Understanding the process numbers returned by BLACS_PNUM (ScaLAPACK)

[OP1]

The following code (built with ifx 2025.2.1 on Windows and run with 6 processes) produce somewhat puzzling information, and we would like to get some clarity on the behavior we observe.

The code uses BLACS_GRIDMAP to create a custom 1x4 grid from processes 2, 3, 4 and 5 (thus, process 0 and 1 are not part of that grid). Then a process that is in that grid (process 2) calls BLACS_PNUM to retrieve the numbers of all the processes that are in that grid.

Instead of the expected output of -1, -1, 2, 3, 4, 5, we get 0, 1, 2, 3, -1, -1. 

Can somebody explain this? Are process numbers defined within the context of a particular grid, or are they global?

PROGRAM P
IMPLICIT NONE (TYPE, EXTERNAL)
EXTERNAL :: BLACS_GET,  BLACS_PINFO, BLACS_PNUM, BLACS_GRIDMAP, BLACS_GRIDINFO
INTEGER(KIND = 8) :: BLACS_PNUM, CTXT_SYS, C_TXT, MY_PROC, N_PROCS, UMAP(1, 4), I, NPROW, NPCOL, MYROW, MYCOL
CALL BLACS_PINFO(MY_PROC, N_PROCS)
IF (N_PROCS /= 6) THEN
    WRITE(*, '(A)') 'Run this program with 6 processes!'
    STOP
END IF
CALL BLACS_GET(0, 0, CTXT_SYS)
C_TXT = CTXT_SYS
UMAP(1, :) = [2, 3, 4, 5]
CALL BLACS_GRIDMAP(C_TXT, UMAP, 1, 1, 4)
IF (MY_PROC == 2) THEN
    CALL BLACS_GRIDINFO(C_TXT, NPROW, NPCOL, MYROW, MYCOL)
    WRITE(*, *) 'Process number : ', MY_PROC, 'Process row: ', MYROW, 'Process column: ', MYCOL
    WRITE(*, *) 'Other processes: ', (BLACS_PNUM(C_TXT, 0, I), I = 0, N_PROCS - 1)
END IF
END PROGRAM P

[sc_Intel]

The output 0, 1, 2, 3, -1, -1 is correct. According to the oneMKL BLACS documentation (https://www.intel.com/content/www/us/en/docs/onemkl/developer-reference-fortran/2025-2/blacs-pnum.html), BLACS_PNUM returns "the system process number of the process at {PROW, PCOL} in the process grid" - meaning the numbering is grid-relative, not global ones.

When you call BLACS_GRIDMAP with processes [2, 3, 4, 5], it creates a new context and remaps them: global process 2 → grid process 0, 3 → 1, and so on. The -1 values indicate invalid positions (your grid is 1×4, so columns 4–5 don’t exist).

This abstraction ensures BLACS operations use clean 0-based grid numbering regardless of the underlying layout.

One correction to the minor issue in the code: INTEGER, EXTERNAL :: BLACS_PNUM instead of INTEGER(KIND=8) :: BLACS_PNUM.

If you need to map between grid and global process numbers, you’ll need to maintain your own mapping array.