How to find common characters in two strings in c code example

Example: how to count the most common letter in a string c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define ENG_LETTERS 26
#define STR_LEN 100

int main()
{
    int countArray[ENG_LETTERS] = {0};
    char str[STR_LEN] = { 0 };
    char engChar = 'a' - 1;
    int length = 0;
    int i , j = 0;
    int max = 0;
    int index = 0;
    printf("Please enter a string:\n");
    fgets(str , STR_LEN , stdin);
    length = strlen(str);
    for(i = 0 ; i <= length;i++)
    {
        if(str[i] == '\n')
        {
            str[i] = '\0';
        }
    }
    for(i = 0;i < ENG_LETTERS;i++)
    {
        engChar++;
        for(j = 0;j <= length - 1;j++)
        {
            if(str[j] == engChar)
            {
                countArray[i]++;
            }
        }
    }
    engChar = 'a'- 1;
    for(i = 0; i <= ENG_LETTERS - 1;i++)
    {
        engChar++;
        printf("There are %d %c letters\n", countArray[i],engChar);

    }

    system("PAUSE");
    return 0;
}