Allocation in an Obstack - The GNU C Library

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##### 3.2.4.3 Allocation in an Obstack

The most direct way to allocate an object in an obstack is with `obstack_alloc`, which is invoked almost like `malloc`.

— Function: void * obstack_alloc (struct obstack *obstack-ptr, int size)

> This allocates an uninitialized block of size bytes in an obstack and returns its address. Here obstack-ptr specifies which obstack to allocate the block in; it is the address of the `struct obstack` object which represents the obstack. Each obstack function or macro requires you to specify an obstack-ptr as the first argument.

> This function calls the obstack's `obstack_chunk_alloc` function if it needs to allocate a new chunk of memory; it calls `obstack_alloc_failed_handler` if allocation of memory by `obstack_chunk_alloc` failed.

For example, here is a function that allocates a copy of a string str in a specific obstack, which is in the variable `string_obstack`:

         struct obstack string_obstack;

         char *
         copystring (char *string)
         {
           size_t len = strlen (string) + 1;
           char *s = (char *) obstack_alloc (&string_obstack, len);
           memcpy (s, string, len);
           return s;
         }

To allocate a block with specified contents, use the function `obstack_copy`, declared like this:

— Function: void * obstack_copy (struct obstack *obstack-ptr, void *address, int size)

> This allocates a block and initializes it by copying size bytes of data starting at address. It calls `obstack_alloc_failed_handler` if allocation of memory by `obstack_chunk_alloc` failed.

— Function: void * obstack_copy0 (struct obstack *obstack-ptr, void *address, int size)

> Like `obstack_copy`, but appends an extra byte containing a null character. This extra byte is not counted in the argument size.

The `obstack_copy0` function is convenient for copying a sequence of characters into an obstack as a null-terminated string. Here is an example of its use:

         char *
         obstack_savestring (char *addr, int size)
         {
           return obstack_copy0 (&myobstack, addr, size);
         }

Contrast this with the previous example of `savestring` using `malloc` (see Basic Allocation).