NAME
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 processes in the communicator's group, and
uniquely identifies 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 communicators 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
communicators 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 Fortran, MPI routines are subroutines, and are invoked with
the call statement.
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
values to be returned (in C and Fortran; this error handler is less
useful in with the C++ MPI bindings. The predefined 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++ bindings 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