Once we import the C library of Darwin into a Swift project, we could use all C function defined there. The library contains most of the functions in C standard library, which provides us some great toolkit in development. It is very easy to import Darwin, just add import Darwin in your Swift code. In fact, Foundation concludes the importing of Darwin, and in app development we always need UIKit, which imports Foundation. Since that, we can use these C functions without doing anything. Moreover, Swift already converts C types in Darwin into Swift types when importing. For example, a trigonometric function will receive and return values of Double instead of C double:

func sin(x: Double) -> Double

All these functions are defined in global scope, so we can use them directly:

sin(M_PI_2)
// Output: 1.0

For third party C code, Swift also supplies interoperability to it. As we know, invoking Objective-C code in Swift is very easy. You can expose your Objective-C header file in {product-module-name}-Bridging-Header.h to Swift. And it is the same with C code. Just import your C header file in the bridging file, so you can call your C method in Swift:

    //test.h
    int test(int a);

    //test.c
    int test(int a) {
        return a + 1;
    }

//Module-Bridging-Header.h
#import "test.h"

//File.swift
func testSwift(input: Int32) {
    let result = test(input)
    print(result)
}

testSwift(1)
// Output: 2

We have a way to import C functions even without header files or Bridging-Header. In Swift, there is a symbol called @asmname, which could map a C function directly to Swift function by the function name. Take the code above for an example, we could delete test.h and Module-Bridging-Header.h, and modify the File.swift file as this:

//File.swift
//Map the `test` function in C to `c_test` in Swift
@asmname("test") func c_test(a: Int32) -> Int32

func testSwift(input: Int32) {
    let result = c_test(input)
    print(result)
}

testSwift(1)
// Output: 2

This is a way to solve conflicting of a third party C function and the standard C library. We could rename the third party function name to make both happy. Of course, we could also depend on the module name and function name to solve it, which might be a better approach.

@asmname is also used to "rename" a Swift class or method name to use in C code. This is similar to the @objc keyword. That is because C will only recognize ascii characters but we could use a much larger character set in Swift.