I am writing a parallel VTK file (pvti) from my fortran CFD solver. The file is really just a list of all the individual files for each piece of the data. Running MPI, if I have each process write the name of its individual file to standard output
print *, name
then I get a nice list of each file, ie
block0.vti
block1.vti
block2.vti
This is exactly the sort of list I want. But if I write to a file
write(9,*) name
then I only get one output in the file. Is there a simple way to replicate the standard output version of this without transferring data?
Apart from having the writes from different ranks well mixed, your problem is that the Fortran OPEN statement probably truncates the file to zero length, thus obliterating the previous content instead of appending to it. I'm with Vladimir F on this and would write this file only in rank 0. There are several possible cases, some of which are listed here:
each rank writes a separate VTK file and the order follows the ranks or the actual order is not significant. In that case you could simply use a DO loop in rank 0 from 0 to #ranks-1 to generate the whole list.
each rank writes a separate VTK file, but the order does not follow the ranks, e.g. rank 0 writes block3.vti, rank 1 writes block12.vti, etc. In that case you can use MPI_GATHER to collect the block number from each process into an array at rank 0 and then loop over the elements of the array.
some ranks write a VTK file, some don't, and the block order does not follow the ranks. It's similar to the previous case - just have the ranks that do not write a block send a negative block number and then rank 0 would skip the negative array elements.
block numbering follows ranks order but not all ranks write a block. In that case you can use MPI_GATHER to collect one LOGICAL value from each rank that indicates if it has written a block or not.