MPI_FILE_READ_AT_ALL(3)

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💅 MPI_FILE_READ_AT_ALL — Open MPI ✨

okay so bestie, welcome to the MPI_FILE_READ_AT_ALL(3) page 🌸 this one lives in the Open MPI manual, and it's all about reading a file at offsets you pick, with the whole squad in on it. blocking, collective, iconic. let's get into it 🫶

✨ NAME ✨

MPI_File_read_at_all — reads a file at explicitly specified offsets, and it's blocking and collective (yes both, she's doing the most 💅).

🎀 SYNTAX 🎀

C Syntax

#include <mpi.h>

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

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_FILE_READ_AT_ALL(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_read_at_all(fh, offset, buf, count, datatype, status, ierror)
     TYPE(MPI_File), INTENT(IN) :: fh
     INTEGER(KIND=MPI_OFFSET_KIND), INTENT(IN) :: offset
     TYPE(*), DIMENSION(..) :: buf
     INTEGER, INTENT(IN) :: count
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     TYPE(MPI_Status) :: status
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

🦋 INPUT PARAMETERS 🦋

  • fh — the file handle (handle). your little pass to the file, key word handle 💖
  • offset — the file offset (integer). where we're reading from, bestie 📍
  • count — number of elements in the buffer (integer). how many lil guys we want ✨
  • datatype — data type of each buffer element (handle). what each lil guy is 🧚

🌸 OUTPUT PARAMETERS 🌸

  • buf — initial address of the buffer (choice). where the data lands, main character energy 👑
  • status — status object (status). the receipts 📝
  • ierror — Fortran only: error status (integer). the "did it work tho" variable 🙈

💜 DESCRIPTION 💜

so MPI_File_read_at_all is a collective routine — meaning everyone in the group shows up, no lurking — and what it does is attempt to read from the file associated with fh, starting at the offset position, a total number of count data items that have datatype type, straight into your very own buffer buf 💫

the offset is measured in etype units relative to the current view, okay? lowkey important: that means holes are not counted when locating an offset. so it's not raw byte math, it's view math, iykyk 🧠✨

the data gets taken out of those parts of the file specified by the current view. and then MPI_File_read_at_all stores the number of datatype elements actually read in status. every other field of status? straight up undefined, don't look at them, they're not ready 💀

not me having to say this but: it is erroneous to call this function if MPI_MODE_SEQUENTIAL mode was specified when the file was opened. like, do not. it's giving undefined behaviour, and not the cute kind 😭

🔥 ERRORS 🔥

almost all MPI routines return an error value — C routines hand it back as the function's return result, and Fortran routines put it in the last argument. classic 💅

before that error value comes back to you, the current MPI error handler associated with the communication object (e.g. communicator, window, file) gets called. and if there's no communication object 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_thread, after MPI_Finalize, or when using the Sessions Model exclusively — the error raises the initial error handler. and that initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or via the mpi_initial_errhandler CLI argument to mpiexec, or the info key to MPI_Comm_spawn/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 you can use, and yes they're a whole vibe:

  • MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. dramatic? yes. effective? also yes 💀
  • 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 was called on that communicator. if called on a window or file, it acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. if called on a session, it aborts only the local process 🎯
  • MPI_ERRORS_RETURN — returns an error code to the application. the chill one 🌈

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

  • MPI_Comm_create_errhandler then MPI_Comm_set_errhandler
  • MPI_File_create_errhandler then MPI_File_set_errhandler
  • MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init
  • MPI_Win_create_errhandler then MPI_Win_set_errhandler
real talk: note that MPI does not guarantee that an MPI program can continue past an error. so like, don't count on it bestie 😭

see the MPI man page for a full list of MPI error codes 📚

see the Error Handling section of the MPI-3.1 standard for more information 🧚

2003-2026, The Open MPI Community

and that's the page, bestie!! 🎀 thank you for coming to my ted talk, this was MPI_FILE_READ_AT_ALL(3) from Open MPI, dated Sep 11, 2026 — slay, read responsibly, and remember: collective means everyone, no cap 💖✨🦋

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