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?

@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<GeoCoord>. Or can it return an unsafe pointer? — lurning too koad
– lurning too koad, Commented Feb 27, 2020 at 17:24
It should be let h3Index = withUnsafePointer(to: geoCoord) { geoToH3($0, 5) } similar to stackoverflow.com/a/60352083/1187415. — Martin R
– Martin R, Commented Feb 27, 2020 at 17:41
You can make this an answer as this answers my question perfectly, thank you! — lurning too koad
– lurning too koad, Commented Feb 27, 2020 at 17:45

Martin R · Accepted Answer · 2020-02-27 18:54:27Z

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.

AMIT · Accepted Answer · 2020-02-27 18:55:32Z

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) } }

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