GNU Info

(gmp.info)Custom Allocation

---

Next: Language Bindings Prev: BSD Compatible Functions Up: Top

---

Enter node , (file) or (file)node

Custom Allocation
*****************

   By default GMP uses `malloc', `realloc' and `free' for memory
allocation, and if they fail GMP prints a message to the standard error
output and terminates the program.

   Alternate functions can be specified to allocate memory in a
different way or to have a different error action on running out of
memory.

   This feature is available in the Berkeley compatibility library
(Note: BSD Compatible Functions) as well as the main GMP library.

 - Function: void mp_set_memory_functions (
          void *(*ALLOC_FUNC_PTR) (size_t),
          void *(*REALLOC_FUNC_PTR) (void *, size_t, size_t),
          void (*FREE_FUNC_PTR) (void *, size_t))
     Replace the current allocation functions from the arguments.  If
     an argument is `NULL', the corresponding default function is used.

     These functions will be used for all memory allocation done by
     GMP, apart from temporary space from `alloca' if that function is
     available and GMP is configured to use it (Note: Build Options).

     *Be sure to call `mp_set_memory_functions' only when there are no
     active GMP objects allocated using the previous memory functions!
     Usually that means calling it before any other GMP function.*

   The functions supplied should fit the following declarations:

 - Function: void * allocate_function (size_t ALLOC_SIZE)
     Return a pointer to newly allocated space with at least ALLOC_SIZE
     bytes.

 - Function: void * reallocate_function (void *PTR, size_t OLD_SIZE,
          size_t NEW_SIZE)
     Resize a previously allocated block PTR of OLD_SIZE bytes to be
     NEW_SIZE bytes.

     The block may be moved if necessary or if desired, and in that
     case the smaller of OLD_SIZE and NEW_SIZE bytes must be copied to
     the new location.  The return value is a pointer to the resized
     block, that being the new location if moved or just PTR if not.

     PTR is never `NULL', it's always a previously allocated block.
     NEW_SIZE may be bigger or smaller than OLD_SIZE.

 - Function: void deallocate_function (void *PTR, size_t SIZE)
     De-allocate the space pointed to by PTR.

     PTR is never `NULL', it's always a previously allocated block of
     SIZE bytes.

   A "byte" here means the unit used by the `sizeof' operator.

   The OLD_SIZE parameters to REALLOCATE_FUNCTION and
DEALLOCATE_FUNCTION are passed for convenience, but of course can be
ignored if not needed.  The default functions using `malloc' and friends
for instance don't use them.

   No error return is allowed from any of these functions, if they
return then they must have performed the specified operation.  In
particular note that ALLOCATE_FUNCTION or REALLOCATE_FUNCTION mustn't
return `NULL'.

   Getting a different fatal error action is a good use for custom
allocation functions, for example giving a graphical dialog rather than
the default print to `stderr'.  How much is possible when genuinely out
of memory is another question though.

   There's currently no defined way for the allocation functions to
recover from an error such as out of memory, they must terminate
program execution.  A `longjmp' or throwing a C++ exception will have
undefined results.  This may change in the future.

   GMP may use allocated blocks to hold pointers to other allocated
blocks.  This will limit the assumptions a conservative garbage
collection scheme can make.

   Since the default GMP allocation uses `malloc' and friends, those
functions will be linked in even if the first thing a program does is an
`mp_set_memory_functions'.  It's necessary to change the GMP sources if
this is a problem.