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. */
     }