18.2.76. MPI_Comm_size — Open MPI 5.0.2 documentation

# 18.2.76. MPI_Comm_size

MPI_Comm_size — Returns the size of the group associated with a communicator.

## 18.2.76.1. SYNTAX

### 18.2.76.1.1. C Syntax

    #include <mpi.h>

    int MPI_Comm_size(MPI_Comm comm, int *size)

### 18.2.76.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_COMM_SIZE(COMM, SIZE, IERROR)
         INTEGER COMM, SIZE, IERROR

### 18.2.76.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Comm_size(comm, size, ierror)
         TYPE(MPI_Comm), INTENT(IN) :: comm
         INTEGER, INTENT(OUT) :: size
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 18.2.76.2. INPUT PARAMETER

- `comm`: Communicator (handle).

## 18.2.76.3. OUTPUT PARAMETERS

- `size`: Number of processes in the group of comm (integer).

- `ierror`: Fortran only: Error status (integer).

## 18.2.76.4. DESCRIPTION

This function indicates the number of processes involved in a communicator. For MPI_COMM_WORLD, it indicates the total number of processes available. This function is equivalent to accessing the communicator’s group with MPI_Comm_group, computing the size using MPI_Group_size, and then freeing the temporary group via MPI_Group_free. If the communicator is an inter-communicator (enables communication between two groups), this function returns the size of the local group. To return the size of the remote group, use the MPI_Comm_remote_size function.

This call is often used with MPI_Comm_rank to determine the amount of concurrency available for a specific library or program. MPI_Comm_rank indicates the rank of the process that calls it in the range from 0 . . . size-1, where size is the return value of MPI_Comm_size.

## 18.2.76.5. NOTE

MPI_COMM_NULL is not considered a valid argument to this function.

## 18.2.76.6. ERRORS

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

- `MPI_ERRORS_ARE_FATAL` Causes the program to abort all connected MPI processes.

- `MPI_ERRORS_ABORT` An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.

- `MPI_ERRORS_RETURN` Returns an error code to the application.

MPI applications can also implement their own error handlers by calling:

- MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

- MPI_File_create_errhandler then MPI_File_set_errhandler

- MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

- MPI_Win_create_errhandler then MPI_Win_set_errhandler

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

See also

- MPI_Comm_group

- MPI_Comm_rank

- MPI_Comm_compare