Question

I am trying to learn implementation of Disk Scheduling Algorithms (SCAN and C-SCAN) in the C language, can somebody please refer to good sources of implementations of these in the C Language or advise me on programming them on C?

Further Into:-
*Objective is to write a program to optimize disk access to read a non contiguous set of pages on disk to memory, for this, I am performing disk scheduling.

*I would want to instruct the disk on read sequence of the pages

Answer 1

The logic for scheduling disk requests in an efficient fashion should be considered in the domain of the disk itself! Optimizing the order you read data from disk sectors is not likely to be portable or efficient.

However, if you have a file of npages * PAGE_SIZE, you could attempt to improve your applications performance when servicing your own internal reads from disk. Given:

#define PAGE_SIZE ...
#define MAX_PAGE_READ ...

struct read_req {
    unsigned int page;
    /* any other book-keeping required */
};

You could internally sort the requests by page (and optionally coalesce neighbor pages):

/* qsort-comparer for two read_req structs */
int cmp_req(const void *a, const void *b)
{
    unsigned int pageA = ((struct read_req*)a)->page,
                 pageB = ((struct read_req*)b)->page;
    return pageA == pageB ? 0 : pageA > pageB ? 1 : -1;
}

int service_reads(struct read_req *reqs, size_t nreqs)
{
    size_t ii = 0;

    /* sort read requests in ascending order */
    qsort(reqs, nreqs, sizeof(reqs[0]), cmp_req);

    while (ii < nreqs)
    {
        unsigned int start = reqs[ii].page;
        size_t size = 1;

        while (++ii < nreqs)
        {
            if (reqs[ii].page != (start + size)) break;

            /* expand our read to include the next page,
             * and break if we've read too much
             */
            if (++size == MAX_PAGE_READ) break;
        }

        your_read_logic(start, size);
    }
}