MPIAllocmem

MPIAllocmem is a function provided by the Message Passing Interface (MPI) standard for allocating memory that is suitable for MPI communication. It differs from routines such as malloc only in that it may provide additional alignment or overhead information useful to the MPI library. The function is defined in the MPI standard and implemented in most MPI distributions such as MPICH and Open MPI.

The syntax of MPIAllocmem is:

```

int MPIAllocmem(size_t size, MPI_Comm comm, void **baseptr);

```

* size: The number of bytes requested for allocation.

* comm: The MPI communicator whose memory attributes are to be respected. In practice MPI_COMM_WORLD is most commonly passed, but other communicators can be used as well.

* baseptr: On output, a pointer to the allocated block of memory.

The routine returns an MPI integer error code—MPI_SUCCESS on success, or a corresponding error code otherwise. If allocation fails, *baseptr is set to NULL. When using MPIAllocmem, the application should later deallocate the memory with MPIFree, not with free, to guarantee consistency with any MPI-specific memory handling.

Typical usage patterns involve allocating homogeneous buffers for collective communication. For example:

```

int *data;

MPIAllocmem(100*sizeof(int), MPI_COMM_WORLD, (void**)&data);

MPI_Bcast(data, 100, MPI_INT, 0, MPI_COMM_WORLD);

MPIFree(data);

```

This guarantees that the buffer is aligned and, when applicable, that it resides in shared or pinned memory accessible by MPI interconnects.

MPIAllocmem is optional; older MPI implementations may not provide it or may map it to standard malloc. The MPI_User_pointer function and MPI_Get_address can be used to inspect allocated addresses for alignment or processor locality. Moreover, MPI communication functions accept memory allocated by MPIAllocmem without any special flags, making it a convenient option for performance-tuned MPI codes.