MPI_File_read(3)

okay bestie, we're deep in the MPI manual, section 3, and today's main character is MPI_File_read. she reads. she slays. let's get into it 💅

---

✨ NAME ✨

MPI_File_read — read stuff using the individual file pointer. yes, individual. she's doing her own thing, not sharing a pointer with the group. periodt 💖

---

🌸 SYNOPSIS 🌸

int MPI_File_read(MPI_File fh, void *buf, int count, MPI_Datatype datatype,
MPI_Status *status)

int MPI_File_read_c(MPI_File fh, void *buf, MPI_Count count,
MPI_Datatype datatype, MPI_Status *status)

two glow-ups of the same function, one with int count and one with MPI_Count count. pick your fighter ✨

---

🎀 INPUT PARAMETERS 🎀

• fh — the file handle (a handle, bestie). it's literally the file you're reading from.
• count — how many elements are chillin' in the buffer (an integer). that's the count of elements, not bytes, iykyk.
• datatype — the datatype of each buffer element (a handle). so the vibe of every lil element in there.

---

🦋 OUTPUT PARAMETERS 🦋

• buf — initial address of the buffer (choice). this is where the data lands. the girls are arriving.
• status — status object (Status). she's holding the receipts of the operation.

---

💖 THREAD AND INTERRUPT SAFETY 💖

okay so this routine is thread-safe. that means multiple threads can use it at the same time with no user-provided thread locks needed. no gatekeeping, no drama. 🫶

BUT. she is not interrupt safe. not even a little. typically this is because of memory allocation routines like malloc or other non-MPICH runtime routines that are themselves not interrupt-safe. so don't come for her in an interrupt, she will not survive it 😭

---

🌈 NOTES FOR FORTRAN 🌈

bestie, Fortran said "we do things differently." so:

• every MPI routine in Fortran (except for MPI_WTIME and MPI_WTICK) has an extra argument called ierr at the end of the argument list. ierr is an integer and it means the same thing as the return value of the routine in C.
• in Fortran, MPI routines are subroutines, and you invoke them with the call statement. very retro, very iconic.
• all MPI objects (like MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. yes, even the fancy ones. 💜

---

👑 ERRORS 👑

so every MPI routine (except MPI_Wtime and MPI_Wtick) returns an error value — C routines as the value of the function, Fortran routines in that last argument. before the value comes back, the current MPI error handler gets called. by default, that error handler aborts the MPI job. that's the dramatic exit, no cap. 💀

you can change the error handler with:

• MPI_Comm_set_errhandler (for communicators)
• MPI_File_set_errhandler (for files)
• MPI_Win_set_errhandler (for RMA windows)

the MPI-1 routine MPI_Errhandler_set may be used, but its use is deprecated. we don't stan deprecated behavior. also, the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned instead of aborting.

note: MPI does not guarantee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible. so like, she might pull through, but no promises. 🙈

here's the error code lineup:

• MPI_SUCCESS — no error; the MPI routine completed successfully. the girls are thriving. ✨
• MPI_ERR_ARG — invalid argument. some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK).
• MPI_ERR_BUFFER — invalid buffer pointer. usually a null buffer where one is not valid. so don't hand her a void and expect a miracle.
• MPI_ERR_COUNT — invalid count argument. count arguments must be non-negative; a count of zero is often valid. zero is fine, negative is not, periodt.
• MPI_ERR_TYPE — invalid datatype argument. additionally, this error can occur if an uncommitted MPI_Datatype (see MPI_Type_commit) is used in a communication call. commit your types, bestie, don't leave them hanging. 💅
• MPI_ERR_OTHER — other error; use MPI_Error_string to get more information about this error code. the catch-all, the mystery, the "it's giving unknown" of errors.

---

and that's the whole page, bestie. go forth and read your files with your individual file pointer. no cap, you got this. 💖

source: MPI — version/date: 4/10/2026 — MPI_File_read(3) — mwah, love u, bye 🎀