Problem with finding the length of a string using pointers in a function in c

In your function signature, you are telling the compiler that lenghtchar() expects a pointer to character strings, or **char in other words.

What you really want to do is to change your function from int lengthchar(char *array1[]) to int lengthchar(char array1[]) or int lengthchar(char *array1). This is a bit tricky since in C, you can address an array by using the address of its first element (aka, by using pointer to its first item).

Expert C Programming has a dedicated chapter on this topic.

Now, coming to your lengthchar() function, I would do some refactoring to eliminate the variable a and use a while loop instead. I have also included another alternative implementation that relies on pointer arithmetic (more fun to me :) )

Note also that I used fgets() instead of gets() which is considered deprecated since it does not do any bounds checking.

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

#define max 100

/* 
 * returns the lenght of the string excluding the terminating 
 * NULL character
 */
int lengthchar(char *array1) {
    int x = 0;
    while (*array1++)
        x++;
    return x-1;
}

int lengthchar1(char *array1){
  char *p;
  for (p = array1; *p; p++)
  ;
  return p - array1 - 1; /* -1 for \0 */
}

int main() {
    char arr1[max];
    int length;

    printf("Enter string\n");
    fgets(arr1, max, stdin);

    length = lengthchar(arr1);

    printf("Length=%d", length);

    return 0;
}

On declaring or defining a function that shall take an array of type T as an argument, use the notation: T *array or T array[]; T may stand for any valid C data type such as char, int, float, double, etc...

As for your loop, the variable a seems to be redundant because it has no effect on any part of the program. The return value of the function lengthchar() is one more than the number of characters inputted.

Also you should avoid the function gets() because it is deemed dangerous and has been removed from the C standard; use fgets() instead.

Your code should look something like this:

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

#define max 100

int lengthchar(char *str)
{
    int len = 0;
    while (*str++)
    {
        len++;
    }
    return len;
}

int main()
{
    char str1[max];

    printf("Enter string:");

    fgets(str1, max, stdin);

    str1[strcspn(str1, "\n")] = 0; // remove new-line character

    int length = lengthchar(str1);

    printf("Length = %d\n", length);
    return 0;
}