Styjun

I am working on a homework problem which consists of making a c program that a user can use to manage memory. Essentially we are trying to mimic what malloc() and free() do in our own way. The function I am currently working on is an initmemory(int size) function that allocates the overall block that the user will use, and from that block, smaller blocks will be allocated as the program calls myalloc() funciton (basically our version of malloc()). My question is, I am trying to access the header portion of the overall block in order to save the size and allocation status of the block, but when I try to perform pointer arithmetic, I end up just moving one bit. How can I access the header in order to save the size and allocation status of the block with my pointer variable startOfMemory

void initmemory(int size)
 printf("this is the initial size: %dn", size);
 //realSize = size + initial padding + anchorHeader + sentinelBlock
 int realSize = size + 12;
 printf("I am the new realSize: %dn", realSize);
 //checks how many remainders are left
 int check = realSize % 8;
 printf("this is the value of check: %dn", check);
 //will only change realSize if check is not zero
 if(check != 0)
 //adds enough bytes to satisfy 8-byte alignment 
 realSize = realSize + (8 - check);
 /*
 * this is only to make sure realSize is 8-byte aligned, it should not run
 * unless the above code for some reason does not run
 */

 check = realSize % 8;
 while(check != 0)
 realSize = realSize + (8-check);
 check = realSize % 8;
 printf("I'm in the while check loop");
 
 
 // initializes the memory to be allocated. 
 void *startOfMemory = malloc(realSize);
 void *placeOfHeader = startOfMemory - 1;

 printf("my memory location is at: %pn", startOfMemory);
 printf("my realSize is: %dn", realSize);
 printf("memory location of placeOfHeader: %pn", placeOfHeader);
 free(startOfMemory);


int main()
 initmemory(5);
 return 0;

the memory location for startOfMemory, which calls the malloc() fuction is at 0x87a3008 (which makes sense because of the 8 byte alignment)

when I do pointer arithmetic, as in the variable place of header, the memory location of placeOfHeader is at 0x87a3007.

placeOfHeader is not somewhere in the allocated region.
You probably want to write something like this.

//alloc(realSize)
void *placeOfHeader = malloc(realSize);
*((size_t*)placeOfHeader) = realSize;
void* startOfMemory = (size_t*)placeOfHeader + 1;
return startOfMemory;

//free(startOfMemory)
void* placeOfHeader = (size_t*)startOfMemory - 1;
size_t realSize = *((size_t*)placeOfHeader);
free(placeOfHeader)