What happens when you try to free() already freed memory in c?

Question

For example:

char * myString = malloc(sizeof(char)*STRING_BUFFER_SIZE);
free(myString);
free(myString);

Are there any adverse side effects

Answer 1

Here's the chapter and verse.

If the argument [to the free function] does not match a pointer earlier returned by the calloc, malloc, or realloc function, or if the space has been deallocated by a call to free or realloc, the behavior is undefined. (ISO 9899:1999 - Programming languages — C, Section 7.20.3.2)

Answer 2

Not so clever. Google for double free vulnerabilities. Set your pointer to NULL after freeing to avoid such bugs.