4-7.c

#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#define BUFFER_SIZE 100

char buffer[100];
int buffer_index = 0;

void unget_character(int character) {
    if (buffer_index >= BUFFER_SIZE)
        printf("\nUngetch : Too many characters");
    else
        buffer[buffer_index++] = character;
}

int get_character() {
    if (buffer_index > 0)
        return buffer[--buffer_index];
    else
        return getchar();
}

void unget_string(char string[]) {
    int size = strlen(string);

    while (size > 0)
        unget_character(string[--size]);
}

int main() {
    unget_string("abcdef");

    char character;

    do {
        character = get_character();
        printf("%c", character);
    } while (character != 'f');

    return 0;
}