the termination status of the process. See signal(2) for a list of termination statuses (signals); 0 status indi-
cates normal termination. A special status (0177) is returned for a stopped process which has not terminated and can be restarted. See
ptrace(2). If the 0200 bit of the termination status is set, a core image of the process was produced by the system.
If the parent process terminates without waiting on its children, the initialization process (process ID = 1) inherits the children.
SEE ALSO
exit(2), fork(2), signal(2)
DIAGNOSTICS
Returns -1 if there are no children not previously waited for.
ASSEMBLER
(wait = 7.)
sys wait
(process ID in r0)
(status in r1)
The high byte of the status is the low byte of r0 in the child at termination.
WAIT(2)