Because memory in obstack chunks is used sequentially, it is possible to build up an object step by step, adding one or more bytes at a time to the end of the object. With this technique, you do not need to know how much data you will put in the object until you come to the end of it. We call this the technique of growing objects. The special macros for adding data to the growing object are described in this section.
You don’t need to do anything special when you start to grow an object.
Using one of the macros to add data to the object automatically
starts it. However, it is necessary to say explicitly when the object is
finished. This is done with
The actual address of the object thus built up is not known until the object is finished. Until then, it always remains possible that you will add so much data that the object must be copied into a new chunk.
While the obstack is in use for a growing object, you cannot use it for ordinary allocation of another object. If you try to do so, the space already added to the growing object will become part of the other object.
The most basic macro for adding to a growing object is
obstack_blank, which adds space without initializing it.
To add a block of initialized space, use
obstack_grow, which is
the growing-object analogue of
obstack_copy. It adds size
bytes of data to the growing object, copying the contents from
This is the growing-object analogue of
obstack_copy0. It adds
size bytes copied from data, followed by an additional null
To add one character at a time, use
It adds a single byte containing c to the growing object.
Adding the value of a pointer one can use
obstack_ptr_grow. It adds
sizeof (void *) bytes
containing the value of data.
A single value of type
int can be added by using
obstack_int_grow. It adds
sizeof (int) bytes to
the growing object and initializes them with the value of data.
When you are finished growing the object, use
obstack_finish to close it off and return its final address.
Once you have finished the object, the obstack is available for ordinary allocation or for growing another object.
When you build an object by growing it, you will probably need to know
afterward how long it became. You need not keep track of this as you grow
the object, because you can find out the length from the obstack
obstack_object_size, before finishing the object.
This macro returns the current size of the growing object, in bytes.
Remember to call
obstack_object_size before finishing the object.
After it is finished,
obstack_object_size will return zero.
If you have started growing an object and wish to cancel it, you should finish it and then free it, like this:
obstack_free (obstack_ptr, obstack_finish (obstack_ptr));
This has no effect if no object was growing.