how to invoke C functions in llvm IR ?

Hi all,
I am a new user of LLVM~.
I want to use LLVM C++ API to generate the LLVM IR.The question is how I can call the C functions in LLVM IR.I did not find the APIs in the documents.

For Example:
Function *func = Function:Create(…) //create a LLVM function
Here is another function I cannot represent with LLVM IR(maybe too complex),so I want to invoke the C function with LLVM IR in func.How can I do it?