## 5.5.3. MPI_APPNUM

Up: Other Functionality Next: Releasing Connections Previous: Singleton MPI_INIT

There is a predefined attribute MPI_APPNUM of MPI_COMM_WORLD. In Fortran, the attribute is an integer value. In C, the attribute is a pointer to an integer value. If a process was spawned with MPI_COMM_SPAWN_MULTIPLE, MPI_APPNUM is the command number that generated the current process. Numbering starts from zero. If a process was spawned with MPI_COMM_SPAWN, it will have MPI_APPNUM equal to zero.

Additionally, if the process was not started by a spawn call, but by an implementation-specific startup mechanism that can handle multiple process specifications, MPI_APPNUM should be set to the number of the corresponding process specification. In particular, if it is started with

        mpiexec spec0 [: spec1 : spec2 : ...]

MPI_APPNUM should be set to the number of the corresponding specification.

If an application was not spawned with MPI_COMM_SPAWN or MPI_COMM_SPAWN_MULTIPLE, and MPI_APPNUM doesn't make sense in the context of the implementation-specific startup mechanism, MPI_APPNUM is not set.

MPI implementations may optionally provide a mechanism to override the value of MPI_APPNUM through the info argument. MPI reserves the following key for all SPAWN calls.

- [ appnum] Value contains an integer that overrides the default value for MPI_APPNUM in the child.

Rationale.

When a single application is started, it is able to figure out how many processes there are by looking at the size of MPI_COMM_WORLD. An application consisting of multiple SPMD sub-applications has no way to find out how many sub-applications there are and to which sub-application the process belongs. While there are ways to figure it out in special cases, there is no general mechanism. MPI_APPNUM provides such a general mechanism. ( End of rationale.)

Up: Other Functionality Next: Releasing Connections Previous: Singleton MPI_INIT

Return to MPI-2 Standard Index
Return to MPI 1.1 Standard Index
Return to MPI Forum Home Page

MPI-2.0 of July 18, 1997
HTML Generated on September 10, 2001