如何修复此replaceSubstring函数？

Question

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

void replaceSubstring(char *str, char *substr);
int main()
{
    int doloop = 1;
    char *str = (char *)(malloc(100)); 
    if (str == NULL)
    {
        printf("No memory allocated");
        return;
    }
    char *substr = (char *)(malloc(100));
    if (substr == NULL)
    {
        printf("No memory allocated");
        return;
    }
    // we gets the text and substring, then we check if the substring is in the text the we change the letters from small to capital .
    while (doloop)
    {
        printf("Enter Text: ");
        gets_s(str, 100);
        //empty text
        if (*str == '\0')
        {
            printf("Finish!\n");
            doloop = 0;
            return;
        }
        printf("Enter substring: ");
        gets_s(substr, 100);
        //empty substring
        if (*substr == '\0')
        {
            printf("Finish!\n");
            doloop = 0;
            return;
        }
        if (*substr)
            replaceSubstring(str, substr);
        puts(str);
    }
    free(substr);
    free(str);
    _getch();
    return 0;
}
/*Function name: replaceSubstring
Input: char *str, char *substr
Output: upercase letter
Algorithm: we used for loop to check every letter if its english letter, and then we replace from lower case to uper case*/

void replaceSubstring(char *str, char *substr)
{
    int i, len;
    char *p;
    p = strstr(str, substr);
    len = strlen(substr);
    while (p)
    {
        for (i = 0; i < len; i++)
        {
            // we check if the letters is small english letters
            if (*p <= 'z'&&*p >= 'a')
            {
                *p = *p - 32; // from small letter to capital letter 
                p++;
            }
            //else p++;
        }
        p = strstr(str, substr);
        replaceSubstring(str, substr);
    }
}

该代码应要求用户输入例如：替换出现的“ r”

然后要求用户输入子字符串，例如： R

然后，代码应检查在文本中找到此子字符串的位置并将其从小写改为大写，但是在这种情况下，当我们输入R时在文本中找不到它，因此答案应与我们输入的文本相同没有改变。 但是当我输入Substring时，代码停止了！