GNU C Library (libc) Programming Guide

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3.2.4.8 Status of an Obstack

Here are functions that provide information on the current status of allocation in an obstack. You can use them to learn about an object while still growing it.

— Function: void * obstack_base (struct obstack *obstack-ptr)

This function returns the tentative address of the beginning of the currently growing object in obstack-ptr. If you finish the object immediately, it will have that address. If you make it larger first, it may outgrow the current chunk—then its address will change!
If no object is growing, this value says where the next object you allocate will start (once again assuming it fits in the current chunk).

— Function: void * obstack_next_free (struct obstack *obstack-ptr)

This function returns the address of the first free byte in the current chunk of obstack obstack-ptr. This is the end of the currently growing object. If no object is growing, obstack_next_free returns the same value as obstack_base.

— Function: int obstack_object_size (struct obstack *obstack-ptr)

This function returns the size in bytes of the currently growing object. This is equivalent to
          obstack_next_free (obstack-ptr) - obstack_base (obstack-ptr)
     