The Wolfram Language supports several levels of interfacing to C and C++ programs. You can create "installable" C programs where C functions are directly connected to Wolfram Language functions. You can use C to call the Wolfram Language through the Wolfram Symbolic Transfer Protocol (WSTP) and get full access to its capabilities. And you can create C program fragments and modify C source code.

Calling C Programs from the Wolfram Language »

Install — load an installable WSTP program

.tm — template file for connecting C and Wolfram Language functions

mcc, mprep — scripts for building WSTP programs

WSEvaluate() — call back into the Wolfram Language

Calling the Wolfram Language from C Programs »

WSPutInteger32()  ▪  WSGetInteger32()  ▪  WSPutString()  ▪  WSPutFunction()  ▪  ...

WSOpenString()  ▪  WSNextPacket()  ▪  WSReady()  ▪  ...

Wolfram LibraryLink »

LibraryFunctionLoad — load a function from a dynamic library into the Wolfram System

LibraryFunction — a function that calls into a dynamic library

LibraryFunctionInformation  ▪  LibraryFunctionUnload  ▪  LibraryLoad  ▪  LibraryUnload  ▪  FindLibrary  ▪  $LibraryPath  ▪  ...

Calling C Compilers from the Wolfram Language »

CreateExecutable — create an executable from C code

CreateLibrary — create a library from C code

Generating C Code »

Export — export a Wolfram Language compiled function into C code

CCodeGenerate — export a Wolfram Language compiled function into C code

CCodeStringGenerate  ▪  SymbolicCGenerate  ▪  LibraryGenerate

CForm — convert an expression to C language form

Symbolic Representation of C Code »

CFunction — symbolic representation of a C function

CCall — symbolic representation of a C function call

CAddress  ▪  CAssign  ▪  CBlock  ▪  CDeclare  ▪  CIf  ▪  CPointerType  ▪  CStruct