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

Yes, you can get a double free error that causes your program to crash. It has to do with malloc's internal data structures to keep track of allocated memory.

Answer 2

It may crash your program, corrupt memory, or have other more subtle negative effects. After you delete memory, it is a good idea to set it to NULL (0). Trying to free a null pointer does nothing, and is guaranteed to be safe. The same holds true for delete in c++.

Answer 3

Don't do that. If the memory that got freed is re-allocated to something else between the calls to free, then things will get messed up.

Answer 4

Answer summary:

Yes, bad things can and probably will happen.

To prevent this do:

free(myString);
myString = NULL;

Note that all references to the memory must be set to NULL if others were created.

Also, calling free() with a NULL results in no action. For more info see: man free

Answer 5

Bad Things (TM)

Really, I think it's undefined so anything at all including playing "Global Thermonuclear War" with NORAD's mainframe

Answer 6

Another interesting situation:

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

if (myString)
{
    free(myString);
    myString = NULL;
}
// Now this one is safe, because we keep to the rule for 
// setting pointers to NULL after deletion ...
if (myString)
{
    free(myString);
    myString = NULL;
}

// But what about this one:
if (yourString)
{
    free(yourString);
    yourString = NULL;
}

//?!? :)