Question

I have difficulty understanding this piece of code:

Particularly this part:

     // check that the stuff we wrote has not changed
     if(n[k][0]!=(unsigned char)(n[k]+s[k]+k))
        printf("Error when checking first byte!\n");
     if(s[k]>1 && n[k][s[k]-1]!=(unsigned char)(n[k]-s[k]-k))
        printf("Error when checking last byte!\n");

The whole program tries to mimic the Windows’ malloc and free function.
It must be run on Windows.

Anyone can explain how those 2 ifs work?

Thanks.

Answer 1

The code makes more sense with a bit more context.

// used to store pointers to allocated memory
unsigned char *n[NO_OF_POINTERS]; 

int s[5000]; // used to store sizes when testing

....

for(i=0;i<NO_OF_ITERATIONS;i++) {
   k=myrand()%NO_OF_POINTERS; // pick a pointer
   if(n[k]) { // if it was allocated then free it
      // check that the stuff we wrote has not changed
      if(n[k][0]!=(unsigned char)(n[k]+s[k]+k))
         printf("Error when checking first byte!\n");
      if(s[k]>1 && n[k][s[k]-1]!=(unsigned char)(n[k]-s[k]-k))
         printf("Error when checking last byte!\n");
      FREE(n[k]);
   }
   size=randomsize(); // pick a random size
             size=1;
   n[k]=(unsigned char *)MALLOC(size); // do the allocation
   s[k]=size; // remember the size
   n[k][0]=(unsigned char)(n[k]+s[k]+k);  // put some data in the first and
   if(size>1) n[k][size-1]=(unsigned char)(n[k]-s[k]-k); // last byte
}

The last two lines set the first and the last byte to values using a formula based on the pointer value (n[k]), the size of the allocation (s[k]), and the pointer index (k). There is no meaning to this formula, it’s just a calculation of a value to be stored that will be different for different pointer allocations.

The if statements you highlighted check that the values of the first (n[k][0]) and last (n[k][s[k]-1]) bytes haven’t changed before freeing the memory. The code is basically a test harness for the VirtualAlloc and VirtualFree functions.