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MPI_Comm_join(3) man page (version 1.4.5)

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MPI_Comm_join - Establishes communication between MPI jobs


C Syntax

#include <mpi.h>
int MPI_Comm_join(int fd, MPI_Comm *intercomm)

Fortran Syntax

INCLUDE ’mpif.h’

C++ Syntax

#include <mpi.h>
MPI::Intercomm MPI::Comm::Join(const int fd)

Input Parameter

socket file descriptor (socket).

Output Parameters

Intercommunicator between processes (handle).
Fortran only: Error status (integer).


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.


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.


Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ functions do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to throw an MPI:Exception object.

Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; 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.

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

See Also


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