https://hejnoah.com/posts/tostring.html

inttostr.c

#include <math.h>
#include <stdlib.h>
#include <stdio.h>

char *inttostr_conventional(int n) {
    char *s = (char*) malloc(sizeof(char) * 42);
    sprintf(s, "%d", n);
    return s;
}

char *inttostr_log(int n) {
    int len;
    if (n > 0) {
        len = (int) floor(log10(n)) + 1;
    } else if (n == 0) {
        len = 1;
    } else if (n < 0) {
        len = (int) floor(log10(-n)) + 2;
    }
    len += 1;

    char *s = (char*) malloc(sizeof(char) * len);
    sprintf(s, "%i", n);
    return s;
}

char *inttostr_magic(int n) {
    int len = snprintf((char*) 0, 0, "%i", n) + 1;
    char *s = (char*) malloc(sizeof(char) * len);
    snprintf(s, len, "%i", n);
    return s;
}

void inttostr_benchmark() {
    char *s = (char*) 0;
    for (int i = -1000000; i < 1000000; i++) {
        s = inttostr_conventional(i); // => 0.365s
        //s = inttostr_log(i);        // => 0.379s
        //s = inttostr_magic(i);      // => 0.544s
    }
}

int main() {
    inttostr_benchmark();
    printf("%s", inttostr_magic(42));
}