Character replacement and substitution in C - c

Question

I'm trying to write a code which will replace character in the string user selects with character he/she does. Eg string london if user picks o and a then the output should be landan.
Here is the code:
#include <stdio.h>
#include <string.h>
#define MAXLEN 100
int function2(char str[], char drop, char sub) {
int i = 0; int num = 0;
while (str != NULL) {
if (str[i] == drop) {
str[i] = sub;
num++;
}
i++;
}
return num;
}
int main() {
char d, s;
char my_string[MAXLEN];
printf("Input your string\n");
scanf("%s", &my_string);
printf("Input character you want to drop\n");
scanf(" %c", &d);
printf("Now character you want to substitute\n");
scanf(" %c", &s);
function2(my_string, d, s);
printf("The string is %s\n", my_string);
return EXIT_SUCCESS;
}
It works up until the point where you actually print the altered string. All I get is Segmentation fault (core dumped). Note that code for function was not mine (I found it on some website, so owner of the original code for function2- Thank you in advance). Any help would be appreciated!

Answer 1

First of all, you should avoid using scanf. If you're interested for the reason and alternatives click here.
But back to your problem
while(str != NULL)
is an infinite loop, because the pointer won't become NULL
while(str[i] != '\0')
should do the trick. It checks each time if you've already arrived at the end of the string.

Answer 2

if (str != null){
while(str[i] != '\0'){
if (str[i] == drop){
str[i] = sub;
num++;
}
i++;
}
}
str is a char array, with str != NULL, you check that the array point to a valid memory address.
With while loop and i++, you loop all characters in array, because the string ends with '\0', you need to stop the loop with while(str[i] != '\0').

Answer 3

Your function runs an infinite loop because str never become NULL, but since i is incremented, str[i] will eventually access memory beyond the end of the string and at some point invalid memory causing a Segmentation fault.
Note also that it is not simple to tell scanf() the maximum number of characters to read into my_string. Using fgets() is much safer and allows a whole phrase to be substituted.
Here is a corrected version:
#include <stdio.h>
#include <stdlib.h>
#define MAXLEN 100
int function2(char str[], char drop, char sub) {
int num = 0;
for (int i = 0; str[i] != '\0'; i++) {
if (str[i] == drop) {
str[i] = sub;
num++;
}
}
return num;
}
int main(void) {
char d, s;
char my_string[MAXLEN];
printf("Input your string\n");
if (!fgets(my_string, MAXLEN, stdin))
return EXIT_FAILURE;
printf("Input character you want to drop\n");
if (scanf(" %c", &d) != 1)
return EXIT_FAILURE;
printf("Now character you want to substitute\n");
if (scanf(" %c", &s) != 1)
return EXIT_FAILURE;
function2(my_string, d, s);
printf("The modified string is %s", my_string);
return EXIT_SUCCESS;
}