CODEWITHSUNDEEP
xamarinxamarin.ios
t9mike
t9mike

Reputation: 1626

VarArgs Binding in MonoTouch / Xamarin.iOS

I have a new MonoTouch binding partially working. But a variable arguments function is causing a crash when run. The .h file:

FOUNDATION_EXPORT void __BFLog(NSInteger lineNumber, NSString *method, NSString *file, 
    BFLogLevel level, NSString *tag, NSString *format, ...);

The C#:

internal static class CFunctions
{
    // extern void __BFLog (NSInteger lineNumber, NSString * method, 
    // NSString * file, BFLogLevel level, NSString * tag, NSString * format, ...);
    [DllImport ("__Internal", EntryPoint = "__BFLog")]
    internal static extern void BFLog (nint lineNumber, string method, string file, 
        LogLevel level, string tag, string format, string arg0);
}

Because I would pass "" in arg0 and really pass the string in format section. But when calling, I see this crash:

critical:   at <unknown> <0xffffffff>
critical:   at (wrapper managed-to-native) BugfenderSDK.CFunctions.BFLog (System.nint,string,string,BugfenderSDK.LogLevel,string,string,string) <0xffffffff>
...

Objective Sharpie put IntPtr varArgs as the last argument by default. I tried this string arg0 and passing in IntPtr.Zero instead, but still crash.

EDIT #1: insteading of worrying about the first vararg -- I was going to just pass "" to it -- I followed TestFlight binding example per ventayol and ignored this, only defining the format in the Dllimport:

 [DllImport ("__Internal", EntryPoint = "__BFLog")]
 internal static extern void BFLog(
     nint lineNumber, /* nint will be marshalled correctly */
     IntPtr method, /* NSString must be declared as IntPtr */
     IntPtr file, /* NSString */
     LogLevel level, /* This may be wrong, depending on the exact LogLevel type */
     IntPtr tag, /* NSString */
     IntPtr format /* NSString */
 );

And the wrapper:

 public static void Log(LogLevel level, nint lineNumber, string method, string file, 
     string tag, string format, params object[] args)
 {
     var nsMethod = new NSString (method);
     var nsFile = new NSString (file);
     var nsTag = new NSString (tag);
     string msg = String.Format(format, args);
     var nsMsg = new NSString(msg);

     BFLog (lineNumber, nsMethod.Handle, nsFile.Handle, level, nsTag.Handle, nsMsg.Handle);

     nsMethod.Dispose ();
     nsFile.Dispose ();
     nsTag.Dispose ();
     nsMsg.Dispose ();            
 }

But I only see tag and others on the backend, no message.

Upvotes: 2

Views: 388

Answers (2)

ventayol
ventayol

Reputation: 635

Maybe you can check what TestFlight did on his binding here: https://github.com/mono/monotouch-bindings/blob/master/TestFlight/binding/testflight-cplusplus.cs

They have a similar function to what you want to do.

Upvotes: 1

Rolf Bjarne Kvinge
Rolf Bjarne Kvinge

Reputation: 19335

This isn't an Objective-C binding, but a standard .NET P/Invoke. This means that Xamarin.iOS will not do the standard C# -> Objective-C type marshalling (like string -> NSString for instance).

So you need to write the P/Invoke like this:

[DllImport ("__Internal", EntryPoint = "__BFLog")]
internal static extern void BFLog (
    nint lineNumber, /* nint will be marshalled correctly */
    IntPtr method, /* NSString must be declared as IntPtr */
    IntPtr file, /* NSString */
    LogLevel level, /* This may be wrong, depending on the exact LogLevel type */
    IntPtr tag, /* NSString */
    IntPtr format, /* NSString */
    string arg0 /* this is a C-style string, char* */)]

and use it like this:

BFLog (
    0, /* nint */
    new NSString (method).Handle, /* IntPtr */
    new NSString (file).Handle, /* IntPtr */
    level, /* LogLevel */
    new NSString (tag).Handle,  /* IntPtr */
    new NSString (format).Handle,  /* IntPtr */
    "arg0" /* string */);

You'll also need to create a separate P/Invoke for every varargs variation you're using. So if you need one that takes two C-style strings, do:

[DllImport ("__Internal", EntryPoint = "__BFLog")]
internal static extern void BFLog (
    nint lineNumber, /* nint will be marshalled correctly */
    IntPtr method, /* NSString must be declared as IntPtr */
    IntPtr file, /* NSString */
    LogLevel level, /* This may be wrong, depending on the exact LogLevel type */
    IntPtr tag, /* NSString */
    IntPtr format, /* NSString */
    string arg0, /* this is a C-style string, char* */
    string arg1 /* second C-style string, char* */)]

Also have in mind that varargs on arm64 has a different calling convention, all varargs are passed on the stack. What this means in practice is that the first varargs argument must be argument #8 in the P/Invoke:

[DllImport ("__Internal", EntryPoint = "__BFLog")]
internal static extern void BFLog_arm64 (
    nint lineNumber, /* 1st */
    IntPtr method, /* 2nd */
    IntPtr file, /* 3rd */
    LogLevel level, /* 4th */
    IntPtr tag, /* 5th */
    IntPtr format, /* 6th */
    IntPtr dummy1, /* 7th */
    string arg0 /* 8th, the first varargs parameter */)]

and then you can provide a wrapper around the BFLog export that will do the right thing depending on the architecture:

using ObjCRuntime;

internal static void Log (nint lineNumber, string method, string file, LogLevel level, string tag, string format, string arg0)
{
    var nsMethod = new NSString (method);
    var nsFile = new NSString (file);
    var nsTag = new NSString (tag);
    var nsFormat = new NSString (format);

    if (Runtime.Arch == ARCH.Device && IntPtr.Size == 8) {
        BFLog_arm64 (lineNumber, nsMethod.Handle, nsFile.Handle, level, nsTag.Handle, nsFormat.Handle, IntPtr.Zero, arg0);
    } else {
        BFLog (lineNumber, nsMethod.Handle, nsFile.Handle, level, nsTag.Handle, nsFormat.Handle, arg0);
    }

    nsMethod.Dispose ();
    nsFile.Dispose ();
    nsTag.Dispose ();
    nsFormat.Dispose ();
}

Upvotes: 6

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