#include <stdio.h>
#include <assert.h>
#include <vector>

inline void on_line(char *p, size_t size) {
    ///....
}

int main() {
    FILE *file = stdin;

    const size_t buf_size = 4096*4;
    size_t n = buf_size;
    char buf[buf_size + 1]; //+1 for fake '/n'
    std::vector<char> dynamic_line; //for lines, which spans buffer

    size_t i, j;
    i = n;
    j = n;
    buf[n] = '\n';
    bool use_dynamic_buffer = false;
    while (1) {
        assert(buf[n] == '\n'); //this assert makes code faster, LOL
        //volatile char p = buf[n];
        while (buf[i] != '\n') {
            ++i;
        }
        if (i < n) {
            assert(buf[i] == '\n');
            if (!use_dynamic_buffer) {
                on_line(buf + j, i - j);
            } else {
                assert(j == 0);
                dynamic_line.insert(dynamic_line.end(), buf, buf + i);
                on_line(&*dynamic_line.begin(), dynamic_line.size());
                dynamic_line.resize(0);
                use_dynamic_buffer = false;
            }
            ++i;
            j = i;
        } else {
            if (i > j) {
                dynamic_line.insert(dynamic_line.end(), buf + j, buf + i);
                use_dynamic_buffer = true;
            }
            j = 0;
            i = 0;
            if (n < buf_size) { //previos fread was last
                break;
            }
            n = fread(buf, 1, buf_size, file);
            buf[n] = '\n';
        }
    }
    if (!use_dynamic_buffer) { //special code for last line without EOL (common corner case for hand-made text files)
        if (i > j)
            on_line(buf + j, i - j);
    } else {
        dynamic_line.insert(dynamic_line.end(), buf + j, buf + i);
        if (dynamic_line.size())
            on_line(&*dynamic_line.begin(), dynamic_line.size());
    }
}