System Workbench for STM32

I have syscall.c in my SW4STM32 project.
I do this below:
"In main.c , #include “stdio.h” , also create the new implementation for this method ;
void __io_putchar(uint8_t ch) {
HAL_UART_Transmit(&huart1, &ch, 1, 1);
}
"

However I get no printf output . Calling HAL_UART_Transmit(&huart3, &ch, 1, 1); directly works.

I get a warning message that probably explains the problem:
'__io_putchar' defined but not used -Wunused-function main.c /myproject/Core/Src line 150 C/C++ Problem

Why is __io_putchar not being used ?