Printf redirect to UART in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this UART printf redirection code?

Consider the following embedded C code that redirects printf output to UART. What will be printed on the UART when the main function runs?

Embedded C

#include <stdio.h>
#include <stdint.h>

// Simulated UART transmit function
void UART_Transmit(char c) {
    // In real hardware, this sends the char over UART
    putchar(c); // For simulation, print to stdout
}

// Redirect putchar to UART_Transmit
int __io_putchar(int ch) {
    UART_Transmit((char)ch);
    return ch;
}

int main() {
    printf("Hello UART!\n");
    return 0;
}

A UART_Transmit called with 'H' only

B Hello UART!\n

C No output, printf not redirected

D Compilation error due to missing function

💡 Hint

Think about how printf uses putchar internally and how __io_putchar redirects output.

The __io_putchar function is used by printf to output each character. Since __io_putchar calls UART_Transmit, which sends characters to UART (simulated here by putchar), the entire string "Hello UART!\n" is printed.

🧠 Conceptual

intermediate

Which function is typically overridden to redirect printf output to UART?

In embedded C, to redirect printf output to UART, which function do you usually need to implement or override?

A printf_uart

B UART_Init

C main

D __io_putchar

💡 Hint

This function is called internally by printf to output each character.

The __io_putchar function is called by printf to output characters. Overriding it to send characters via UART redirects printf output to UART.

🔧 Debug

advanced

Why does this UART printf redirection code fail to output anything?

Examine the code below. Why does printf not send any output to UART?

Embedded C

#include <stdio.h>

void UART_SendChar(char c) {
    // UART send implementation
}

int __io_putchar(int ch) {
    UART_SendChar(ch);
    return 0;
}

int main() {
    printf("Test\n");
    return 0;
}

A Because __io_putchar returns 0 instead of ch, printf buffering fails

B UART_SendChar is empty and does not send data

C printf requires UART_Init call before use

D Missing #include <stdint.h> causes failure

💡 Hint

Check the return value of __io_putchar and how printf uses it.

printf expects __io_putchar to return the character written. Returning 0 breaks printf's internal buffering, so no output appears.

📝 Syntax

advanced

Which option correctly implements __io_putchar for UART redirection?

Select the correct implementation of __io_putchar that sends a character over UART and returns the character.

A int __io_putchar(int ch) { UART_Send((char)ch); return ch; }

B void __io_putchar(int ch) { UART_Send(ch); }

C int __io_putchar(char ch) { UART_Send(ch); return ch; }

D int __io_putchar(int ch) { UART_Send(ch); return 0; }

💡 Hint

Check function return type and parameter types carefully.

Option A matches the expected signature and returns the character. Option A has wrong return type, C has wrong parameter type, A returns 0 which breaks printf.

🚀 Application

expert

How many characters are sent over UART by this code?

Given the code below, how many characters will UART_Transmit send when main runs?

Embedded C

#include <stdio.h>

void UART_Transmit(char c) {
    // Sends character over UART
}

int __io_putchar(int ch) {
    UART_Transmit((char)ch);
    return ch;
}

int main() {
    printf("Line1\nLine2\n");
    return 0;
}

A 11

B 10

C 12

D 13

💡 Hint

Count all characters including newline characters in the string.

The string "Line1\nLine2\n" has 5 characters + 1 newline + 5 characters + 1 newline = 12 characters total sent.