Why does my number variable change after running the 'insert' function?

Question

I have been looking at this code of mine for my schoolwork but I cannot understand why my integer variable has changed after I passed it into a function by value although I did not alter it in the function.

#include <stdio.h>
#include <stdlib.h>

void Q1();
void Q1_Remove (char **ptr_string);
void Q1_Insert (char **ptr_string, int length);

int main()
{
    Q1();

    return 0;
}

void Q1 () {
    int number;
    printf("How many characters do you want to input: ");
    scanf("%d", &number);
    char *string = malloc(number + 1);
    printf("Input the string: ");
    scanf("%s", string);
    printf("The string is: %s\n", string);

    int option;
    do {
        printf("Do you want to 1-Insert, 2-Remove or 3-Quit: ");
        scanf("%d", &option);
        switch (option) {
            case 1:
                Q1_Insert(&string, number);
                break;
            case 2:
                Q1_Remove(&string);
                break;
        }
        if (option == 1 || option == 2) {
            printf("Resulting string: %s\n", string);
        }
    } while (option == 1 || option == 2);

    free(string);
}

void Q1_Remove (char **ptr_string) {
    (*ptr_string)++;
}

void Q1_Insert (char **ptr_string, int length) {
    int curr_len = strlen(*ptr_string);
    int i;
    printf("length is %d and curr_len is %d", length, curr_len);
    if (curr_len == length) {
        for (i = curr_len - 1; i > 0; --i) {
            *(ptr_string + i) = *(ptr_string + i - 1);
        }
    } else {
        (*ptr_string)--;
    }
    char to_insert;
    printf("What is the character you want to insert: ");
    scanf(" %c", &to_insert);
    **ptr_string = to_insert;
}

My terminal input and output:

How many characters do you want to input: 5
Input the string: abcde
The string is: abcde
Do you want to 1-Insert, 2-Remove or 3-Quit: 1
length is 5 and curr_len is 5
What is the character you want to insert: a
Resulting string: abcde
Do you want to 1-Insert, 2-Remove or 3-Quit: 1
length is 11212224 and curr_len is 5                     // length is weird here
What is the character you want to insert:

(EDIT 2) Removed constant for variable number and added in the declaration on the top. Edited to add in the headers!

Answer 1

Primarily the issue is how you are shuffling characters in the string. You are doing this:

for (i = curr_len - 1; i > 0; --i) {
    *(ptr_string + i) = *(ptr_string + i - 1);
}

However, the actual string pointer is *ptr_string . Since you are using ptr_string + i , it's treating ptr_string as an array of pointers, and you are writing all over the stack memory following your string pointer defined in main . This will be what is trashing your number value, among other things.

Instead, you need to do this:

*(*ptr_string + i) = *(*ptr_string + i - 1);

Personally, I'd use array notation instead, which is far more readable:

(*ptr_string)[i] = (*ptr_string)[i - 1];

Or simply scrap the loop entirely and use memmove .