Is it double free in C?

Question

Usually, if the pointer is freed twice, it is double free. For instance,

char *ptr; ptr=malloc(5 * sizeof(*ptr)); free(ptr); free(ptr); 

The above code is considered double. Are the following, also considered double, free?

 char *ptr; char *ptr1; ptr=malloc(5 * sizeof(*ptr)); ptr1=ptr; free(ptr); free(ptr1); 

Thanks.

Answer 1

Yes. The library doesn’t care what name you gave varaible in your source code (it took a long time to run the code). All that matters is the value, in which case the values ​​passed to free() will be the same.

Answer 2

Yes. You free the same memory twice.

Answer 3

Yeah. Double freedom is an attempt to free an already freed block of memory. Both ptr and ptr1 point to the same memory block, so the second call to free tries to free the already freed memory block.

Answer 4

Yes, because pointers point to the same address and therefore pass the same address to a free one.

You can show how important the pointer is by typing it

 printf( "%p", ptr); 

or see it in the debugger

Answer 5

Yes, this is double free (and a very bad thing). Ptr1 is a pointer to the memory allocated by malloc. This is the same place ptr was pointing to. By freeing ptr and ptr1, you free the same memory twice.