Printf redirect to UART in Embedded C - Step-by-Step Execution

Concept Flow - Printf redirect to UART

Start

↓

Call printf

↓

printf calls _write

↓

_write sends data to UART

↓

UART transmits data

↓

Data appears on terminal

↓

End

When printf is called, it internally calls the _write function, which we redirect to send characters over UART, so output appears on a terminal.

Execution Sample

Embedded C

int _write(int file, char *ptr, int len) {
  for (int i = 0; i < len; i++) {
    UART_SendChar(ptr[i]);
  }
  return len;
}

This code redirects printf output by sending each character to UART.

Execution Table

Step	Action	Input	UART_SendChar Called With	Return Value
1	printf called	"Hello\n"
2	printf calls _write	"Hello\n", length=6
3	Loop i=0	ptr[0] = 'H'	'H'
4	Loop i=1	ptr[1] = 'e'	'e'
5	Loop i=2	ptr[2] = 'l'	'l'
6	Loop i=3	ptr[3] = 'l'	'l'
7	Loop i=4	ptr[4] = 'o'	'o'
8	Loop i=5	ptr[5] = '\n'	'\n'
9	_write returns			6
10	printf completes

💡 All characters sent to UART, _write returns length, printf finishes.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	Final
i	0	1	2	3	4	5	6	6
ptr[i]	'H'	'e'	'l'	'l'	'o'	'\n'

Key Moments - 3 Insights

Why does printf call _write instead of sending characters directly?

printf uses _write as a low-level output function. Redirecting _write lets us control where printf sends its output, as shown in steps 2-9 of the execution_table.

What happens if UART_SendChar is not implemented correctly?

If UART_SendChar fails, characters won't be sent, so output won't appear on the terminal. The loop in steps 3-8 depends on UART_SendChar working properly.

Why does _write return the length of characters sent?

_write returns the number of characters sent to signal success to printf, as seen in step 9. This tells printf all data was handled.

Visual Quiz - 3 Questions

Look at the execution_table, what character does UART_SendChar send at step 5?

A 'l'

B 'H'

C 'e'

D 'o'

💡 Hint

Check the 'UART_SendChar Called With' column at step 5 in execution_table.

At which step does _write finish sending all characters?

A Step 8

B Step 9

C Step 10

D Step 6

💡 Hint

Look for when _write returns in the execution_table.

If the input string length was 4 instead of 6, how would the variable 'i' final value change?

A Final value would be 6

B Final value would be 0

C Final value would be 4

D Final value would be 5

💡 Hint

Check variable_tracker for 'i' values and relate to string length.

Concept Snapshot

Redirect printf output by implementing _write function.
_write sends each character to UART using UART_SendChar.
printf calls _write internally to output data.
Return number of characters sent from _write.
This lets printf output appear on UART terminal.

Full Transcript

When you use printf in embedded C, it doesn't send characters directly. Instead, it calls a function named _write. By writing your own _write function, you can send each character to UART using UART_SendChar. This way, all printf output goes through UART and appears on your terminal. The _write function loops over each character in the string, sends it via UART, and returns the total number of characters sent. This process is shown step-by-step in the execution table, tracking each character sent and the loop variable. Understanding this flow helps you redirect printf output to UART easily.