.TH MPI_Comm_join 3 "6/24/2006" "LAM/MPI 7.1.4" "LAM/MPI"
.SH NAME
MPI_Comm_join \-  Connect two MPI processed joined by a socket 
.SH SYNOPSIS
.nf
#include <mpi.h>
int
MPI_Comm_join(int fd, MPI_Comm *newcomm)
.fi
.SH INPUT PARAMETER
.PD 0
.TP
.B fd 
- socket file descriptor
.PD 1

.SH OUTPUT PARAMETER
.PD 0
.TP
.B newcomm 
- intercommunicator with client as remote group
.PD 1

.SH DESCRIPTION

This function only works between two LAM/MPI processes that are
connected by a socket.  They either must have the same endian
orientation, or not have used the homogeneous flag to 
.I mpirun
(1)
(-O).  Both processes must be in a single LAM universe -- they must
share LAM daemons that are already connected to each other.  That is,
they were either initially lamboot(1)ed together, or a lamgrow(1)
command was given to grow an initial LAM universe such that the
resulting set includes the two hosts in question.

The socket may not be used by the calling application until
.I MPI_Comm_join
has completed.  When 
.I MPI_Comm_join
completes, 
.I fd
may be used by the calling application.  Per MPI-2, the socket is only
used for initial handshaking -- it will not be used as the
communications channel between the newly connected processes.

.SH SAMPLE

The following code shows an abbreviated example using 
.I MPI_Comm_join
since no example is provided in the MPI-2 standard.  Note that only
one rank in each MPI application should call this function since each
socket can only have two endpoints (i.e., an endpoint in each
process).  Upon successful completion, the intercommunicator that is
returned will have two members -- the local process and the remote
process.

.nf
MPI_Comm
do_join(char* server_name, int port) {
int sfd, fd = -1;
unsigned char addr[4];
MPI_Comm intercomm;
if (server_name != NULL) {
if (getinetaddr(server_name, &addr) == 0)
fd = open_socket_to_server(addr, port);
} else {
if ((sfd = open_listening_socket(&port)) >= 0) {
fd = accept_client_socket(sfd, -1);
close(sfd);
}
}
if (fd < 0)
MPI_Abort(MPI_COMM_WORLD, 0);
MPI_Comm_join(fd, &intercomm);
return intercomm;
}
.fi


.SH NOTES FOR FORTRAN

All MPI routines in Fortran (except for 
.I MPI_WTIME
and 
.I MPI_WTICK
)
have an additional argument 
.I ierr
at the end of the argument list.
.I ierr
is an integer and has the same meaning as the return value of
the routine in C.  In Fortran, MPI routines are subroutines, and are
invoked with the 
.I call
statement.

All MPI objects (e.g., 
.I MPI_Datatype
, 
.I MPI_Comm
) are of type
.I INTEGER
in Fortran.

.SH USAGE WITH IMPI EXTENSIONS

The IMPI standard only supports MPI-1 functions.  Hence, this function
is currently not designed to operate within an IMPI job.

.SH ERRORS

If an error occurs in an MPI function, the current MPI error handler
is called to handle it.  By default, this error handler aborts the
MPI job.  The error handler may be changed with 
.I MPI_Errhandler_set
;
the predefined error handler 
.I MPI_ERRORS_RETURN
may be used to cause
error values to be returned (in C and Fortran; this error handler is
less useful in with the C++ MPI bindings.  The predefined error
handler 
.I MPI::ERRORS_THROW_EXCEPTIONS
should be used in C++ if the
error value needs to be recovered).  Note that MPI does 
.I not
guarantee that an MPI program can continue past an error.

All MPI routines (except 
.I MPI_Wtime
and 
.I MPI_Wtick
) return an error
value; C routines as the value of the function and Fortran routines
in the last argument.  The C++ bindings for MPI do not return error
values; instead, error values are communicated by throwing exceptions
of type 
.I MPI::Exception
(but not by default).  Exceptions are only
thrown if the error value is not 
.I MPI::SUCCESS
\&.


Note that if the 
.I MPI::ERRORS_RETURN
handler is set in C++, while
MPI functions will return upon an error, there will be no way to
recover what the actual error value was.
.PD 0
.TP
.B MPI_SUCCESS 
- No error; MPI routine completed successfully.
.PD 1
.PD 0
.TP
.B MPI_ERR_ARG 
- Invalid argument.  Some argument is invalid and is not
identified by a specific error class.  This is typically a NULL
pointer or other such error.
.PD 1
.PD 0
.TP
.B MPI_ERR_INTERN 
- An internal error has been detected.  This is
fatal.  Please send a bug report to the LAM mailing list (see
.I http://www.lam-mpi.org/contact.php
). 
.PD 1
.PD 0
.TP
.B MPI_ERR_OTHER 
- Other error; use 
.I MPI_Error_string
to get more
information about this error code.
.PD 1

.SH SEE ALSO
lamboot(1), lamgrow(1), mpirun(1), MPI_Finalize(3)
.br

.SH MORE INFORMATION

For more information, please see the official MPI Forum web site,
which contains the text of both the MPI-1 and MPI-2 standards.  These
documents contain detailed information about each MPI function (most
of which is not duplicated in these man pages).

.I http://www.mpi-forum.org/
.SH LOCATION
join.c