C++ Fast file reader

input.txt

16
1
2 3
4 5 6
 7 
8	  9		
		  10 11
   12
 13    14 15
     1234567890    
     
     
5
   123456789123456   
     135792648013579		101010101010101
120120120120120120 210210210210210210
	
	
5
abcdefghijklme  AMcvdinVCWew
	.-,vè092367][``~€¶Ð§łß
aknafsna 			15f3d1bg35fgb

main.c

/* 
 * (really) FAST READING
 * 
 * How to:
 * 1. Set the buffer size:       BUFFER_SIZE = ???
 * 2. Open the file:             _fd = open("filename.txt", O_RDONLY);
 * 3. Load the buffer:           loadBuffer();
 * -------------
 * 4. Read int:                  int x = readint();
 * 5. Read long long int:        long long y = readlonglong();
 * 6. Read string (no space):    int len = readstring(str);
 */ 

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

#define BUFFER_SIZE 500
#define DEBUG 0

#if DEBUG
#define _D(x) x
#else
#define _D(x)
#endif

char _buffer[BUFFER_SIZE];
int _buf_pointer;
int _buf_size;
int _fd;
// load the buffer
void loadBuffer() {
	_buf_size = read(_fd, _buffer, BUFFER_SIZE);
	_buf_pointer = 0;
	_D(printf("load buffer: %d chars loaded\n", _buf_size);)
}
// skip all spaces
int skipspaces() {
	char c;
    // skip spaces
	while (((c = _buffer[_buf_pointer++]) == ' ' || c == '\n' || c == '\t') && c != '\0') {
		_D(printf("Skipped: %d\n", (int)c);)
		if (_buf_pointer >= _buf_size) 
			loadBuffer();
	}
	// if EOF stop
	if (c == '\0') {
		_D(printf("EOF!!!\n");)
		return 0;
	}
	// unskip last char
	_buf_pointer--;
	return 1;
}
// read an int from the buffer
int readint() {
    int n = 0;
    char c;
    
    if (!skipspaces()) return -2000000000;
	
	// parse the int from the buffer
    do {
        c = _buffer[_buf_pointer++];
        if (_buf_pointer >= _buf_size) loadBuffer();
        if (c >= '0' && c <= '9')
           n = n*10 + (c-'0');
    } while (c >= '0' && c <= '9');
    return n;
}
// read a long long int from the buffer
long long readlonglong() {
	long long n = 0;
    char c;
    
    if (!skipspaces()) return -2000000000000000;
	
	// parse the int from the buffer
    do {
        c = _buffer[_buf_pointer++];
        if (_buf_pointer >= _buf_size) loadBuffer();
        if (c >= '0' && c <= '9')
           n = n*10 + (c-'0');
    } while (c >= '0' && c <= '9');
    return n;
}

// read a string from the buffer
int readstring(char* str) {
	char c;
	int l = 0;
	if (!skipspaces()) {
		str[0] = 0;
		return 0;
	}
	// read the string
	do {
		c = _buffer[_buf_pointer++];
		str[l++] = c;
		if (_buf_pointer >= _buf_size) loadBuffer();
	// until a space
	} while (c != ' ' && c != '\n' && c != '\t' && c != '\0');
	// string terminator
	str[--l] = '\0';
	return l;
}


int main() {
	// see the sample input.txt!!
	_fd = open("input.txt", O_RDONLY);
	loadBuffer();
	
	int N = readint();
	printf("N = %d\n", N);
	while (N--)
	    printf("%d\n", readint());
	    
	int M = readint();
	printf("M = %d\n", M);
	while (M--)
		printf("%lld\n", readlonglong());
		
	int S = readint();
	char str[100];
	printf("S = %d\n", S);
	while (S--) {
		int l = readstring(str);
		printf("%s  [%d charaters]\n", str, l);
	}
	return 0;
}