.TH MPI_Allgather 3 "11/10/2017" " " "MPI"

.SH NAME

MPI_Allgather \-  Gathers data from all tasks and distribute the combined data to all tasks 

.SH SYNOPSIS

.nf

int MPI_Allgather(const void *sendbuf, int sendcount, MPI_Datatype sendtype,

                  void *recvbuf, int recvcount, MPI_Datatype recvtype,

                  MPI_Comm comm)

.fi

.SH INPUT PARAMETERS

.PD 0

.TP

.B sendbuf 

- starting address of send buffer (choice) 

.PD 1

.PD 0

.TP

.B sendcount 

- number of elements in send buffer (integer) 

.PD 1

.PD 0

.TP

.B sendtype 

- data type of send buffer elements (handle) 

.PD 1

.PD 0

.TP

.B recvcount 

- number of elements received from any process (integer) 

.PD 1

.PD 0

.TP

.B recvtype 

- data type of receive buffer elements (handle) 

.PD 1

.PD 0

.TP

.B comm 

- communicator (handle) 

.PD 1

.SH OUTPUT PARAMETERS

.PD 0

.TP

.B recvbuf 

- address of receive buffer (choice) 

.PD 1

.SH NOTES

The MPI standard (1.0 and 1.1) says that

.br

.br

The jth block of data sent from  each process is received by every process

and placed in the jth block of the buffer 

.I recvbuf

\&.

.br

.br

This is misleading; a better description is

.br

.br

The block of data sent from the jth process is received by every

process and placed in the jth block of the buffer 

.I recvbuf

\&.

.br

.br

This text was suggested by Rajeev Thakur and has been adopted as a

clarification by the MPI Forum.

.SH 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 

.I malloc

or other non-MPICH runtime routines that are themselves not interrupt-safe.

.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 ERRORS

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.  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 

.I MPI_Comm_set_errhandler

(for communicators),

.I MPI_File_set_errhandler

(for files), and 

.I MPI_Win_set_errhandler

(for

RMA windows).  The MPI-1 routine 

.I MPI_Errhandler_set

may be used but

its use is deprecated.  The predefined error handler

.I MPI_ERRORS_RETURN

may be used to cause error values to be returned.

Note that MPI does 

.B not

guarentee that an MPI program can continue past

an error; however, MPI implementations will attempt to continue whenever

possible.

.PD 0

.TP

.B MPI_ERR_COMM 

- Invalid communicator.  A common error is to use a null

communicator in a call (not even allowed in 

.I MPI_Comm_rank

).

.PD 1

.PD 0

.TP

.B MPI_ERR_COUNT 

- Invalid count argument.  Count arguments must be 

non-negative; a count of zero is often valid.

.PD 1

.PD 0

.TP

.B MPI_ERR_TYPE 

- Invalid datatype argument.  Additionally, this error can

occur if an uncommitted MPI_Datatype (see 

.I MPI_Type_commit

) is used

in a communication call.

.PD 1

.PD 0

.TP

.B MPI_ERR_BUFFER 

- Invalid buffer pointer.  Usually a null buffer where

one is not valid.

.PD 1