Using printf Functions

What’s the deal with printf?

The printf and related functions (sprintf, vsprintf, etc.) are based on a relatively small implementation. However, they contribute around 8 KiB to the resultant program. It is highly recommended to not use printf and related functions at all because of this. If you insist on using these functions, this page details how to do so in the next section.

Alternatively, a limited sprintf implementation is baked into the OS which doesn’t add any extra size to the resultant program. Only the ‘c’, ‘d’, ‘u’, ‘x’, and ‘s’ format specifiers will probably work. To disable all other printf functions and use this sprintf implementation, add the following line to the Makefile:


If you just need to convert a float to a string, which the OS sprintf does not support, you can utilize the following float2str function instead.

void float2str(float value, char *str)
    real_t tmp_real = os_FloatToReal(value);
    os_RealToStr(str, &tmp_real, 8, 1, 2);

Using the printf functions

All the printf and related functions are defined in the standard stdio.h header, and can be used just as they would in any normal C program.

The output of character text is done via the outchar function, which is a special toolchain function that by default prints to the OS homescreen. It’s prototype looks like this:

void outchar(char character);

Reimplement this function in your program in any source file to change how characters are printed. For example, a horrid implementation of this function might look like the one below, which just prints to the OS homescreen.

void outchar(char character)
    char str[2] = { character, '\0' };


As an additional note, the outchar function is used by the standard putchar and puts functions – even if you have disabled the other printf functions, these two will still work as expected.