When we call the memory management functions (malloc, calloc or realloc) then these functions keep extra bytes for bookkeeping. Whenever we call the free function and pass the pointer that is pointing to allocated memory, the free function gets the bookkeeping information and release the allocated memory. Anyhow if you or your program change the value of the pointer that is pointing to the allocated address, the calling of the free function gives the undefined result.
____ The allocated block ____
/ \
+--------+--------------------+
| Header | Your data area ... |
+--------+--------------------+
^
|
+-- Returned Address
Let us see a program to understand the above concept. The behavior of the below program is not defined.
#include
#include
int main()
{
char *pcBuffer = NULL;
//Allocate the memory
pcBuffer = malloc(sizeof(char) * 16);
//make sure piBuffer is valid or not
if (pcBuffer == NULL)
{
// allocation failed, exit from the program
fprintf(stderr, "Out of memory!\n");
exit(1);
}
//Increment the pointer
pcBuffer++;
//Call free function to release the allocated memory
free(pcBuffer);
return 0;
}
OutPut: Undefined Result
