MPI_File_read_all_begin(3)

💖 MPI_File_read_all_begin(3) 💖

okay bestie, welcome to the MPI corner of the man pages 🌸 this one's called MPI_File_read_all_begin and it's about kicking off a split collective read using the individual file pointer. yes, "split collective" — it's giving ✨fancy✨, we'll get there. iykyk.

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🌈 NAME 🌈

MPI_File_read_all_begin — begin a split collective read using individual file pointer 🫶

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✨ SYNOPSIS ✨

int MPI_File_read_all_begin(MPI_File fh, void *buf, int count,
MPI_Datatype datatype)

int MPI_File_read_all_begin_c(MPI_File fh, void *buf, MPI_Count count,
MPI_Datatype datatype)

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🎀 INPUT PARAMETERS 🎀

• fh — the file handle, it's a handle type. your lil key to the file, periodt 🔑
• count — how many elements are chillin' in the buffer, it's an integer. yes you have to tell it, no it won't guess 🙈
• datatype — the datatype of each buffer element, it's a handle. like, what ARE these things we're reading, bestie 🦋

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💅 OUTPUT PARAMETERS 💅

• buf — initial address of buffer, it's a choice. this is where the goods land 🍓

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👑 THREAD AND INTERRUPT SAFETY 👑

so this routine is thread-safe, and that's iconic of it 💯 what that means: multiple threads can use it at the same time and you do NOT need to bring your own thread locks. no user-provided lock drama required, we love that for us ✨

BUT — and this is the plot twist 😭 — it is not interrupt safe. typically that's because it uses memory allocation routines like malloc, or other non-MPICH runtime routines that themselves are not interrupt-safe. so don't @ it with interrupts, it will simply not cope 💀

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🌸 NOTES FOR FORTRAN 🌸

all the MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) get an extra argument ierr tacked on at the very end of the argument list 👍 ierr is an integer and it means the exact 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 — that's just the vibe over there 🧚

also! all MPI objects (like MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. no cap 📌

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💜 ERRORS 💜

so literally every MPI routine (except MPI_Wtime and MPI_Wtick) returns an error value — C routines hand it back as the value of the function, Fortran routines put it 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 😭 dramatic, right? but you can change it:

• 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 ⚠️ (so like… maybe don't, bestie).

the predefined error handler MPI_ERRORS_RETURN can be used to make error values actually get returned to you instead of everything exploding ✨

heads up: MPI does not guarantee that an MPI program can continue past an error. however, MPI implementations will attempt to continue whenever possible. so it might survive, it might not — no promises 💀

here's the error code lineup:

• MPI_SUCCESS — no error, the MPI routine completed successfully. slay 🎉
• MPI_ERR_BUFFER — invalid buffer pointer. usually it's a null buffer where a null one is not valid. like girl, a NULL? here? 🙈
• MPI_ERR_COUNT — invalid count argument. count arguments must be non-negative; a count of zero is often valid. so no negative counts, that's the rule 📏
• 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 first, it's not optional 🎀
• MPI_ERR_OTHER — other error; use MPI_Error_string to get more information about this error code. the mysterious one 👀

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that's the whole page, bestie 🫶 go forth and split-collectively-read. this was the MPI manual, page dated 4/10/2026 — MPI_File_read_all_begin(3), mwah, we're done here 💖✨