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Asked: 2026-05-30T01:54:00+00:00

I am using MPI2.2 standard to write parallel program in C. I have 64

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I am using MPI2.2 standard to write parallel program in C. I have 64 bit machine.

/* MPI offset is long long*/

MPI_Offset my_offset; printf ("%3d: my offset = %lld\n", my_rank, my_offset);

int count; 
MPI_Get_count(&status, MPI_BYTE, &count);

printf ("%3d: read =%d\n", my_rank, count);

I am reading a file of very large size byte by byte. To read the file parallely i am setting the offset for each process using offset variable. I am having confusion for the data-type of MPI_offset type, that "whither it is signed or unsigned" long ?

My second question is about limitation of the “range of count variable” which is used in MPI_Get_count() function. since this function is used here to read all the elements from each process’s buffer so i think it should also be of the long long type to read such a very large file.

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1 Answer

  1. Editorial Team

    MPI_Offset‘s size isn’t defined by the standard – it is, roughly, as large as possible. ROMIO, a widely-used underlying implemetation of MPI-IO, uses 8-byte integers on systems which support them. You can probably find out for sure by looking in your system’s mpi.h.

    MPI_Offset is very definitely signed; there are functions like MPI_File_seek where it is perfectly reasonable to have values of type MPI_Offset take negative values.

    MPI_Get_count returns an integer, of normal integer size, and this can certainly cause problems for some large file IO strategies.

    Generally, it’s better for a number of reasons not to use small low-level units of IO like bytes when doing MPI-IO; it’s better in terms of performance and code readability to express the IO in units of your underlying data types. In doing so, these size limitations become less of an issue. If your underlying data type really is bytes, though, thre aren’t many options.

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