If you write a subroutine in ASM, you would generally document:
1. The parameters used
2. The function performed and result that is returned
3. The registers and other resources that are used
4. The side-effects e.g. registers that are trashed

You (or someone else) can read the documentation and call this subroutine by just obeying the rules.

A C compiler uses exactly the same process. You will find how parameters are passed, registers and memory used etc from the C compiler manual.
The actual function signature is documented by the C library. You just obey the rules.

It is normally a C program that would link with an ASM function. Providing all the rules are followed then everything is fine.

An ASM program can link with compiled C functions in principle. You follow the rules.

The linker must be able to understand the .OBJ modules from both C and ASM. It is a lot easier to use a compiler and assembler from the same toolset. Or you and your assembler must understand the compiler generated ASM.

Neither method is for the faint-hearted. You need an intimate knowledge of both (a) your ASM style and (b) the C compiler style.

When (a) is unknown or undocumented you will have little success.

David.