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.

Practice

1. What is the main purpose of overriding the fputc function in embedded C when redirecting printf output to UART?

easy

A. To clear the UART receive buffer

B. To change the baud rate of the UART communication

C. To initialize the UART peripheral before use

D. To send each character from printf to the UART hardware for transmission

Solution

1. Step 1: Understand printf output mechanism

   The printf function sends characters one by one to fputc for output.

2. Step 2: Role of overriding fputc

   By overriding fputc, we redirect each character to UART transmit function, sending it out via UART.

3. Final Answer:

   To send each character from printf to the UART hardware for transmission -> Option D

4. Quick Check:

   Overriding fputc redirects output to UART [OK]

Hint: Remember: printf calls fputc for output [OK]

Common Mistakes:

• Confusing fputc with UART initialization
• Trying to change UART settings inside fputc
• Ignoring that printf uses fputc internally

2. Which of the following is the correct prototype for overriding fputc to redirect printf output to UART in embedded C?

easy

A. char fputc(int ch, FILE *f);

B. void fputc(char ch);

C. int fputc(FILE *f, char ch);

D. int fputc(char ch, FILE *f);

Solution

1. Step 1: Recall standard fputc signature

   The standard fputc function prototype is int fputc(int ch, FILE *stream);.

2. Step 2: Match options with correct signature

   char fputc(int ch, FILE *f); matches the correct order and types: returns char (though standard is int), takes int and FILE pointer.

3. Final Answer:

   char fputc(int ch, FILE *f); -> Option A

4. Quick Check:

   Correct fputc prototype returns int and takes int, FILE* [OK]

Hint: Prototype must match standard fputc signature [OK]

Common Mistakes:

• Swapping parameter order
• Using void return type
• Using wrong parameter types

3. Given the following code snippet, what will be the output on the UART terminal?

int fputc(int ch, FILE *f) {
    UART_SendChar((char)ch);
    return ch;
}

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

medium

A. Only 'H' is sent to UART

B. No output because UART_SendChar is not defined

C. Hi followed by a newline on UART terminal

D. Compilation error due to missing UART initialization

Solution

1. Step 1: Understand fputc role in printf

   Each character of "Hi\n" is sent to fputc, which calls UART_SendChar to send it.

2. Step 2: Output on UART terminal

   Characters 'H', 'i', and newline '\n' are sent sequentially, so "Hi" and a newline appear on UART terminal.

3. Final Answer:

   Hi followed by a newline on UART terminal -> Option C

4. Quick Check:

   printf output redirected via fputc to UART [OK]

Hint: Each printf char calls fputc, which sends via UART [OK]

Common Mistakes:

• Assuming no output without UART initialization code shown
• Thinking only first char is sent
• Confusing compile errors with runtime behavior

4. Identify the error in this fputc override code that tries to redirect printf to UART:

int fputc(int ch, FILE *f) {
    UART_SendChar(ch);
    return 0;
}

medium

A. Returning 0 instead of the character causes incorrect printf behavior

B. Passing ch directly to UART_SendChar causes type mismatch

C. Missing UART initialization before sending characters

D. Function should return void, not int

Solution

1. Step 1: Check return value of fputc

   The fputc function must return the character written on success, or EOF on failure.

2. Step 2: Analyze the code's return statement

   Returning 0 (null char) instead of the character can cause printf to misbehave or stop output prematurely.

3. Final Answer:

   Returning 0 instead of the character causes incorrect printf behavior -> Option A

4. Quick Check:

   Return written char from fputc to keep printf working [OK]

Hint: Always return the character from fputc [OK]

Common Mistakes:

• Returning 0 instead of the character
• Ignoring return value importance
• Assuming void return type is allowed

5. You want to redirect printf output to UART and also support multiple UART ports. Which approach correctly extends fputc to select UART port dynamically?

hard

A. Create separate fputc functions for each UART port and call them manually

B. Use a global variable to store current UART port and send chars accordingly inside fputc

C. Modify printf to accept UART port as parameter

D. Use interrupts to switch UART ports automatically during fputc

Solution

1. Step 1: Understand fputc limitation

   Only one fputc function can be linked, so multiple versions are not possible.

2. Step 2: Use global variable for UART selection

   By storing the current UART port in a global variable, fputc can send characters to the selected UART dynamically.

3. Step 3: Evaluate other options

   Modifying printf is complex; interrupts do not switch ports automatically; multiple fputc functions cause conflicts.

4. Final Answer:

   Use a global variable to store current UART port and send chars accordingly inside fputc -> Option B

5. Quick Check:

   Global UART selector enables dynamic port choice in fputc [OK]

Hint: Use global UART variable to select port inside single fputc [OK]

Common Mistakes:

• Trying multiple fputc functions
• Changing printf signature
• Relying on interrupts to switch UART ports