Unable to free const pointers in C

Question

How can I free a const char*? I allocated new memory using malloc, and when I\'m trying to free it I always receive the error \"incompatible pointe

Answer 1

If you are talking about pure C and you are in complete control of the memory allocation you can use the following trick to cast (const char *) to (char *) which will not give you any warning in compiler:

const char *const_str = (const char *)malloc(...);
char *str = NULL;

union {
  char *mutable_field_p;
  const char *const_field_p;
} u;

u.const_field_p = const_str;
str = u.mutable_field_p;

Now you can use free(str); to free the memory.

But BEWARE that this is evil beyond words and should be only used in strictly controlled environment (e.g. library which allocates and frees strings, but doesn't want to allow user to modify them) Otherwise you will end up with your program crashing when someone provides compile time "STRING" to your free function.