How to pass argument in Swift to C function that takes an UnsafePointer?

Question

I've imported a C function called geoToH3 that returns an H3Index (which is just a UInt64).

let h3Index = geoToH3(g: UnsafePointer<GeoCoord>!, r: Int32)

The function takes an Int32 and a GeoCoord object, which is just an object with a pair of Double.

let geoCoord = GeoCoord(lat: 45.0, lon: -90.0)

How do I pass the geoCoord argument to this function since it takes an UnsafePointer?

Does this help? https://stackoverflow.com/questions/60351981/swift-regards-a-c-const-pointer-as-mutable — Martin R, Feb 27 '20 at 17:08
@MartinR the `withUnsafePointer` generic function would return a type of `GeoCoord`, as far as I understand how it works, but the C function is expecting an argument of type `UnsafePointer`. Or can it return an unsafe pointer? — lurning too koad, Feb 27 '20 at 17:24
It should be `let h3Index = withUnsafePointer(to: geoCoord) { geoToH3($0, 5) }` similar to https://stackoverflow.com/a/60352083/1187415. — Martin R, Feb 27 '20 at 17:41
You can make this an answer as this answers my question perfectly, thank you! — lurning too koad, Feb 27 '20 at 17:45

Martin R · Accepted Answer · 2020-02-27T18:54:27.430

withUnsafePointer(to:) must be used here:

let h3Index = withUnsafePointer(to: geoCoord) {
     (pointer: UnsafePointer<GeoCoord>) -> H3Index in
     return geoToH3($0, 5)
}

or shorter (using shorthand parameter syntax and implicit return):

let h3Index = withUnsafePointer(to: geoCoord) { geoToH3($0, 5) }

withUnsafePointer(to:) calls the closure with a pointer to the geoCoord value.
The C function is called with that pointer as the first argument.
The return value from the C function is the return value of withUnsafePointer(to:) and assigned to h3Index.

It is important that the pointer is only valid during the execution of withUnsafePointer(to:) and must not be stored or returned for later use.

As an example, the following would be undefined behaviour:

let pointer = withUnsafePointer(to: geoCoord) { return $0 }
let h3Index = geoToH3(pointer, 5)

I have one more, just posted it, I hope you look at it... you are a god. — lurning too koad, Feb 28 '20 at 01:32

AMIT · Answer 2 · 2020-02-27T18:55:32.070

Just simply call with a student ref there contain UnsafePointer name.

let stu = student(id: 1, name: input?.cString(using: .utf8), percentage: 10.0)
passByStudent(stu)

Try this example:

MyC.c

#include <stdio.h>

void changeInput(int *output) {
    *output = 5;
}

typedef struct student Student ;

void passByStudent(Student stu);

struct student {
    int id;
    const char *name;
    float percentage;
};

void passByStudent(Student stu) {
    stu.id = 5;
}

ViewController.swift

import UIKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        var output: CInt = 0
        changeInput(&output)
        print("output: \(output)")


        let input: String? = "strudent name"
        let stu = student(id: 1, name: input?.cString(using: .utf8), percentage: 10.0)
        passByStudent(stu)
    }
}

Follow this example step-by-step:

Create a Swift Project
Create MyC.c file and write code.

Click on your project.
Click the Build Setting Tab.
Select all tab
search with Objective-c Bridging Header
Double click Here.
After that left-click on MyC.c file, drag and drop inside popup layer.

You can define your function inside Practice-Bridging-Header.h

void changeInput(int *output);

Write code inside your ViewController.

Code:

import UIKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        var output: CInt = 0
        changeInput(&output)
        print("output: \(output)")


        let input: String? = "strudent name"
        let stu = student(id: 1, name: input?.cString(using: .utf8), percentage: 10.0)
        passByStudent(stu)
    }
}

How to pass argument in Swift to C function that takes an UnsafePointer?

2 Answers2