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);

Answer 2

In short: "Undefined Behavior".

(Now, what that can include and why that is the case the others have already said. I just though it was worth mentioning the term here as it is quite common).

Answer 3

Depending on which system you run it on, nothing will happen, the program will crash, memory will be corrupted, or any other number of interesting effects.

Answer 4

Always set a pointer to NULL after freeing it. It is safe to attempt to free a null pointer.

It's worth writing your own free wrapper to do this automatically.

Answer 5

It (potentially) makes demons fly out of your nose.

Answer 6

One of nothing, silent memory corruption, or segmentation fault.