How can I communicate between ASM and C code?

Previous Assembly and C

Q: Maybe I am quite stupid, but I cannot catch anything about communicating information between the C code and the assembly code... I'm not really familiar with mixing high-level languages with assembly language; all I really know is that the asm keyword allows one to include assembly instructions in C source code. But how would would you write an assembly function that C code could utilize?
A: This is explained in details in this release of TIGCC documentation. See section about Extended Asm.