MPI_Allgather

Gathers data from all tasks and distribute the combined data to all tasks

Synopsis



int MPI_Allgather(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
                  void *recvbuf, int recvcount, MPI_Datatype recvtype,
                  MPI_Comm comm)




int MPI_Allgather_c(const void *sendbuf, MPI_Count sendcount,
                    MPI_Datatype sendtype, void *recvbuf, MPI_Count recvcount,
                    MPI_Datatype recvtype, MPI_Comm comm)






Input Parameters




sendbuf  
 starting address of send buffer (choice)


sendcount  
 number of elements in send buffer (non-negative integer)


sendtype  
 data type of send buffer elements (handle)


recvcount  
 number of elements received from any process (non-negative integer)


recvtype  
 data type of receive buffer elements (handle)


comm  
 communicator (handle)





Output Parameters




recvbuf  
 address of receive buffer (choice)





Notes

The MPI standard (1.0 and 1.1) says that








The jth block of data sent from  each process is received by every process
and placed in the jth block of the buffer recvbuf.








This is misleading; a better description is








The block of data sent from the jth process is received by every
process and placed in the jth block of the buffer recvbuf.








This text was suggested by Rajeev Thakur and has been adopted as a
clarification by the MPI Forum.



Thread and Interrupt Safety



This routine is thread-safe.  This means that this routine may be
safely used by multiple threads without the need for any user-provided
thread locks.  However, the routine is not interrupt safe.  Typically,
this is due to the use of memory allocation routines such as malloc
or other non-MPICH runtime routines that are themselves not interrupt-safe.



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



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.  Before the value is returned, the current MPI error handler is
called.  By default, this error handler aborts the MPI job.  The error handler
may be changed with MPI_Comm_set_errhandler (for communicators),
MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for
RMA windows).  The MPI-1 routine MPI_Errhandler_set may be used but
its use is deprecated.  The predefined error handler
MPI_ERRORS_RETURN may be used to cause error values to be returned.
Note that MPI does not guarantee that an MPI program can continue past
an error; however, MPI implementations will attempt to continue whenever
possible.





MPI_SUCCESS  
 No error; MPI routine completed successfully.





MPI_ERR_BUFFER  
 Invalid buffer pointer.  Usually a null buffer where
one is not valid.





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_COUNT  
 Invalid count argument.  Count arguments must be 
non-negative; a count of zero is often valid.





MPI_ERR_TYPE  
 Invalid datatype argument.  Additionally, this error can
occur if an uncommitted MPI_Datatype (see MPI_Type_commit) is used
in a communication call.





MPI_ERR_OTHER  
 Other error; use MPI_Error_string to get more information
about this error code.