Disk scheduling on Linux using C++

readdisk.cpp

#include <fstream>
#include <cerrno>
#include <stdexcept>
#include <cstring>
#include <vector>
#include <iostream>
#include <time.h>
#include <stdlib.h>

#define MAX_REQUESTS 1000000
#define SECTOR_SIZE 512
#define SECTOR_UB 1800000

int request[MAX_REQUESTS];

void execrequests(){
	int sector = 0;
	clock_t t; 

  	// Which disk?
  	char diskName[] = "/dev/sda";
  	std::string diskError = std::string() + diskName + ": ";

  	// Open device file
  	std::ifstream disk(diskName, std::ios_base::binary);

  	if(!disk)
    	throw(std::runtime_error(diskError + std::strerror(errno)));

    int length = disk.tellg(); // tellg() returns the distance from the beginning of the file

	std::vector<char> buffer(SECTOR_SIZE);
	disk.clear();
	
	// Start clock
	t = clock();

	for(sector=0; sector<MAX_REQUESTS; sector++){
	  	// Seek to the sector
		// std::cout << "Seeking sector: " << request[sector] << "\n";
		disk.seekg(SECTOR_SIZE*request[sector]);
		if(!disk)
			throw(std::runtime_error(diskError + std::strerror(errno)));

		// Read in one sector
		// std::cout << "Reading sector: " << request[sector] << "\n";
	  	disk.read(&buffer[0], SECTOR_SIZE);
	  	if(!disk)
			throw(std::runtime_error(diskError + std::strerror(errno)));
		
		//Print the sector 
		/*
		for(std::vector<char>::const_iterator i = buffer.begin(); i != buffer.end(); ++i)
			std::cout << *i;
		*/
		
	}

	// Stop clock
	t = clock()-t;

	std::cout <<"Time taken: " <<(float)t/CLOCKS_PER_SEC << "\n";
}

void mergesort(int a[], int low, int high){
	int i = 0;
	int length = high - low + 1;
	int pivot  = 0;
	int merge1 = 0;
	int merge2 = 0;
	int working[length];

	if(low == high)
		return;

	pivot  = (low + high) / 2;

	mergesort(a, low, pivot);
	mergesort(a, pivot + 1, high);
 
	for(i = 0; i < length; i++)
		working[i] = a[low + i];

	merge1 = 0;
	merge2 = pivot - low + 1;

	for(i = 0; i < length; i++) {
		if(merge2 <= high - low)
			if(merge1 <= pivot - low)
				if(working[merge1] > working[merge2])
					a[i + low] = working[merge2++];
				else
					a[i + low] = working[merge1++];
			else
				a[i + low] = working[merge2++];
		else
			a[i + low] = working[merge1++];
	}
}

void genrequests(){
	srand(time(NULL));
	for(int i=0; i<MAX_REQUESTS; i++){
		request[i] = rand()%SECTOR_UB + 1;
	}
}

void FCFS(){
	std::cout << "\n" << "Algorithm: FCFS";
	std::cout << "\n" << "---------------" << "\n";
	execrequests();
}

void SCAN(){
	std::cout << "\n" << "Algorithm: SCAN";
	std::cout << "\n" << "---------------" << "\n";
	mergesort(request, 0, MAX_REQUESTS - 1);
	execrequests();
}

int main(){
	genrequests();
	std::cout << "\n" << "General Details";
	std::cout << "\n" << "---------------" << "\n";
	int sectorsize = SECTOR_SIZE;
	std::cout << "All requests made to the sectors in the partition /dev/sda" << "\n";
	std::cout << "Sector size: " << sectorsize << "\n";
	int totalrequests = MAX_REQUESTS;
	std::cout << "Total requests: " << totalrequests << "\n";
	int upperbound = SECTOR_UB;
	std::cout << "Sectors requested between 1 and " << upperbound << "\n";
	FCFS();
	SCAN();
}