This is a libc implementation which is mostly compliant with the c89 standard. I've been working on this for a while as part of a disk-based OS for the TI, but this can still be used as an independent library.
I chose this standard since more recent ones mainly add support for wide characters, complex floating-point data types and other stuff which is probably not useful for a TI99/4a system.
All of the functions commonly used in stdio.h have been implemented, but there are two unresolved symbols: console_write and console_read. These are intended to provide text-based screen output and keyboard input respectively. In my OS project, these are provided by device drivers, but Tursi's libti99 would be better for anyone intending to use the stdio functions.
Hopefully this library is useful for someone. Feel free to do whatever you want with this.