Page 1 of 1
memory...bytes.....c....but?
Posted: Mon Aug 22, 2005 8:47 am
by datajack
how would i say...copy 4096 bytes of data from pointer dest, to pointer src? and explaination would be really nice
Re:memory...bytes.....c....but?
Posted: Mon Aug 22, 2005 9:11 am
by AR
That is an extremely basic operation, just set the contents of destination pointer to the contents of the source pointer, incrementing both as you go, ie.
Code: Select all
for(size_t i = 0 ; i < NUMBYTES ; ++i)
destination[i] = source[i];
Or the more optimized version:
Code: Select all
__asm__ ("rep\n\t"
"movsb\n\t" : : "eDi"(destination), "eSi"(source), "ecx"(NumBytes));
Re:memory...bytes.....c....but?
Posted: Mon Aug 22, 2005 9:57 am
by datajack
shouldent it be *dest = *src?
and, is size_t important or can it be int i?
Re:memory...bytes.....c....but?
Posted: Mon Aug 22, 2005 10:30 am
by durand
size_t is defined as an unsigned int.
Code: Select all
typdef unsigned int size_t;
void* memcpy( char *dest, const char *src, size_t n )
{
size_t i;
for ( i = 0; i < n; i++ )
dest[i] = src[i];
return dest;
}
It won't be *dest
because the is already indexing into the array at dest.
Try and find some tutorials on pointers. It will save you a lot of headaches.
Code: Select all
int a = 5; // a is now == 5
int *b = &a; // b is now pointing to a
int **d = &b; // d is now a pointer to a pointer. *d == b and **d == a.
printf("%i %i %i", a, *b, **d ); // prints three 5's.
a = 2; // everything points to a, so this will change the output for everything.
printf("%i %i %i", a, *b, **d ); // prints three 2's.
printf("%p", b ); // prints b which is a pointer to a.
// So, it's printing the memory location of a.
printf("%p", d ); // prints d which is a pointer to b.
// So, it's printing the memory location of b.
printf("%p", *d ); // prints *d which is the value of b which is a pointer to a.
// So, it's printing the memory location of a.
Re:memory...bytes.....c....but?
Posted: Mon Aug 29, 2005 6:38 pm
by Joel (not logged in)
shouldent it be *dest = *src?
and, is size_t important or can it be int i?
size_t is whatever the appropriate data type for storing a size on the current platform is. In general, it's probably better to use size_t and not go around it by using what it's typedef'd as.
As for the memory copying, unless you're implementing the C standard library or doing a programming assignment, the standard library function memcpy already does this for you, and (unless you have a useless compiler vendor) it's probably faster than anything you would implement by hand.
Re:memory...bytes.....c....but?
Posted: Mon Aug 29, 2005 8:51 pm
by AR
He created this after being told that the question was more C-like then OS Dev-like, so... he's implementing the C library.