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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?

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3 Answers 3

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18

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); }
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1 Comment

Altough this answer is correct, I had some troubles getting things to work. That's why I made a little article.
0

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!) }}
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1 Comment

Good example, but unless I'm mistaken mySwiftFunction returns a dangling pointer in your example -- retmsg gets de-allocated at the end of the function, but the address of a pointer inside it gets returned. If you're lucky, -[NSString UTF8String] has to create a copy of the string though and uses an autoreleased NSData to store the UTF8 bytes, so the pointer might still be retained by the NSAutoreleasePool. But I don't think one should rely on that behavior never changing.
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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]; }
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5 Comments

OP asked about calling C functions in swift, not how to call an Objective-C method
Author want to call Swift code from C. "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." @pbush25
Probably I can extend my comment. SwiftFunctionWrapper() function can be called from any c file.
@Rolan is right … doing it that way though requires an extra layer of wrapping to bridge C to Swift via Objective-C … using callbacks at least skips that layer. Might there be some good resources out there?
This answer is not cross-platform and would only work on Darwin platforms where Objective-C interop is available, while it won't work on Linux or Windows. The question did not specify a specific platform. A correct answer should be usable on all platform that Swift supports.

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