[mpich-discuss] Redirection and Pipes

[Diego Fernández Slezak]

Thanks Dave.
I don't know what's happening, but I solved it editing the code, and
avoiding all the unnecessary output.
Now it works perfectly ;)

On Fri, Aug 20, 2010 at 1:30 PM, Dave Goodell <goodell at mcs.anl.gov> wrote:

> Is your read done repeatedly, or just once?  Your code won't be portable in
> general if it assumes that you'll always get MAX_LEN out of the read() call.
>  Do you flush your stdout explicitly at any point?
>
> I'm not sure why the buffering behavior would change when run under MPI or
> not.  If you are on Linux you can try to use strace to watch for relevant
> calls (ioctl, fcntl, etc). I think you can also use ltrace or a debugger to
> watch for calls to fflush, setvbuf, and similar, which will affect the
> buffering behavior of the standard library.
>
> You might also try to isolate whether it is being caused by using the MPI
> library (i.e. calling MPI_Init/MPI_Finalize/etc), or by being executed via
> mpiexec, or both.
>
> It looks like we call "setbuf(stdout,NULL)" at MPI_Init time, so that might
> be relevant.  Do you perform this redirection before or after MPI_Init time?
>
> -Dave
>
> On Aug 20, 2010, at 9:30 AM CDT, Diego Fernández Slezak wrote:
>
> > Hello,
> >
> > I have a working code that redirects stdout to a pipe, runs some code and
> then process the stdout (through the pipe):
> >
> >
> >               /// Connect STDOUT to pipe
> >               // Save old stdout.
> >               saved_stdout = dup(STDOUT_FILENO);  // save stdout for
> display later
> >               // create pipe
> >               if( pipe(out_pipe) != 0 ) {          // make a pipe
> >                       exit(1);
> >               }
> >               // connect stdout to pipe.
> >               dup2(out_pipe[1], STDOUT_FILENO);   // redirect stdout to
> the pipe
> >
> > ...
> > ...
> > //at some point:
> >               int read_bytes = read(out_pipe[0], buffer, MAX_LEN); //
> read from pipe into buffer
> >
> > ...
> > ...
> >
> >               dup2(saved_stdout, STDOUT_FILENO);  // reconnect stdout for
> testing
> >
> >
> > Now, if I try to run exactly the same code in MPI environment, the "read"
> function only retrieves part of the message.
> > I think this could be because of some MPI redefinition of pipe buffer
> sizes or similar.
> >
> > Any ideas?
>
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