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(libc.info)Extra Fast Growing

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Extra Fast Growing Objects
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   The usual functions for growing objects incur overhead for checking
whether there is room for the new growth in the current chunk.  If you
are frequently constructing objects in small steps of growth, this
overhead can be significant.

   You can reduce the overhead by using special "fast growth" functions
that grow the object without checking.  In order to have a robust
program, you must do the checking yourself.  If you do this checking in
the simplest way each time you are about to add data to the object, you
have not saved anything, because that is what the ordinary growth
functions do.  But if you can arrange to check less often, or check
more efficiently, then you make the program faster.

   The function `obstack_room' returns the amount of room available in
the current chunk.  It is declared as follows:

 - Function: int obstack_room (struct obstack *OBSTACK-PTR)
     This returns the number of bytes that can be added safely to the
     current growing object (or to an object about to be started) in
     obstack OBSTACK using the fast growth functions.

   While you know there is room, you can use these fast growth functions
for adding data to a growing object:

 - Function: void obstack_1grow_fast (struct obstack *OBSTACK-PTR, char
          C)
     The function `obstack_1grow_fast' adds one byte containing the
     character C to the growing object in obstack OBSTACK-PTR.

 - Function: void obstack_ptr_grow_fast (struct obstack *OBSTACK-PTR,
          void *DATA)
     The function `obstack_ptr_grow_fast' adds `sizeof (void *)' bytes
     containing the value of DATA to the growing object in obstack
     OBSTACK-PTR.

 - Function: void obstack_int_grow_fast (struct obstack *OBSTACK-PTR,
          int DATA)
     The function `obstack_int_grow_fast' adds `sizeof (int)' bytes
     containing the value of DATA to the growing object in obstack
     OBSTACK-PTR.

 - Function: void obstack_blank_fast (struct obstack *OBSTACK-PTR, int
          SIZE)
     The function `obstack_blank_fast' adds SIZE bytes to the growing
     object in obstack OBSTACK-PTR without initializing them.

   When you check for space using `obstack_room' and there is not
enough room for what you want to add, the fast growth functions are not
safe.  In this case, simply use the corresponding ordinary growth
function instead.  Very soon this will copy the object to a new chunk;
then there will be lots of room available again.

   So, each time you use an ordinary growth function, check afterward
for sufficient space using `obstack_room'.  Once the object is copied
to a new chunk, there will be plenty of space again, so the program will
start using the fast growth functions again.

   Here is an example:

     void
     add_string (struct obstack *obstack, const char *ptr, int len)
     {
       while (len > 0)
         {
           int room = obstack_room (obstack);
           if (room == 0)
             {
               /* Not enough room. Add one character slowly,
                  which may copy to a new chunk and make room.  */
               obstack_1grow (obstack, *ptr++);
               len--;
             }
           else
             {
               if (room > len)
                 room = len;
               /* Add fast as much as we have room for. */
               len -= room;
               while (room-- > 0)
                 obstack_1grow_fast (obstack, *ptr++);
             }
         }
     }