(flex.info)Overriding The Default Memory Management

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21.2 Overriding The Default Memory Management
=============================================

Flex calls the functions 'yyalloc', 'yyrealloc', and 'yyfree' when it
needs to allocate or free memory.  By default, these functions are
wrappers around the standard C functions, 'malloc', 'realloc', and
'free', respectively.  You can override the default implementations by
telling flex that you will provide your own implementations.

   To override the default implementations, you must do two things:

  1. Suppress the default implementations by specifying one or more of
     the following options:

        * '%option noyyalloc'
        * '%option noyyrealloc'
        * '%option noyyfree'.

  2. Provide your own implementation of the following functions: (1)

          // For a non-reentrant scanner
          void * yyalloc (size_t bytes);
          void * yyrealloc (void * ptr, size_t bytes);
          void   yyfree (void * ptr);
          
          // For a reentrant scanner
          void * yyalloc (size_t bytes, void * yyscanner);
          void * yyrealloc (void * ptr, size_t bytes, void * yyscanner);
          void   yyfree (void * ptr, void * yyscanner);

   In the following example, we will override all three memory routines.
We assume that there is a custom allocator with garbage collection.  In
order to make this example interesting, we will use a reentrant scanner,
passing a pointer to the custom allocator through 'yyextra'.

     %{
     #include "some_allocator.h"
     %}
     
     /* Suppress the default implementations. */
     %option noyyalloc noyyrealloc noyyfree
     %option reentrant
     
     /* Initialize the allocator. */
     %{
     #define YY_EXTRA_TYPE  struct allocator*
     #define YY_USER_INIT  yyextra = allocator_create();
     %}
     
     %%
     .|\n   ;
     %%
     
     /* Provide our own implementations. */
     void * yyalloc (size_t bytes, void* yyscanner) {
         return allocator_alloc (yyextra, bytes);
     }
     
     void * yyrealloc (void * ptr, size_t bytes, void* yyscanner) {
         return allocator_realloc (yyextra, bytes);
     }
     
     void yyfree (void * ptr, void * yyscanner) {      
         /* Do nothing -- we leave it to the garbage collector. */
     }
     

   ---------- Footnotes ----------

   (1) It is not necessary to override all (or any) of the memory
management routines.  You may, for example, override 'yyrealloc', but
not 'yyfree' or 'yyalloc'.