MPI_FILE_IWRITE_AT(3)				     Open MPI				     MPI_FILE_IWRITE_AT(3)

MPI_File_iwrite_at — Writes a file at an explicitly specified offset (nonblocking, noncollective).

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_File_iwrite_at(MPI_File fh, MPI_Offset offset,
	       const void *buf, int count, MPI_Datatype datatype, MPI_Request *request)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_FILE_IWRITE_AT(FH, OFFSET, BUF, COUNT, DATATYPE, REQUEST, IERROR)
	       <type>  BUF(*)
	       INTEGER FH, COUNT, DATATYPE, REQUEST, IERROR
	       INTEGER(KIND=MPI_OFFSET_KIND)   OFFSET

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_File_iwrite_at(fh, offset, buf, count, datatype, request, ierror)
	       TYPE(MPI_File), INTENT(IN) :: fh
	       INTEGER(KIND=MPI_OFFSET_KIND), INTENT(IN) :: offset
	       TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: buf
	       INTEGER, INTENT(IN) :: count
	       TYPE(MPI_Datatype), INTENT(IN) :: datatype
	       TYPE(MPI_Request), INTENT(OUT) :: request
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT/OUTPUT PARAMETER
       • fh: File handle (handle).

INPUT PARAMETERS
       • offset: File offset (integer).

       • buf: Initial address of buffer (choice).

       • count: Number of elements in buffer (integer).

       • datatype: Data type of each buffer element (handle).

OUTPUT PARAMETERS
       • request: Request object (handle).

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

DESCRIPTION
       MPI_File_iwrite_at is a nonblocking version of MPI_File_write_at <#mpi-file-write-at>. It attempts to write
       into  the  file	associated  with  fh  (at  the	offset position) a total number of count data items having
       datatype type from the user’s buffer buf. The offset is in etype units relative to the current  view.  That
       is,  holes  are	not  counted  when  locating  an  offset. The data is written into those parts of the file
       specified by the current view. MPI_File_iwrite_at stores the number of datatype elements	 actually  written
       in  status.  All	 other	fields of status are undefined. The request structure can be passed to MPI_Wait <#
       mpi-wait> or MPI_Test <#mpi-test>, which will return a status with the number of bytes actually accessed.

       It is erroneous to call this function if MPI_MODE_SEQUENTIAL mode was specified when the file was open.

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.

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				     MPI_FILE_IWRITE_AT(3)
