In many situations when working with STM32 microcontrollers, you will want to output text strings. There is no need to write specialized functions that output specially formatted strings as it is hard to keep up with various cases. It is convenient to use standard I/O streams and its library functions that allows sending formatted data streams. Arm GCC toolchain comes with newlib C library from Redhat, and so it isn’t specially designed for embedded toolchain. To use stdio functions we have to take care of several syscals so-called “stub functions.” These functions usually are provided by operating systems like you would write C programs in Windows or Linux. In our case, we aren’t using any OS, os to avoid error messages while compiling we have to provide these function declarations where most of them are dummy implementations. It’s not something new pick one that you find on the internet. I noticed that it was written for STM32 Discovery.