How to count number characters of strings without using strlen

Question

I have the task to count the number of letters in random words until "End" is entered. I'm not allowed to use the strlen(); function. That's my solution so far:

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

int stringLength(char string[]){
    unsigned int length = sizeof(*string) / sizeof(char);
    return length;
}

int main(){
    char input[40];
    
    while (strcmp(input, "End") != 0) {
        printf("Please enter characters.\n");
        scanf("%s", &input[0]);
        while (getchar() != '\n');
        printf("You've entered the following %s. Your input has a length of %d characters.\n", input, stringLength(input));
    }
}

The stringLength value isn't correct. What am I doing wrong?

Answer 1

The %n specifier could also be used to capture the number of characters.
Using %39s will prevent writing too many characters into the array input[40].

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

int main( void)
{
    char input[40] = {'\0'};
    int count = 0;

    do {
        printf("Please enter characters or End to quit.\n");
        scanf("%39s%n", input, &count);
        while (getchar() != '\n');
        printf("You've entered the following %s. Your input has a length of %d characters.\n", input, count);
    } while (strcmp(input, "End") != 0);

    return 0;
}

EDIT to correct flaw pointed out by @chux.
using " %n to record leading space and %n" record total characters this should record the number of leading whitespace and the total characters parsed.

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

int main( int argc, char *argv[])
{
    char input[40] = {'\0'};
    int count = 0;
    int leading = 0;

    do {
        printf("Please enter characters. Enter End to quit.\n");
        if ( ( scanf(" %n%39s%n", &leading, input, &count)) != 1) {
            break;
        }
        while (getchar() != '\n');
        printf("You've entered %s, with a length of %d characters.\n", input, count - leading);
    } while (strcmp(input, "End") != 0);

    return 0;
}

EDIT stringLength() function to return length

int stringLength(char string[]){
    unsigned int length = 0;
    while ( string[length]) {// true until string[length] is '\0'
        length++;
    }
    return length;
}