---
name: "MPI_Comm_rank"
section: "3"
platform: "linux"
package: "openmpi"
version: "5.0.11-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/MPI_Comm_rank"
---

# MPI\_Comm\_rank(3)

*MPI\_Comm\_rank* — Determines the rank of the calling process in the communicator.

## SYNTAX

### C Syntax

```
#include <mpi.h>
int MPI_Comm_rank(MPI_Comm comm, int *rank)
```

### Fortran Syntax

```
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_COMM_RANK(COMM, RANK, IERROR)
     INTEGER COMM, RANK, IERROR
```

### Fortran 2008 Syntax

```
USE mpi_f08
MPI_Comm_rank(comm, rank, ierror)
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, INTENT(OUT) :: rank
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror
```

## INPUT PARAMETERS

- •

  **comm**: Communicator (handle).

## OUTPUT PARAMETERS

- •

  **rank**: Rank of the calling process in group of comm (integer).

- •

  **ierror**: Fortran only: Error status (integer).

## DESCRIPTION

This function gives the rank of the process in the particular communicator’s group. It is equivalent to accessing the communicator’s group with *MPI\_Comm\_group*, computing the rank using *MPI\_Group\_rank*, and then freeing the temporary group via *MPI\_Group\_free*.

Many programs will be written with the manager-worker model, where one process (such as the rank-zero process) will play a supervisory role, and the other processes will serve as compute nodes. In this framework, *MPI\_Comm\_size* and *MPI\_Comm\_rank* are useful for determining the roles of the various processes of a communicator.

## 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\_size*

- •

  *MPI\_Comm\_compare*

## COPYRIGHT

2003-2026, The Open MPI Community