Standard input in C is typical line-buffered. This means data entered via the keyboard is stored in a temporary memory area—the input buffer—until the Enter key is pressed. Only then is the entire line, including the newline character (\n), made available to reading functions.

Funcsions like getchar() interact directly with this buffer. When called, getchar() attempts to read a single character. If the buffer is empty, the program blocks, waiting for user input. Once a line is entered and buffered, successive calls to getchar() will consume characters from the buffer sequentially until it is empty, at which point the program will block again for new input.

This behavior is key to solving problems where prompt output must appear in a specific sequence relative to input. Consider a task to implement a Caesar cipher where the output label "Ciphertext: " must be printed after the user begins typing but before the final encrypted text is displayed, all without using arrays or scanf().

The solution leverages the blocking nature of getchar(). The first call to getchar() forces the program to wait for the enitial user input line. Once entered, this first character is consumed. Immediately after, printf() can be used to output the "Ciphertext: " label. Subsequent calls to getchar() within a loop will then process the remaining buffered characters from the same input line.

Here is a revised implementation of the Caesar cipher demonstrating this technique, with restructured logic and renamed variables:

#include <stdio.h>

char caesar_shift(char input_char, int direction) {
    int offset = 3; // Shift value for Caesar cipher
    if (direction == 0) offset = -offset; // For decryption

    // Process only alphabetic characters
    if (input_char >= 'a' && input_char <= 'z') {
        return ((input_char - 'a' + offset + 26) % 26) + 'a';
    }
    if (input_char >= 'A' && input_char <= 'Z') {
        return ((input_char - 'A' + offset + 26) % 26) + 'A';
    }
    // Return non-alphabetic characters unchanged
    return input_char;
}

void process_stream_cipher() {
    int encrypt_mode = 1; // 1 for encrypt
    char current_char;

    printf("Plaintext: ");
    // First getchar() blocks, waiting for user input line
    current_char = getchar();

    printf("Ciphertext: ");
    // Process all characters from the buffer until newline
    while (current_char != '\n') {
        if ((current_char >= 'a' && current_char <= 'z') || 
            (current_char >= 'A' && current_char <= 'Z')) {
            current_char = caesar_shift(current_char, encrypt_mode);
        }
        putchar(current_char);
        current_char = getchar(); // Gets next char from buffer
    }
    putchar('\n');
}

int main() {
    process_stream_cipher();
    return 0;
}

Execution flow:

1. Program prints "Plaintext: ".
2. current_char = getchar(); executes, causing the program to wait.
3. User types ABCabcxyzXYZ and presses Enter. The entire string "ABCabcxyzXYZ\n" is placed into the input buffer.
4. The first getchar() returns 'A' and removes it from the buffer.
5. Program prints "Ciphertext: ".
6. The while loop begins, calling getchar() repeatedly. Each call instantly retrieves the next character ('B', 'C', 'a', ...) from the buffer with out further user interaction.
7. Each character is shifted and printed.
8. The loop ends when getchar() finally retrieves the '\n' character.

This approach effectively separates the input prompt from the output label using the buffer's blocking mechanism, meeting the constraint of no array usage.