SHMALLOC(3) Open MPI SHMALLOC(3)
NAME
shmalloc(3), shfree(3), shmemalign(3), shrealloc(3) - Symmetric heap
memory management functions.
SYNOPSIS
C or C++:
#include <mpp/shmem.h>
void *shmalloc(size_t size);
void shfree(void *ptr);
void *shrealloc(void *ptr, size_t size);
void *shmemalign(size_t alignment, size_t size);
extern long malloc_error;
DESCRIPTION
The shmalloc function returns a pointer to a block of at least size
bytes suitably aligned for any use. This space is allocated from the
symmetric heap (in contrast to malloc(3C), which allocates from the
private heap).
The shmemalign function allocates a block in the symmetric heap that
has a byte alignment specified by the alignment argument.
The shfree function causes the block to which ptr points to, to be de-
allocated, that is, made available for further allocation. If ptr is a
null pointer, no action occurs; otherwise, if the argument does not
match a pointer earlier returned by a symmetric heap function, or if
the space has already been deallocated, malloc_error is set to indicate
the error, and shfree returns.
The shrealloc function changes the size of the block to which ptr
points to, to the size (in bytes) specified by size.
The contents of the block are unchanged up to the lesser of the new and
old sizes. If the new size is larger, the value of the newly allocated
portion of the block is indeterminate. If ptr is a null pointer, the
shrealloc function behaves like the shmalloc function for the specified
size. If size is 0 and ptr is not a null pointer, the block to which it
points to is freed. Otherwise, if ptr does not match a pointer earlier
returned by a symmetric heap function, or if the space has already been
deallocated, the malloc_error variable is set to indicate the error,
and shrealloc returns a null pointer. If the space cannot be allocated,
the block to which ptr points to is unchanged.
The shmalloc, shfree, and shrealloc functions are provided so that mul-
tiple PEs in an application can allocate symmetric, remotely accessible
memory blocks. These memory blocks can then be used with (shmem) commu-
nication routines. Each of these functions call the shmem_bar-
rier_all(3) function before returning; this ensures that all PEs par-
ticipate in the memory allocation, and that the memory on other PEs can
be used as soon as the local PE returns.
The user is responsible for calling these functions with identical ar-
gument(s) on all PEs; if differing size arguments are used, subsequent
calls may not return the same symmetric heap address on all PEs.
NOTES
The total size of the symmetric heap is determined at job startup. One
can adjust the size of the heap using the SHMEM_SYMMETRIC_HEAP_SIZE en-
vironment variable. See the intro_shmem(3) man page for futher details.
The shmalloc, shfree, and shrealloc functions differ from the private
heap allocation functions in that all PEs in an application must call
them (a barrier is used to ensure this).
RETURN VALUES
The shmalloc function returns a pointer to the allocated space (which
should be identical on all PEs); otherwise, it returns a null pointer
(with malloc_error set). The shfree function returns no value. The
shrealloc function returns a pointer to the allocated space (which may
have moved); otherwise, it returns a null pointer (with malloc_error
set).
SEE ALSO
intro_shmem(3), my_pe(3I), start_pes(3)
4.1.2 Nov 24, 2021 SHMALLOC(3)
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