Two important notes:

1) Semihosting calls are extremely slow. They halt the processor until the debug probe polls the target, notices that it's stopped at a semihosting breakpoint, handles that, and resumes it. This can take tens to hundreds of milliseconds per call.

2) You technically need to initialize the semihosting file descriptors with an open() call before using them. Many debuggers will allow you to use the descriptors without initialization, but not all will.