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

The admittedly strange macro below is a useful drop-in replacement for wiping out a few classes of security vulnerabilities as well as aid debugging since accesses to free()'d regions are more likely to segfault instead of silently corrupting memory.

#define my_free(x) do { free(x); x = NULL; } while (0)

The do-while loop is to help surrounding code more easily digest the multiple-statements. e.g. if (done) my_free(x);