What is the best way to call into Swift from C? - c

Question

Calling into C from Swift is pretty simple, however I'm looking into making a bi-directional wrapper in C, so my C has to call Swift functions.
Right now, I can do this by declaring function pointers in C, and having my C functions call them after the Swift side has set them up to call code in Swift.
My C header file:
typedef void (*callback_t)(void);
void callBackIntoSwift( callback_t cb );
My C implementation file:
#include "stuff.h"
#include <stdio.h>
void callBackIntoSwift( callback_t cb )
{
printf( "Will call back into Swift\n" );
cb();
printf( "Did call back into Swift\n" );
}
After including my C header file in the bridging header, I can do the following on the Swift side:
let cb: callback_t = {
someKindOfSwiftFunction()
}
callBackIntoSwift( cb )
Or even:
callBackIntoSwift {
someKindOfSwiftFunction()
}
Is there a better way to do this, where function pointers and callbacks are not needed? I'd like to let the C-side call someKindOfSwiftFunction directly … but when I try to apply #convention (c) to function declarations I get the message that the attribute can only be applied to types, and not declarations.
Any ideas or codebases in e.g. Github I can take a look at?

Answer 1

According to Joe Groff:
There’s no official way yet. Aside from name mangling, Swift functions use a different calling convention from C. Unofficially, if you’re willing to deal with more than the usual amount of code breakage and compiler bugs, there’s an unofficial attribute #_cdecl that does this:
#_cdecl("mymodule_foo")
func foo(x: Int) -> Int {
return x * 2
}
which you can then call from C:
#include <stdint.h>
intptr_t mymodule_foo(intptr_t);
intptr_t invoke_foo(intptr_t x) {
return mymodule_foo(x);
}

Answer 2

You can do something like this:
FileSwift.swift
public class SwiftTest: NSObject {
#objc public static func testMethod() {
print("Test")
}
}
FileSwiftWrapper.h
void SwiftFunctionWrapper();
FileSwiftWrapper.m
#import "ProductModuleName-Swift.h"
void SwiftFunctionWrapper() {
[SwiftTest testMethod];
}

Answer 3

Passing values between C and Swift
In C
extern char *mySwiftFunction(char *valuefromc);
int checkSwiftFuncation(void )
{
char *retval = mySwiftFunction("samplevalue");
printf("value %s\n",retval);
return 0;
}
In Swift
#_silgen_name("mySwiftFunction") // vital for the function being visible from C
func mySwiftFunction(valuefromc: UnsafePointer<Int8>) -> UnsafeMutablePointer<Int8>
{
let value = String(cString: valuefromc, encoding: .utf8)
print ("mySwiftFUnction called in Swift with \(value!)")
let retmsg = "return message"
return retmsg.charpointer
}
extension String {
var charpointer: UnsafeMutablePointer<Int8> {
return UnsafeMutablePointer(mutating: (self as NSString).utf8String!)
}}