memory corruption

Question

i was running a small c program:

#include
int main()
{
char *p;
p = (char *)malloc(10);
free(p);
free(p);
free(p);
printf(\"\\npointer is freed!!\         


        

Answer 1

There are multiple issues with your program:

1. Since you're using malloc() and free(), you should do #include <stdlib.h> before calling any of those functions.
2. There's no need to cast the return value from malloc(): it returns a void *, which can be assigned to any other pointer type safely (except function pointers). So, you can do: p = malloc(10);
3. Once you free a pointer allocated by malloc() or realloc(), using the pointer value in any way is bad: in particular, you cannot call free() on it again.
4. int main() is better written as int main(void).
5. Since main() returns int, you should return a value from it. Traditionally, 0 means success.

Of course, the main (no pun intended) problem with your program is freeing it many times, but other issues mentioned above are important too. Once you've free()'d a pointer successfully, calling free() on it is undefined behavior: the program can do anything, including (unfortunately), seeming to not do anything bad. I say "unfortunately" because it might give you a sense of security that it's okay to free() a pointer more than once.