Extracted from Unix Network Programming Vol1 Third Edition Section 5.10 wait and waitpid functions
#include "unp.h"
void
sig_chld(int signo)
{
pid_t pid;
int stat;
while ( (pid = waitpid(-1, &stat, WNOHANG)) > 0) {
printf("child %d terminated\n", pid);
}
return;
}
...
// in server code
Signal(SIGCHLD, sig_chld); // used to prevent any zombies from being left around
...
..
// in client code
The client establishes five connection with the server and then immediately exit
...
Reference waitpid:
Return Value
waitpid(): on success, returns the process ID of the child whose state has changed; if WNOHANG was specified and one or more child(ren) specified by pid exist, but have not yet changed state, then 0 is returned. On error, -1 is returned.
Based on the above document, waitpid
will return 0
if at the moment no child process has terminated. If I understood correctly, this will cause the function sig_chld
break from the while
statement.
Question> Thus how can we guarantee that this signal handler can make sure all terminated children processes are collected?