MPI_COMM_JOIN(3)				     Open MPI					  MPI_COMM_JOIN(3)

MPI_Comm_join — Establishes communication between MPI jobs

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Comm_join(int fd, MPI_Comm *intercomm)

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

   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

INPUT PARAMETER
       • fd: socket file descriptor (socket).

OUTPUT PARAMETERS
       • intercomm: Intercommunicator between processes (handle).

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

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.

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.

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>/MPI_Init_thread  <#mpi-init-thread>,
       after  MPI_Finalize  <#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	<#
       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>/MPI_Comm_spawn_multiple	<#
       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 <#mpi-abort> was called on that communicator. If called
	 on a window or file, acts as if MPI_Abort <#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	 <#mpi-comm-create-errhandler>	   then	    MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	<#mpi-file-create-errhandler>	  then	   MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

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

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

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

       See also:

	  • socket(3SOCKET)

	  • MPI_Comm_create <#mpi-comm-create>

	  • MPI_Comm_group <#mpi-comm-group>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					  MPI_COMM_JOIN(3)
