linux-kernel - Re: parent process behaviour to signal after vfork()

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Date:	Wed, 29 Oct 2008 08:17:36 -0500
From:	"Michael Kerrisk" <mtk.manpages@...il.com>
To:	"Halesh S" <halesh.s@...ia.com>
Cc:	linux-kernel@...r.kernel.org,
	"Michael Kerrisk" <mtk.manpages@...il.com>
Subject: Re: parent process behaviour to signal after vfork()

On Tue, Oct 28, 2008 at 1:43 AM, Halesh S <halesh.s@...ia.com> wrote:
> Hi,
>
> Please find the below test code...
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <signal.h>
> #include <unistd.h>
> int x = 0;
> typedef void (*sighandler_t)(int);
> void fun()
> {
>  printf("SIGNAL CAUGHT\n");
> }
>
> int main()
> {
>  int pid;
>
>  signal(SIGINT, (sighandler_t)fun);
>  signal(SIGSEGV, (sighandler_t)fun);
>  pid = vfork();
>
>  if ( pid == 0 ) {
>    printf(" I am child - %d \n", pid);
>    x++;
>    raise(SIGINT);
>    printf("x = %d\n", x);
>    exit(0);
>  }
>  else {
>    printf(" I am parent - %d\n", pid);
>    raise(SIGSEGV);
>    printf("x = %d\n", x);
>  }
> }
>
> Why the parent process is not able to handle/respond the signals, when a child
> once handles the signal, child is created using vfork().
>
> In vfork() man page -
>       "Signals to the parent arrive after the child releases the parent's
>       memory."
>
> How to make sure that child has released the parent memory..??

That happens when the child does execve(2) or _exit(2).  The man-page could be a little clearer on that point (it implies it, but not very clearly).  For man-pages-3.12, I've made the change below.

Cheers,

Michael

diff --git a/man2/vfork.2 b/man2/vfork.2
index 55044ad..8a7ed50 100644
--- a/man2/vfork.2
+++ b/man2/vfork.2
@@ -94,7 +94,10 @@ but may call
 .PP
 Signal handlers are inherited, but not shared.
 Signals to the parent
-arrive after the child releases the parent's memory.
+arrive after the child releases the parent's memory (i.e., after the child calls
+.BR _exit (2)
+or
+.BR execve (2)).
 .SS "Historic Description"
 Under Linux,
 .BR fork (2)

-- 
Michael Kerrisk Linux man-pages maintainer;
http://www.kernel.org/doc/man-pages/ Found a documentation bug?
http://www.kernel.org/doc/man-pages/reporting_bugs.html

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