Help understanding why my UART printf actually works - linker query

[John123]

NUCLEO-C031C6 board.

My printf over the UART works fine - would anyone be able to explain the 'bold' comment below.

How does such a simple bit of code overload the whole printf function.

What is 'small printf' ?

Regards, John.

#if defined(__GNUC__)

/* With GCC, small printf (option LD Linker->Libraries->Small printf

set to 'Yes') calls __io_putchar() */

#define PUTCHAR_PROTOTYPE int __io_putchar(int ch)

#endif

.

.

PUTCHAR_PROTOTYPE

{

/* Place your implementation of fputc here */

/* e.g. write a character to the USART2 and Loop until the end of transmission */

HAL_UART_Transmit(&huart2, (uint8_t *)&ch, 1, 0xFFFF);

return ch;

}

[STea · Accepted solution]

Hello @John123 ,

the printf implementation in the standard library is using low-level output function like __write which uses the __io_putchar() function to put characters in the std output so by surcharging this function to call the uart_transmit() function we will get printf printing over UART with the Virtual Com Port (VCP).

so, by redefining the __io_putchar() function declared as weak in the syscalls.c into your main you simply are redirecting the function that output characters in the standard output to output to UART.

BR

[John123]

Many thanks