Solution for https://habr.com/ru/post/540136/

customprint_incr.c

#include <stdio.h>
#include <string.h>

#define LIMIT 1000000000
#define CHUNK_COUNT 35000
#define CHUNK_NUMS 15 * CHUNK_COUNT
// max size of buffer for CHUNK_NUMS numbers (except 1B)
#define CHUNK_SIZE  (119 * CHUNK_COUNT)

void print();

char numbuf[13];
char* num;
int numlen;

int main(void) {
    // Keep trailing '\0' - useful for debug
    numbuf[11] = '\n';
    numbuf[10] = '1';
    num = numbuf + 10;
    // A digit and '\n'
    numlen = 2;

    int i;
    for (i = 1; i < LIMIT - CHUNK_NUMS; i += CHUNK_NUMS) {
        print();
    }
    while (i <= LIMIT) {
        if (0 == i % 3) {
            if (0 == i % 5) {
                printf("FizzBuzz\n");
            } else {
                printf("Fizz\n");
            }
        } else if (0 == i % 5) {
            printf("Buzz\n");
        } else {
            printf("%d\n", i);
        }
        i++;
    }

    return 0;
}

// 1 <= delta <= 3
void inc(int delta) {
    int carry = delta;
    char *cur;
    for (cur = numbuf + 10; cur >= num && carry; cur--) {
        int add = *cur - '0' + carry;
        carry = add >= 10;
        *cur = add - 10 * carry + '0';
    }
    *(num - 1) = '0' + carry;
    num -= carry;
    numlen += carry;
}

#define NUM do { memcpy(cur, num, numlen); cur += numlen; } while(0)
#define FIZZ do { memcpy(cur, "Fizz\n", 5); cur += 5; } while (0)
#define BUZZ do { memcpy(cur, "Buzz\n", 5); cur += 5; } while (0)
#define FIZZBUZZ do { memcpy(cur, "FizzBuzz\n", 9); cur += 9; } while (0)

void print() {
    static char wrkbuf[CHUNK_SIZE];

    char *cur = wrkbuf;
    int i;
    for (i = 0; i < CHUNK_COUNT; i++) {
        NUM;
        inc(1);
        
        NUM;
        FIZZ;
        inc(2);
        
        NUM;
        BUZZ;
        FIZZ;
        inc(3);
        
        NUM;
        inc(1);
        
        NUM;
        FIZZ;
        BUZZ;
        inc(3);

        NUM;
        FIZZ;
        inc(2);
        
        NUM;
        inc(1);
        
        NUM;
        FIZZBUZZ;
        inc(2);
    }
    fwrite(wrkbuf, cur - wrkbuf, 1, stdout);
}