# 17.2.68. MPI_Comm_join

MPI_Comm_join — Establishes communication between MPI jobs

## 17.2.68.1. SYNTAX

### 17.2.68.1.1. C Syntax

    #include <mpi.h>

    int MPI_Comm_join(int fd, MPI_Comm *intercomm)

### 17.2.68.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_COMM_JOIN(FD, INTERCOMM, IERROR)
         INTEGER FD, INTERCOMM, IERROR

### 17.2.68.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Comm_join(fd, intercomm, ierror)
         INTEGER, INTENT(IN) :: fd
         TYPE(MPI_Comm), INTENT(OUT) :: intercomm
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 17.2.68.2. INPUT PARAMETER

- `fd`: socket file descriptor (socket).

## 17.2.68.3. OUTPUT PARAMETERS

- `intercomm`: Intercommunicator between processes (handle).

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

## 17.2.68.4. DESCRIPTION

MPI_Comm_join creates an intercommunicator from the union of two MPI processes that are connected by a socket. fd is a file descriptor representing a socket of type SOCK_STREAM (a two-way reliable byte-stream connection). Nonblocking I/O and asynchronous notification via SIGIO must not be enabled for the socket. The socket must be in a connected state, and must be quiescent when MPI_Comm_join is called.

MPI_Comm_join must be called by the process at each end of the socket. It does not return until both processes have called MPI_Comm_join.

## 17.2.68.5. NOTES

There are no MPI library calls for opening and manipulating a socket. The socket fd can be opened using standard socket API calls. MPI uses the socket to bootstrap creation of the intercommunicator, and for nothing else. Upon return, the file descriptor will be open and quiescent.

In a multithreaded process, the application must ensure that other threads do not access the socket while one is in the midst of calling MPI_Comm_join.

The returned communicator will contain the two processes connected by the socket, and may be used to establish MPI communication with additional processes, through the usual MPI communicator-creation mechanisms.

## 17.2.68.6. 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

- socket(3SOCKET)

- MPI_Comm_create

- MPI_Comm_group