"undefined reference to `__putchar'" error message

I'm getting an "undefined reference to `__putchar'" error message, what does this mean?

This error indicates that you are using a stdio output function such as printf and that you haven't implemented the __putchar function that is required to actually carry out the output.

As an example, the following code demonstrates an implementation of __putchar that outputs to a UART:

void 
__putchar(int ch)
{
  if (ch == '\n')
    UARTWriteChar('\r');
  UARTWriteChar(ch);
}

Note: If all want to do is make the stdio functions output to the debug terminal rather than hardware you should set the Library Options > I/O Library Name project property to debug or, alternatively, use the debug I/O functions directly (for example use debug_printf rather than printf).