MPIL_Comm_id - LAM/MPI-specific function to return the
context ID of the communicator
SYNOPSIS
#include <mpi.h>
int
MPIL_Comm_id(MPI_Comm comm, int *pcid)
INPUT PARAMETER
comm - communicator (handle)
OUTPUT PARAMETER
pcid - communicator context id (integer)
NOTES
In the LAM implementation of MPI, each communicator has an
integer context ID associated with it for synchronizing on
different contexts. This ID is global to all the pro
cesses in the communicator's group, and uniquely identi
fies that communicator for each process. These properties
allow the processes to safely exchange messages without
interference from operations on other communicators.
The MPI standard does not provide a way to access/view
this implementation-dependent synchronization since commu
nicators are opaque objects. Users do not need such
access for normal MPI operations. On the other hand, when
debugging MPI applications, the opaque nature of communi
cators hinders the user's efforts. This is especially
true on fully observable systems such as LAM, where users
can monitor the full state of the processes and message
queues, which includes the context ID (see mpitask (1) and
mpimsg (1)).
This is a LAM/MPI-specific function and is intended mainly
for debugging. If this function is used, it should be
used in conjunction with the LAM_MPI C preprocessor macro
#if LAM_MPI
int cid;
MPIL_Comm_id(MPI_COMM_WORLD, &cid);
#endif
NOTES FOR FORTRAN
All MPI routines in Fortran (except for MPI_WTIME and
MPI_WTICK ) have an additional argument ierr at the end of
the argument list. ierr is an integer and has the same
meaning as the return value of the routine in C. In For
tran, MPI routines are subroutines, and are invoked with
All MPI objects (e.g., MPI_Datatype , MPI_Comm ) are of
type INTEGER in Fortran.
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 MPI_Errhandler_set ; the predefined error
handler MPI_ERRORS_RETURN may be used to cause error val
ues to be returned (in C and Fortran; this error handler
is less useful in with the C++ MPI bindings. The prede
fined error handler MPI::ERRORS_THROW_EXCEPTIONS should be
used in C++ if the error value needs to be recovered).
Note that MPI does not guarantee that an MPI program can
continue past an error.
All MPI routines (except MPI_Wtime and MPI_Wtick ) return
an error value; C routines as the value of the function
and Fortran routines in the last argument. The C++ bind
ings for MPI do not return error values; instead, error
values are communicated by throwing exceptions of type
MPI::Exception (but not by default). Exceptions are only
thrown if the error value is not MPI::SUCCESS .
Note that if the 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.
MPI_SUCCESS
- No error; MPI routine completed successfully.
MPI_ERR_COMM
- Invalid communicator. A common error is to use a
null communicator in a call (not even allowed in
MPI_Comm_rank ).
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.
SEE ALSO
MPIL_Comm_gps, MPIL_Type_id
LOCATION
mpil_id.c
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