MPI_FILE_WRITE_AT(3)				     Open MPI				      MPI_FILE_WRITE_AT(3)

MPI_File_write_at — Writes a file at an explicitly specified offset (blocking, noncollective).

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_File_write_at(MPI_File fh, MPI_Offset offset, const void *buf,
	       int count, MPI_Datatype datatype, MPI_Status *status)

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

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

INPUT PARAMETERS
       • fh: File handle (handle).

       • 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
       • status: Status object (status).

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

DESCRIPTION
       MPI_File_write_at  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_write_at stores the number of datatype
       elements actually written in status. All other fields of status are undefined.

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

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_File_iwrite_at <#mpi-file-iwrite-at>

	  • MPI_File_write_at_all <#mpi-file-write-at-all>

	  • MPI_File_write_at_all_begin <#mpi-file-write-at-all-begin>

	  • MPI_File_write_at_all_end <#mpi-file-write-at-all-end>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				      MPI_FILE_WRITE_AT(3)
