How to use mmap for integer input? - c++

Question

I have coded a program that uses mmap as input to fill a integer 2D vector from a .txt file. The code is part of a larger program and will be submitted to a competition. Is there a way to improve the speed using mmap, or by using a different way all together? Here is the code:
#include <fstream>
#include <vector>
#include <algorithm>
#include <cstring>
#include <iostream>
// for mmap:
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
using namespace std;
const char* map_file(const char* fname, size_t& length);
int main()
{
auto start = std::chrono::high_resolution_clock::now();
size_t length;
auto f = map_file("erasmus.in", length);
auto l = f + length;
int i = 0;
bool flag = false;
string lines;
vector<vector<int> > students(10000); //The number of lines is predefined
const char* temp;
while (f && f!=l) {
string element = "";
temp = static_cast<const char*>(memchr(f, '\n', l-f));
for(f = f; f<=temp; f++)
{
if(!isspace(*f))
{
element += *f;
flag = true;
}
if(isspace(*f) && flag == true)
{
flag = false;
int assigned_element = stoi(element);
students[i].push_back(assigned_element);
element = "";
}
}
i++;
temp++;
}
auto finish = std::chrono::high_resolution_clock::now();
std::chrono::duration<double> elapsed1 = finish - start;
FILE* output = fopen("erasmus.out", "w");
for (int i = 0; i < students.size(); i++)
{
for (int j = 0; j < students[i].size(); j++)
{
fprintf(output, "%d ", students[i][j]);
}
fprintf(output, "\n");
}
std::cout << "Elapsed time: " << elapsed1.count() << " s\n";
return 0;
}
void handle_error(const char* msg) {
perror(msg);
exit(255);
}
const char* map_file(const char* directory, size_t& length)
{
int fileDirectory = open(directory, O_RDONLY);
if (fileDirectory == -1)
handle_error("open");
// obtain file size
struct stat sb;
if (fstat(fileDirectory, &sb) == -1)
handle_error("fstat");
length = sb.st_size;
const char* map = static_cast<const char*>(mmap(NULL, length, PROT_READ, MAP_PRIVATE, fileDirectory, 0u));
if (map == MAP_FAILED)
handle_error("mmap");
return map;
}
The file will be executed on a linux system, if this helps to find the optimal answer. At the end of each line of the .txt
there is a space character (' ') and a newline('\n')