How does a program written for an os find os functions?

[vinny]

I plan on writing an operating system, mainly in c, and I want the user to be able to run custom programs. How does the program use operating system functions without having to recompile the os?

Do I need to make custom header files and linker scripts for the cross compiler?

[klange]

While designs for user programs can vary, generally they do not link to the kernel or "use operating system functions" directly - they use system calls.