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Message-ID: <20090803152146.GJ29252@hawkmoon.kerlabs.com>
Date: Mon, 3 Aug 2009 17:21:46 +0200
From: Louis Rilling <Louis.Rilling@...labs.com>
To: Nicholas Vinen <hb@...6.org>
Cc: linux-kernel@...r.kernel.org
Subject: Re: Possible kernel bug with SYS_clone / CLONE_PARENT
On 03/08/09 14:32 +1000, Nicholas Vinen wrote:
> Hello,
>
> I have a program where process A forks process B and process B forks
> process C. I want process A to be notified if/when process C terminates.
> (Process B terminates almost immediately after forking process C and a
> number of other siblings.)
>
> According to the manual page for clone() I should be able to do this
> with CLONE_PARENT:
>
> -----------------
> CLONE_PARENT (since Linux 2.3.12)
> If CLONE_PARENT is set, then the parent of the new child (as
> returned by getppid(2)) will be the same as that of the calling process.
> If CLONE_PARENT is not set, then (as with fork(2)) the child's
> parent is the calling process.
> Note that it is the parent process, as returned by getppid(2), which
> is signaled when the child terminates, so that if CLONE_PARENT is set,
> then the parent of the calling process, rather than the calling process
> itself, will be signaled.
> -----------------
>
>
> I am using kernel 2.6.29-gentoo-r5 (not the most recent, I know, but
> relatively new), glibc 2.9_p20081201-r2 and gcc 4.3.2-r3. Here is my
> test program:
>
> -----------------
> #include <asm/unistd.h>
> #include <sys/syscall.h>
> #include <sched.h>
> #include <signal.h>
> #include <stdio.h>
> #include <string.h>
> #include <stdlib.h>
>
> int fork_but_keep_ppid() {
> return syscall(SYS_clone, CLONE_PARENT, (void*)0);
> }
You should use CLONE_PARENT|SIGCHLD. CLONE_PARENT alone deactivates
signal notification to original parent.
Louis
>
> void sigchld_handler(int signum, siginfo_t* info, void* ucontext) {
> fprintf(stderr, "SIGCHLD (PID=%d)\n", info->si_pid);
> }
>
> int main(void) {
> struct sigaction act;
> memset(&act, 0, sizeof(act));
> act.sa_sigaction = sigchld_handler;
> act.sa_flags = SA_NOCLDSTOP|SA_NOCLDWAIT|SA_SIGINFO;
> sigaction(SIGCHLD, &act, 0);
>
> fprintf(stderr, "Main PID = %d\n", syscall(SYS_getpid));
>
> pid_t a = fork();
> if( a == 0 ) {
> fprintf(stderr, "first fork PID = %d, PPID = %d\n",
> syscall(SYS_getpid), syscall(SYS_getppid));
> pid_t b = fork_but_keep_ppid();
> if( b == 0 ) {
> fprintf(stderr, "second fork PID = %d, PPID = %d\n",
> syscall(SYS_getpid), syscall(SYS_getppid));
> exit(-2);
> } else {
> fprintf(stderr, "second fork returned %d\n", b);
> }
> exit(-1);
> } else {
> fprintf(stderr, "first fork returned %d\n", a);
> }
> sleep(1);
> fprintf(stderr, "Creating & terminating another child to check
> SIGCHILD still works...\n");
> pid_t c = fork();
> if( c == 0 ) {
> fprintf(stderr, "Child's PID is %d\n", syscall(SYS_getpid));
> exit(-3);
> }
> sleep(1);
>
> return 0;
> }
> -----------------
>
> Note that I am making the syscall directly because I want fork-like
> semantics and these are not provided by the clone() call. I also don't
> want to have to provide a separate stack for the child and it seems that
> (at least according to the man page) for clone() you have to, even if
> you don't use CLONE_VM.
>
> The output from this program is:
>
> -----------------
> Main PID = 13125
> first fork returned 13126
> first fork PID = 13126, PPID = 13125
> second fork PID = 13127, PPID = 13125
> second fork returned 13127
> SIGCHLD (PID=13126)
> Creating & terminating another child to check SIGCHILD still works...
> Child's PID is 13128
> SIGCHLD (PID=13128)
> -----------------
>
>
> There should be a SIGCHLD for the second fork's child PID but there
> isn't. ps shows the process is a zombie with PPID=13125 (in this case)
> while the test program is still running, yet the "parent" does not seem
> to receive the SIGCHLD.
>
> I could be doing something wrong but I can't see what it might be. Any
> suggestions? Please CC me on any reply as I am not currently subscribed
> to LKML.
>
>
>
> Thanks,
>
> Nicholas.
>
>
> --
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--
Dr Louis Rilling Kerlabs
Skype: louis.rilling Batiment Germanium
Phone: (+33|0) 6 80 89 08 23 80 avenue des Buttes de Coesmes
http://www.kerlabs.com/ 35700 Rennes
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