fork and pipe problem

[Duncan Roe]

On Sat, Feb 22, 2003 at 06:22:44PM +1100, Alex Pollard wrote:
> Hi,
> 
> I'm debugging some software on two machines. One is an athlon 686 RedHat 8.0
>  with kernel 2.4.18-14, the other a pentium with RedHat 7.1 and kernel
>  2.4.9-31
> 
> The code works fine on the athlon without the fcntl(), but it is required for 
> the pentium.
> 
>   if (!(childPid = fork())) {
>     // close stdout and dup the writing end of the pipe
*** So *close* STDOUT_FILENO before the dup2()
>     if (dup2(childToParentPipe[1],STDOUT_FILENO)==-1) {
>       fprintf(stderr, "Failed to dup2 in proc = %d\n",(int)getpid());
>     }
>     //fcntl(STDOUT_FILENO, F_SETFD,FD_CLOEXEC);
*** You *never* want to to that. It will close the pipe, if the dup2 call worked
> 
>    // close unused end of pipe
> 
>     ::_close(childToParentPipe[0]);
> 
>     fprintf(stderr,"execing on ");
>     fprintf(stderr,"%s -p %s -r %s \n",command,portString,theDirector.url()
>  );
>     if (execl(command,command,"-p",portString,"-r",theDirector.url(),NULL)<0)
>  { fprintf(stderr,"exec on %s -p %s -r %s failed with error
>  %d\n",command,portString,theDirector.url(),errno ); delete this;
> 	exit(-4);
>     }
> 
>   }
> 
> Without the fcntl() on the athlon the child process's output to STDOUT is 
> received at the parent's end of the pipe. Without the fcntl() on the pentium, 
> the child process's output appears in the terminal, but there is no error 
> reported for the dup2() or anything else.
> 
> But the problem with including fcntl() in either version is that the parent 
> never receives notification of data put into the pipe via a select call.
> 
> I've found that 
> 
> fcntl(STDOUT_FILENO, F_SETFD,0);
> 
> is equivalent to not using fcntl() at all.
> 
> 
> Is there some difference between how these kernels handle fork() and execl()?
> 
> Also, is it OK for multiple child processes to use the same pipe to send data
>  to their common parent? Or should there be a new pipe for each child? I'm 
> ignoring the problem of zombie children for the moment.
> 
> Alex
> 
>