MPI_ALLOC_MEM(3)

💅 MPI_ALLOC_MEM ✨

okay so bestie, welcome to Open MPI's MPI_Alloc_mem — the little function that goes "here's a chunk of memory, you're welcome" 🎀. It allocates a specified memory segment, no cap.

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🌸 SYNTAX 🌸

✨ C Syntax

#include <mpi.h>

int MPI_Alloc_mem(MPI_Aint size, MPI_Info info, void *baseptr)

✨ Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_ALLOC_MEM(SIZE, INFO, BASEPTR, IERROR)
     INTEGER INFO, IERROR
     INTEGER(KIND=MPI_ADDRESS_KIND) SIZE, BASEPTR

✨ Fortran 2008 Syntax

USE mpi_f08
MPI_Alloc_mem(size, info, baseptr, ierror)
     USE, INTRINSIC :: ISO_C_BINDING, ONLY
     INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) :: size
     TYPE(MPI_Info), INTENT(IN) :: info
     TYPE(C_PTR), INTENT(OUT) :: baseptr
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🫶 INPUT PARAMETERS 🫶

• size — the size of the memory segment, in bytes (nonnegative integer, so like, no funny business with negatives pls 🙈).
• info — the info argument (handle).

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💖 OUTPUT PARAMETERS 💖

• baseptr — pointer to the beginning of the memory segment that got allocated. slay.
• ierror — Fortran only: error status (integer).

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🦋 DESCRIPTION 🦋

MPI_Alloc_mem allocates size bytes of memory. The starting address of this memory is returned in the variable baseptr. literally that's it, that's the whole vibe 💅

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🍓 C NOTES 🍓

okay so the parameter baseptr is of type void * so that you can pass, like, any pointer object for this parameter. What you should actually hand it is a pointer to a pointer of arbitrary type (e.g., void **). iykyk 👑

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🧚 FORTRAN NOTES 🧚

the MPI_Alloc_mem calls require the use of the iso_c_binding module because of the whole TYPE(C_PTR) situation. no skipping, bestie:

use iso_c_binding

type(c_ptr) :: alloc_ptr
integer :: size, ierr
double precision, pointer :: array(:,:)

! A 2D array of 100 elements
size = 10 * 10
call MPI_Alloc_Mem(size * 8, MPI_INFO_NULL, alloc_ptr, ierr)

! Point to the array
call c_f_pointer(alloc_ptr, array, [10, 10])

! ... use the array ...

! Free the memory, no need for the alloc_ptr
call MPI_Free_mem(array, ierr)

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😭 ERRORS 😭

So basically almost all MPI routines return an error value — C routines as the return result of the function, and Fortran routines in the last argument. iconic behavior fr fr.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) gets 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_thread, after 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 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_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. yeah. everyone. gone. 💀
• 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, 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, aborts only the local process. versatile queen 👑
• MPI_ERRORS_RETURN — returns an error code to the application. chill one. respectful. 🫶

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

⚠️ note that MPI does not guarantee that an MPI program can continue past an error. like, it might just not. so don't come for me.

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.

✨ SEE ALSO ✨

• MPI_Free_mem

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👑 COPYRIGHT 👑

okay that's literally the whole page, bestie — hope it ate 💅✨

and for the cute sign-off: this page is © 2003-2026, The Open MPI Community, dated Sep 11, 2026. go forth and allocate responsibly 🎀🦋💜