C program to implement substring function

/*C program to implement substring function.*/

#include <stdio.h>

/*
function : substring
argument list:
1) source - source string
2) from - starting index from where you want to 
get substring.
3) n - number of characters to be copied in substring
4) target - target string in which you want to store
targe string.
Return type - int : 0 on success
*/

int substring(char* source, int from, int n, char* target)
{
    int length, i;
    //get string length
    for (length = 0; source[length] != '\0'; length++)
        ;

    if (from > length) {
        printf("Starting index is invalid.\n");
        return 1;
    }

    if ((from + n) > length) {
        //get substring till end
        n = (length - from);
    }

    //get substring in target
    for (i = 0; i < n; i++) {
        target[i] = source[from + i];
    }
    target[i] = '\0'; //assign null at last

    return 0;
}

int main()
{
    char str[100], targetString[100];
    int from, n;

    printf("Enter any string:");
    gets(str);

    printf("Enter from index (count from 0): ");
    scanf("%d", &from);

    printf("Enter number of characters: ");
    scanf("%d", &n);

    if (substring(str, from, n, targetString) == 0) {
        printf("Substring is: %s\n", targetString);
    }

    return 0;
}