#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
void add_symbol(char **line, char symb, int len)
{
    int new_size = (len+2)*sizeof(char); //+1 for symbol, +2 for \0
    char *line_copy = (char*) malloc(new_size);
    strcpy(line_copy, *line);
    line_copy[len] = symb;
    line_copy[len+1] = '\0';
    *line = (char *) realloc(*line, new_size);
    strcpy(*line, line_copy);
    free(line_copy);
}
int main()
{
    char input;
    char *final_line = (char*) malloc(sizeof(char)*1);
    final_line[0] = '\0';
    while ((input = getchar()) != '0') //input != '0'
    {
        if (isalpha(input))
            add_symbol(&final_line, input, strlen(final_line));
        else if (input != '\n') //\n, \r and other != EOF
            printf("Invalid input: %c\n", input); //
    }
    printf("Final line: \"%s\"\n", final_line);
    free(final_line);
    return 0;
}