MPIL_Comm_id(3)

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