Thread: malloc & cover function

I want to write a cover function for malloc, so that each time malloc() is invoked, the program execute fmalloc() instead. In fmalloc() I will execute the real malloc(). How do I do that??

Code:

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

int main (void)
{
	char *pt,*lt,*gt;
	
	pt=(char*)malloc(40);
	if (pt==NULL)
		{
		perror("fmalloc");
		exit(1);
		}
	
	lt=(char*)malloc(40);
	if (lt==NULL)
		{
		perror("fmalloc");
		exit(1);
		}

	gt=(char*)malloc(40);
	if (gt==NULL)
		{
		perror("fmalloc");
		exit(1);
		}

	strcpy(pt, "abcdefghijklmnopqrstuvxyz");
	strcpy(lt, "abcdefghijklmnopqrstuvxyz");
	strcpy(gt, "abcdefghijklmnopqrstuvxyz");


	printf("Address allocated is %p\n",pt);
	printf("Address allocated is %p\n",lt);
	printf("Address allocated is %p\n",gt);

	free(pt);
	free(lt);
	free(gt);

	printf("Address freed is %p\n",pt);
	printf("Address freed is %p\n",lt);
	printf("Address freed is %p\n",gt);
	
	return 0;
}

Maybe something like the following would work.

Code:

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

void *fmalloc(size_t size)
{
   void *start = malloc(size);
   if(start)
   {
      char *end = start;
      end += size;
      printf("fmalloc:%p-%p\n", start, end);
   }
   else
   {
      puts("fmalloc failed");
   }
   return(start);
}

#define malloc fmalloc

int main()
{
   char *ptr = malloc(64);
   free(ptr);
   return 0;
}

/* my output
fmalloc:007A305C-007A309C
*/

I don't see anything too wrong with writing your own malloc. However, of the standard library functions malloc is probably the most frustrating to write.