Number format for thousand separators in C programming language

number_format.c

#include <stdio.h>
#include <stdint.h>

/**
 * @brief Format a number in the format of 123.456.789
 * 
 * @param[out] outbuf output buffer
 * @param[in/out] outcaplen output buffer capacity + return length
 * @param[in] num number to format
 * @param[in] thousep thousands separator
 * 
 * @return error code (zero if success, -1 otherwise)
 **/
int number_format(char *outbuf, size_t *outcaplen, uint64_t num, char thousep)
{
    uint16_t t[10] = {0};
    uint8_t l = 0;
    while (num / 1000 != 0) {
        t[l++] = num % 1000;
        num /= 1000;
    }
    t[l++] = num % 1000;
    if (*outcaplen < l * 4) {
        *outcaplen = 0;
        return -1;
    }
    sprintf(outbuf, "%d", t[l---1]);
    uint8_t fo = strlen(outbuf);
    if (l > 0) {
        outbuf[fo++] = thousep;
        outbuf[fo] = 0;
        uint8_t j = 0;
        if (l > 1) {
            for (int i = l - 1; i > 0; --i) {
                sprintf(outbuf+fo+(j++*4), "%0*d%c", 3, t[i], thousep);
            }
        }
        sprintf(outbuf+fo+j*4, "%0*d", 3, t[0]);
    }
    *outcaplen = strlen(outbuf);
    return 0;
}

you need to include the following :
#include <string.h>
if you leave it out, you will get a warning!