MPI_FILE_IREAD_AT(3)				     Open MPI				      MPI_FILE_IREAD_AT(3)

MPI_File_iread_at — Reads a file at an explicitly specified offset (nonblocking, noncollective).

SYNTAX
   C Syntax
	  #include <mpi.h>

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

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_FILE_IREAD_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_iread_at(fh, offset, buf, count, datatype, request, ierror)
	       TYPE(MPI_File), INTENT(IN) :: fh
	       INTEGER(KIND=MPI_OFFSET_KIND), INTENT(IN) :: offset
	       TYPE(*), DIMENSION(..), ASYNCHRONOUS :: buf
	       INTEGER, INTENT(IN) :: count
	       TYPE(MPI_Datatype), INTENT(IN) :: datatype
	       TYPE(MPI_Request), INTENT(OUT) :: request
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS
       • fh: File handle (handle).

       • offset: File offset (integer).

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

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

OUTPUT PARAMETERS
       • buf: Initial address of the buffer (choice).

       • request: Request object (handle).

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

DESCRIPTION
       MPI_File_iread_at is the nonblocking version of MPI_File_read_at <#mpi-file-read-at>.

       MPI_File_iread_at  is  a	 nonblocking routine that attempts to read from the file associated with fh at the
       offset position a total number of count data items having datatype type into 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 taken out of those parts of the file specified by the current  view.	 MPI_File_iread_at
       stores the number of datatype elements actually read in status. All other fields of status are undefined.

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_IREAD_AT(3)
