Reversing a string in c, problem using strlen function

Question

I wrote a code to reverse a string, but the strlen function is giving me the wrong length of string, that's why the reversing of the string is not done properly. Here is the code I wrote:

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

void reversestring(char string[], int start, int end);

int main() {
    char str[500];
    int n;
    n = strlen(str);
    reversestring(str, 0, n - 1);
    printf("%d\n", n);
    printf("The reverse string is %s", str);
    return 0;
}

void reversestring(char string[], int start, int end) {
    printf("enter the string:\n");
    scanf("%s", string);
    int temp;
    while (start < end) {
        //printf("insidewhile\n");
        temp = string[start];
        string[start] = string[end];
        string[end] = temp;
        start++;
        end --;
    }
}

Answer 1

strlen() can´t give you the length of a string, when its argument does not point to a valid string, which is the case on your example:

char str[500];
int n;
n = strlen(str);

str isn´t initialized with a string.

Providing a pointer to a char array which doesn´t contain a string as argument to strlen() causes undefined behavior.

Also strlen() doesn´t return an int. It´s return value is of type size_t.

Also use fgets() instead of scanf() when input a string. It is a little bit more safe.

---

Solution:

Let the string get entered in main() into str, then use strlen() and thereafter call the reversestring() function:

char str[500];         
size_t n;

printf("enter the string:\n");
fgets(str,500,stdin);

n = strlen(str);   
reversestring(str, 0, n-1);

I also edited the function declaration and the printf() accordingly to take care of the size_t type.

---

Here is the full code (Online Example):

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

void reversestring(char string[], size_t start, size_t end);

int main()
{
    char str[500];
    size_t n;

    printf("enter the string:\n");
    fgets(str,500,stdin);
    str[strcspn(str, "\n")] = 0;    // removing trailing newline from fgets

    n = strlen(str);   
    reversestring(str, 0, n-1);

    printf("%zu\n", n);
    printf("The reverse string is %s", str);
    return 0;
}


void reversestring(char string[], size_t start, size_t end)
{
    int temp;
    while(start < end)
    {   //printf("insidewhile\n");
        temp = string[start];
        string[start] = string[end];
        string[end] = temp;
        start++;
        end --;
    }
}

Output:

enter the string:             
helloworld     
10                     
The reverse string is dlrowolleh