Re: 2.6.14-rc1 wait()/SIG_CHILD bevahiour

[Kyle Moffett]

On Sep 19, 2005, at 13:39:33, Badari Pulavarty wrote:
> Hi,
>
> I am looking at a problem where the parent process doesn't seem to  
> cleanup the exited children (with a webserver). We narrowed it down  
> to a simple testcase. Seems more like a lost SIG_CHILD.

You don't get one SIG_CHLD per child that quits.  The kernel may and  
probably will merge SIG_CHLD signals together if it has several  
queued before it gets a chance to deliver them to your process.  This  
is true of _all_ signals.  If you "kill -STOP 1234", then "kill -QUIT  
1234", "kill -QUIT 1234", "kill -QUIT 1234", "kill -CONT 1234", the  
PID 1234 will have 3 signals delivered:  The original untrappable  
SIGSTOP, the SIGCONT that causes it to resume, and a single SIGQUIT  
immediately following it.  The correct and portable way to handle  
this is to put a loop in your SIGCHLD signal handler:

#include <sys/types.h>
#include <sys/wait.h>

void sigchld_handler(int signal) {
    pid_t pid;
    int status;

    while( -1 != (child = waitpid(-1, &status, WNOHANG)) ) {
        /*
         * Now "status" is the exit status of the child and
         * "pid" is its pid.  See the waitpid() manpage for
         * macros you can use to get information from the
         * status variable.
         */
        do_some_processing_of_exited_child(pid,status);
    }
}

Cheers,
Kyle Moffett

--
There is no way to make Linux robust with unreliable memory  
subsystems, sorry.  It would be like trying to make a human more  
robust with an unreliable O2 supply. Memory just has to work.
  -- Andi Kleen


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