Maximum number of elements that can be stored in an array in c

You mix two notions: the notion of arrays and the notion of strings.

In this declaration

char character_array[4];

there is declared an array that can store 4 objects of type char. It is not important what values the objects will have.

On the other hand the array can contain a string: a sequence of characters limited with a terminating zero.

For example you can initialize the array above in C the following way

char character_array[4] = { 'A', 'B', 'C', 'D' };

or

char character_array[4] = "ABCD";

or

char character_array[4] = { '\0' };

or

char character_array[4] = "";

and so on.

In all these cases the array has 4 objects of type char. In the last two cases you may suppose that the array contains strings (empty strings) because the array has an element with zero character ( '\0' ). That is in the last two cases you may apply to the array functions that deal with strings.

Or another example

char character_array[4] = { 'A', 'B', '\0', 'C' };

You can deal with the array as if it had a string "AB" or just four objects.

Consider this demonstrative program

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

int main( void )
{
    char character_array[4] = { 'A', 'B', '\0', 'C' };

    char *p = strchr(character_array, 'C');

    if (p == NULL)
    {
        printf("Character '%c' is not found in the array\n", 'C');
    }
    else
    {
        printf("Character '%c' is found in the array at position %zu\n",
            'C',
            (size_t)(p - character_array));
    }

    p = ( char * )memchr(character_array, 'C', sizeof(character_array));

    if (p == NULL)
    {
        printf("Character '%c' is not found in the array\n", 'C');
    }
    else
    {
        printf("Character '%c' is found in the array at position %zu\n",
            'C',
            (size_t)(p - character_array));
    }
}

The program output is

Character 'C' is not found in the array
Character 'C' is found in the array at position 3

In the first part of the program it is assumed that the array contains a string. The standard string function strchr just ignores all elements of the array after encountering the element with the value '\0'.

In the second part of the program it is assumed that the array contains a sequence of objects with the length of 4. The standard function memchr knows nothing about strings.

Conclusion.

This array

char character_array[4];

can contain 4 objects of type character. It is so declared.

The array can contain a string if to interpret its content as a string provided that at least one element of the array is equal to '\0'.

For example if to declare the array like

char character_array[4] = "A";

that is equivalent to

char character_array[4] = { 'A', '\0', '\0', '\0' };

then it may be said that the array contains the string "A" with the length equal to 1. On the other hand the array actually contain 4 object of type char as the second equivalent declaration shows.

You just reserve 4 bytes to fill with. If you write to _array[4] (the fifth character) you have a so called buffer overflow, means you write to non-reserved memory.

If you store a string in 4 characters, you have actually just 3 characters for printable characters (_array[0], ..., _array[2]) and the last one (_array[3]) is just for keeping the string termination '\0'.

For instance, in your case the function strlen() parses until such string termination '\0' and returns length=3.