For my reference:

I've found libapreq a library for uploading files to Apache, I can use to accept uploaded RTF files in mod_virgule.

I also found the apache API listing on dev.apache.org.

Also found *NIX programming guide. Which I needed for the system call I need in order to launch the converter.

I've been Win32 API programming too long...

By the way, I sniffed out the error that was causing the the RTF converter to seg-fault... It had the smell of a stack overrun - and sure enough there it was a printf writing a number into a locally declared string (char str[64]) which was big enough until the number exceeds 9... result stack gets overwritten and program crashes when the function returns. It only seemed to affect the Linux version, not the Windows one. Probably would crash either/or at random if you tried converting enough documents...