MPI_IALLGATHER(3)				     Open MPI					 MPI_IALLGATHER(3)

       MPI_Allgather  <#mpi-allgather>,	 MPI_Iallgather,  MPI_Allgather_init  <#mpi-allgather-init> - Gathers data
       from all processes and distributes it to all processes

SYNTAX
   C Syntax
	  #include <mpi.h>

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

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

	  int MPI_Allgather_init(const void *sendbuf, int  sendcount,
		MPI_Datatype sendtype, void *recvbuf, int recvcount,
		MPI_Datatype recvtype, MPI_Comm comm, MPI_Info info, MPI_Request *request)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_ALLGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
		       RECVTYPE, COMM, IERROR)
	       <type>  SENDBUF (*), RECVBUF (*)
	       INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM,
	       INTEGER IERROR

	  MPI_IALLGATHER(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
		       RECVTYPE, COMM, REQUEST, IERROR)
	       <type>  SENDBUF(*), RECVBUF (*)
	       INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM
	       INTEGER REQUEST, IERROR

	  MPI_ALLGATHER_INIT(SENDBUF, SENDCOUNT, SENDTYPE, RECVBUF, RECVCOUNT,
			       RECVTYPE, COMM, INFO, REQUEST, IERROR)
	       <type>  SENDBUF(*), RECVBUF (*)
	       INTEGER SENDCOUNT, SENDTYPE, RECVCOUNT, RECVTYPE, COMM
	       INTEGER INFO, REQUEST, IERROR

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Allgather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
		       comm, ierror)
	       TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
	       TYPE(*), DIMENSION(..) :: recvbuf
	       INTEGER, INTENT(IN) :: sendcount, recvcount
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Iallgather(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
		       comm, request, ierror)
	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN) :: sendcount, recvcount
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       TYPE(MPI_Request), INTENT(OUT) :: request
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

	  MPI_Allgather_init(sendbuf, sendcount, sendtype, recvbuf, recvcount, recvtype,
			       comm, info, request, ierror)
	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
	       INTEGER, INTENT(IN) :: sendcount, recvcount
	       TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       TYPE(MPI_Info), INTENT(IN) :: info
	       TYPE(MPI_Request), INTENT(OUT) :: request
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS
       • sendbuf: Starting address of send buffer (choice).

       • sendcount: Number of elements in send buffer (integer).

       • sendtype: Datatype of send buffer elements (handle).

       • recvbuf: Starting address of recv buffer (choice).

       • recvcount: Number of elements received from any process (integer).

       • recvtype: Datatype of receive buffer elements (handle).

       • comm: Communicator (handle).

       • info: Info (handle, persistent only).

OUTPUT PARAMETERS
       • recvbuf: Address of receive buffer (choice).

       • request: Request (handle, non-blocking only).

       • ierror: Fortran only: Error status (integer).

DESCRIPTION
       MPI_Allgather <#mpi-allgather> is similar to MPI_Gather <#mpi-gather>, except that  all	processes  receive
       the  result,  instead  of  just	the  root. In other words, all processes contribute to the result, and all
       processes receive the result.

       The type signature associated with sendcount, sendtype at a process must be equal  to  the  type	 signature
       associated with recvcount, recvtype at any other process.

       The outcome of a call to MPI_Allgather <#mpi-allgather> is as if all processes executed n calls to

	  MPI_Gather(sendbuf, sendcount, sendtype, recvbuf, recvcount,
		     recvtype, root, comm);

	  // for root = 0 , ..., n-1.

       The rules for correct usage of MPI_Allgather <#mpi-allgather> are easily found from the corresponding rules
       for MPI_Gather <#mpi-gather>.

       Example:	 The  all-gather  version  of  Example	1  in  MPI_Gather  <#mpi-gather>.  Using  MPI_Allgather <#
       mpi-allgather>, we will gather 100 ints from every process in the group to every process.

	  MPI_Comm comm;
	  int gsize,sendarray[100];
	  int *rbuf;
	  ...
	  MPI_Comm_size( comm, &gsize);
	  rbuf = (int *)malloc(gsize*100*sizeof(int));
	  MPI_Allgather( sendarray, 100, MPI_INT, rbuf, 100, MPI_INT, comm);

       After the call, every process has the group-wide concatenation of the sets of data.

USE OF IN-PLACE OPTION
       When the communicator is an intracommunicator, you can perform an all-gather operation in-place (the output
       buffer is used as the input buffer). Use the variable MPI_IN_PLACE as the value of sendbuf. In  this  case,
       sendcount  and sendtype are ignored. The input data of each process is assumed to be in the area where that
       process would receive its own contribution to the receive buffer. Specifically, the outcome of  a  call	to
       MPI_Allgather  <#mpi-allgather>	that  used  the	 in-place  option  is  identical  to the case in which all
       processes executed n calls to

	  MPI_Allgather( MPI_IN_PLACE, 0, MPI_DATATYPE_NULL, recvbuf,
			 recvcount, recvtype, root, comm )

	  // for root =0, ... , n-1.

       Note that MPI_IN_PLACE is a special kind of value; it has the same restrictions on its use as MPI_BOTTOM.

       Because the in-place option converts the receive buffer into a send-and-receive buffer, a  Fortran  binding
       that includes INTENT must mark these as INOUT, not OUT.

WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR
       When  the  communicator	is  an	inter-communicator, the gather operation occurs in two phases. The data is
       gathered from all the members of the first group and received by all the members of the second group.  Then
       the data is gathered from all the members of the second group and received by all the members of the first.
       The  operation,	however,  need	not be symmetric. The number of items sent by the processes in first group
       need not be equal to the number of items sent by the the processes in the second group. You can	move  data
       in only one direction by giving sendcount a value of 0 for communication in the reverse direction.

ERRORS
       Almost  all MPI routines return an error value; C routines as the return result of the function and Fortran
       routines in the last argument.

       Before the error value is returned, the current MPI error handler associated with the communication  object
       (e.g.,  communicator, window, file) is called.  If no communication object is associated with the MPI call,
       then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler.  When
       MPI_COMM_SELF  is  not  initialized  (i.e., before MPI_Init <#mpi-init>/MPI_Init_thread <#mpi-init-thread>,
       after MPI_Finalize <#mpi-finalize>, or when using the Sessions Model  exclusively)  the	error  raises  the
       initial	error  handler.	 The  initial  error  handler can be changed by calling MPI_Comm_set_errhandler <#
       mpi-comm-set-errhandler> on MPI_COMM_SELF when using the World model,  or  the  mpi_initial_errhandler  CLI
       argument	  to   mpiexec	 or   info  key	 to  MPI_Comm_spawn  <#mpi-comm-spawn>/MPI_Comm_spawn_multiple	<#
       mpi-comm-spawn-multiple>.  If no other appropriate error handler has been set, then  the	 MPI_ERRORS_RETURN
       error  handler  is  called  for	MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all
       other MPI functions.

       Open MPI includes three predefined error handlers that can be used:

       • MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.

       • MPI_ERRORS_ABORT An error handler that can be invoked on a communicator, window, file, or  session.  When
	 called on a communicator, it acts as if MPI_Abort <#mpi-abort> was called on that communicator. If called
	 on  a window or file, acts as if MPI_Abort <#mpi-abort> was called on a communicator containing the group
	 of processes in the corresponding window or file. If called on a session, aborts only the local process.

       • MPI_ERRORS_RETURN Returns an error code to the application.

       MPI applications can also implement their own error handlers by calling:

       • MPI_Comm_create_errhandler	<#mpi-comm-create-errhandler>	  then	   MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	 <#mpi-file-create-errhandler>	   then	    MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

       See the Error Handling section of the MPI-3.1 standard for more information.

       See also:

	  • MPI_Allgatherv <#mpi-allgatherv>

	  • MPI_Gather <#mpi-gather>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					 MPI_IALLGATHER(3)
