Add commas to string of digits

addcommas.c

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

// Cleanup: This is just for Rodney to see what's happening via printf()
void addCommas_debug(const char *num, char *buffer)
{
    char temp[100] = {0};
    int src = strlen(num) - 1;
    int dst = 0;
    
    while (src >= 0)
    {
        temp[dst] = num[src];
        printf("temp: %s\n", temp);
        
        src--;
        dst++;
        
        if (src < 0)
        {
            break;
        }
        
        if ((src - 1) % 3 == 0) {
            temp[dst] = ',';
            dst++;
        }
    }

    printf("temp buffer filled, src: %d dst: %d\n", src, dst);
    
    src = dst - 1;
    dst = 0;
    
    printf("reset indices src: %d dst: %d\n", src, dst);
    
    while (src >= 0)
    {
        buffer[dst++] = temp[src--];
        printf("%s\n", buffer);
    }
}

// Inserts commas every third digit of num, result is written to buffer.
void addCommas(const char *num, char *buffer)
{
    char temp[100] = {0};
    int src = strlen(num) - 1;
    int dst = 0;
    
    // Copy num into temp from back-to-front.
    // Add a comma every 3rd digit.
    while (src >= 0)
    {
        // Copy current char from num to temp
        temp[dst++] = num[src--];
        
        // End of num, break now to prevent extra comma
        if (src < 0)
            break;
        
        // Insert comma every third digit
        if ((src - 1) % 3 == 0) {
            temp[dst] = ',';
            dst++;
        }
    }

    // Reuse src and dst indices to reverse-copy temp buffer into final buffer
    src = dst - 1;
    dst = 0;
    while (src >= 0) {
        buffer[dst++] = temp[src--];
    }
}

int main()
{
    char num[20] = "12345678";
    char buffer[100] = {0};

    addCommas_debug(num, buffer);
    addCommas(num, buffer);

    printf(" input: %s\n", num);
    printf("result: %s\n", buffer);
    return 0;
}