Thread: strings & arrays question

Code:

#include <stdio.h>

void readLine (char buffer[])  //function to read what I type in the terminal
{
    int i = 0, character;
    
    do
    {
        character = getchar ();
        buffer[i] = character;
        ++i;
    }
    while (character != '\n');
    
    buffer[i-1] = '\0';
}
    
int lenghtString (char source[]) // function to return the lenght of the string
{
    int count = 0;
    
    while (source[count] != '\0')
        
        ++count;
    
    return count;
}

void insertString (char source[], char insert[], int position) //Function to insert a string inside another string from a given position
{
    int i, j, charKeep, lenght;
    int lenghtString (char source[]);
    
    lenght = lenghtString (source) + lenghtString (insert);
    
    for (i = position, j = 0; i < lenght;)
    {
   
         if (insert[j] != '\0')
         {
            charKeep = source[i];
            source[i] = insert[j];
            insert[j] = charKeep;
            ++j;
            ++i;
         }
         else
             j = 0;
    }
         
    source[lenght] = '\0';
}

void showString (char source[]) //Function to show a string
{
    int i;
    
    for (i = 0; source[i] != '\0'; ++i)
    
        printf ("%c", source[i]);
    
    printf ("\n");
}

int main (void)
{
    char source[81], insert[81];
    void insertString (char source[], char insert[], int position), readLine (char buffer[]), showString (char source[]);
    int position;
    
    printf ("Ingrese la/s palabra/s:\n"); //type in the string
    readLine (source);
    
    printf ("Ingrese la palabra a insertar:\n"); //type in the string to be inserted
    readLine (insert);
    
    printf ("Ingrese la posicion en la cual desea insertarla: "); //type the position from where the previous string will be inserted
    scanf ("%i", &position);
    
    insertString (source, insert, position);

    showString (source);
    
    return 0;
}

This program is to insert a string inside another from a given position.
My question is why the function insertString works fine. I thought that an array, even if I set 81 to be the size, will "compress" somehow to the lenght of the string I type in the terminal and the new size is (less than 81) cant be expanded anymore. (pd: sorry for my english. If something is not clear let me know)