To use the standard output functions putchar, puts, and printf, you need to customize the way that characters are written to the standard output device. These output functions rely on a function __putchar that outputs a character and returns an indication of whether it was successfully written.

The prototype for __putchar is

    int __putchar(int ch);

###### Sending all output to the CrossStudio virtual terminal

You can send all output to the CrossStudio virtual terminal by supplying the following implementation of the__putchar function in your code:

    #include <__cross_studio_io.h>

    int __putchar(int ch)
    {
      return debug_putchar(ch);
    }

This hands off output of the character ch to the low-level debug output routine, debug_putchar.

Whilst this is an adequate implementation of __putchar, it does consume stack space for an unnecessary nested call and associated register saving. A better way of achieving the same result is to define the low-level symbol for __putchar to be equivalent to the low-level symbol for debug_putchar. To do this, we need to instruct the linker to make the symbols equivalent. To do this:

- Select the project node in the Project Explorer.
- Display the Properties Window.
- Enter the text “-D___putchar=_debug_putchar” into the Additional Options property of the Linker Options group.

Note that there are three leading underscores in ___putchar and a single leading underscore in _debug_putchar because the C compiler automatically prepends an underscore to all global symbols.

###### Sending all output to another device

If you need to output to a physical device, such as a UART, the following notes will help you:

- If the character cannot be written for any reason, putchar must return EOF.
- The higher layers of the library do not translate C's end of line character '\n' before passing it to putchar. If you are directing output to a serial line connected to a terminal, for instance, you will most likely need to output a carriage return and line feed when given the character '\n' (ASCII code 10).