[Beowulf] Stupid MPI programming question

Vincent Diepeveen diep at xs4all.nl
Thu Sep 28 09:54:08 EDT 2006


----- Original Message ----- 
From: "Joe Landman" <landman at scalableinformatics.com>
To: "Clements, Brent M (SAIC)" <brent.clements at bp.com>
Cc: <beowulf at beowulf.org>
Sent: Thursday, September 28, 2006 2:34 AM
Subject: Re: [Beowulf] Stupid MPI programming question


> Hi Brent
>
> Clements, Brent M (SAIC) wrote:
>> Hey Guys,
>>
>> I've been sitting here working for the past 48 hours and I'm fighting
>> a stupid bug in some mpi code I'm working on
>>
>> How do I broadcast a char string to my slave mpi processes? And how
>> do I receive that char string and print it out on my slave mpi
>> process.
>>
>>
>> This is what I have in my code(some things have been removed)
>>
>> #define MASTER_RANK 0
>>
>> char* mystring;
>
> Hmmm....  This makes *mystring a string (ok, a character array), and
> mystring a pointer to a character array.
>
>>
>> mystring = "some test";
>
> ok ... I haven't played with "strings" recently in C, though I seem to
> remember needing a "\0" at the end.  RGB is more likely a C language
> guru/lawyer than I am, I don't remember off the top of my head if this
> is strictly necessary.

If we move to C++ then you have more ways to mess up,
but also methods similar to the above.

Actually in C it would be

#include <string.h>
..

char mystring[1024];
strcpy(mystring,"some test");



>>
>> MPI_Bcast(&mystring, sizeof(basedir), MPI_CHAR, MASTER_RANK,
>> MPI_COMM_WORLD);
>
> Lets put on our parser hat.  The first argument is
>
> pointer to (pointer to mystring)
>
> mystring is a pointer, *mystring is the (technically) first character in
> the array, and &mystring is a pointer to mystring, which is a pointer to
> a pointer to the first character in the array.
>
> Ok, if you want to see more of this sort of detail, have a look at
> http://course.scalableinformatics.com/lectures/lecture-5/lecture-5.pdf
> around page 62 for an example with BCast and Send/Recv pairs.  The files
> for this lecture are in
> http://course.scalableinformatics.com/lectures/lecture-5/source/
>
>>
>> in my slave process code I have this
>>
>> printf("my string I received: %s", mystring);
>>
>>
>> Am I doing something wrong? Or if someone has some sample code they
>> can point me to on how to send a string to a slave mpi process and
>> have that slave mpi process print out the string..that would be even
>> better. That way..I relearn MPI basics 101
>
> Lots of tutorials on the net, my course and material are up on the net
> above.
>
> Joe
>
> -- 
> Joseph Landman, Ph.D
> Founder and CEO
> Scalable Informatics LLC,
> email: landman at scalableinformatics.com
> web  : http://www.scalableinformatics.com
> phone: +1 734 786 8423
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