[Beowulf] Stupid MPI programming question

[Joe Landman]

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.

> 
> 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

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Joseph Landman, Ph.D
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