MPI_Comm_size(3) MPI MPI_Comm_size(3) NAME MPI_Comm_size - Determines the size of the group associated with a commu- nicator SYNOPSIS int MPI_Comm_size(MPI_Comm comm, int *size) INPUT PARAMETERS comm - communicator (handle) OUTPUT PARAMETERS size - number of processes in the group of comm (integer) NULL HANDLES The MPI 1.1 specification, in the section on opaque objects, explicitly disallows freeing a null communicator. The text from the standard is: A null handle argument is an erroneous IN argument in MPI calls, unless an exception is explicitly stated in the text that defines the function. Such exception is allowed for handles to request objects in Wait and Test calls (sections Communication Completion and Multiple Completions ). Otherwise, a null handle can only be passed to a function that allocates a new object and returns a reference to it in the handle. THREAD AND INTERRUPT SAFETY This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as malloc or other non-MPICH runtime routines that are themselves not interrupt-safe. NOTES FOR FORTRAN All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK ) have an additional argument ierr at the end of the argument list. ierr is an inte- ger and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the call state- ment. All MPI objects (e.g., MPI_Datatype , MPI_Comm ) are of type INTEGER in Fortran. ERRORS All MPI routines (except MPI_Wtime and MPI_Wtick ) return an error value; C routines as the value of the function and Fortran routines in the last ar- gument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error han- dler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible. MPI_SUCCESS - No error; MPI routine completed successfully. MPI_ERR_ARG - Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK ). MPI_ERR_COMM - Invalid communicator. A common error is to use a null communica- tor in a call (not even allowed in MPI_Comm_rank ). MPI_ERR_OTHER - Other error; use MPI_Error_string to get more information about this error code. 4/10/2026 MPI_Comm_size(3)
NAME | SYNOPSIS | INPUT PARAMETERS | OUTPUT PARAMETERS | NULL HANDLES | THREAD AND INTERRUPT SAFETY | NOTES FOR FORTRAN | ERRORS
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