PROGRAMMING - Hacking: The Art of Exploitation - page 80

function chooses to reclaim freed space for new allocations. Often, simple
informative printf() statements and a little experimentation can reveal many
things about the underlying system.

0x273 Error-Checked malloc()

In heap_example.c, there were several error checks for the malloc() calls.
Even though the malloc() calls never failed, it's important to handle all
potential cases when coding in C. But with multiple malloc() calls, this error-
checking code needs to appear in multiple places. This usually makes the
code look sloppy, and it's inconvenient if changes need to be made to the
error-checking code or if new malloc() calls are needed. Since all the error-
checking code is basically the same for every malloc() call, this is a perfect
place to use a function instead of repeating the same instructions in multiple
places. Take a look at errorchecked_heap.c for an example.

errorchecked_heap.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void *errorchecked_malloc(unsigned int); // Function prototype for errorchecked_malloc()

int main(int argc, char *argv[]) {
char *char_ptr; // A char pointer
int *int_ptr; // An integer pointer
int mem_size;

if (argc < 2) // If there aren't command-line arguments,
mem_size = 50; // use 50 as the default value.
else
mem_size = atoi(argv[1]);

printf("\t[+] allocating %d bytes of memory on the heap for char_ptr\n", mem_size);
char_ptr = (char *) errorchecked_malloc(mem_size); // Allocating heap memory

strcpy(char_ptr, "This is memory is located on the heap.");
printf("char_ptr (%p) --> '%s'\n", char_ptr, char_ptr);
printf("\t[+] allocating 12 bytes of memory on the heap for int_ptr\n");
int_ptr = (int *) errorchecked_malloc(12); // Allocated heap memory again

*int_ptr = 31337; // Put the value of 31337 where int_ptr is pointing.
printf("int_ptr (%p) --> %d\n", int_ptr, *int_ptr);

printf("\t[-] freeing char_ptr's heap memory...\n");
free(char_ptr); // Freeing heap memory

printf("\t[+] allocating another 15 bytes for char_ptr\n");
char_ptr = (char *) errorchecked_malloc(15); // Allocating more heap memory

strcpy(char_ptr, "new memory");