NSInvocation has garbage return value, and even trashes target?

[andyvn22]

It seems that any NSInvocation with a return value that's a struct gives a garbage return value. Furthermore, as demonstrated by the little test below, it seems to even mess up its target object when it is invoked. The logging of [vertex location] segfaults after the invocation, but works before. If I comment that out, thus allowing the code to log the return value, that prints incorrect x and y values.

Any idea what's going on here or at least where I could start debugging this?

Thank you!

@interface IRVertex : NSObject
{
    NSPoint location;
}
@property (nonatomic,assign) NSPoint location;
@end
@implementation IRVertex
@synthesize location;
@end
int main(int argc, char *argv[])
{
    NSAutoreleasePool* pool = [NSAutoreleasePool new];
    
    IRVertex* vertex = [[IRVertex new] autorelease];
    [vertex setLocation:NSMakePoint(4,2)];
    NSLog(@"Value before invocation:");
    NSLog(@"%@",NSStringFromPoint([vertex location]));
    
    SEL sel = @selector(location);
    id sig = [vertex methodSignatureForSelector:sel];
    id inv = [NSInvocation invocationWithMethodSignature:sig];
    [inv setSelector:sel];
    [inv invokeWithTarget:vertex];
    
    NSUInteger returnLength=[sig methodReturnLength];
    void *returnValue=__builtin_alloca(returnLength);
    [inv getReturnValue:returnValue];
    
    NSLog(@"Value after invocation:");
    NSLog(@"%@",NSStringFromPoint([vertex location]));
    NSLog(@"Invocation return value:");
    NSLog(@"%@",NSStringFromPoint(*(NSPoint*)returnValue));
    
    [pool drain];
    
    return NSApplicationMain(argc, (const char **) argv);
}

[Elder William]

Does   [inv getReturnValue:&returnValue]; work?

In my experience you have to pass the address of the variable using &... for both get and setReturnValue....(this is on apples implementation though)

Andre

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[Christopher Lloyd]

If 'target' is getting trashed I'd guess -methodReturnLength is returning the wrong value. In general NSInvocation could use some work especially for struct returns, although NSPoint should fall into the non-struct return category.

Start putting some logs in there :)

Chris

[andyvn22]

Hum, -methodReturnLength is a reasonable value according to NSGetSizeAndAlignment(@encode(NSPoint)...)--it also matches the Mac 32-bit value.

Through some logging, I tracked down the problem to NSInvocation.m:448:

else if (size <= sizeof(long long)) {
    long long (*function)() = msgSendv;
    NSLog(@"1: %@",NSStringFromPoint([[self target] location])); //Result: 1: {4, 2}
    long long value = function([self target], [self selector], _argumentFrameSize, _argumentFrame);
    NSLog(@"2: %@",NSStringFromPoint([[self target] location])); //Result: Segmentation fault
    [self setReturnValue:&value];
}

What does this mean? Possibly incorrect _argumentFrameSize? It seems to be 8, same as -methodReturnLength.

Thank you!

[Christopher Lloyd]

Ah, you need to call NSInitializeProcess(argc,argv) right after main. The runtime is not initialized properly.

Chris

[andyvn22]

Oh--thank you! I wasn't aware of that. Unfortunately, adding it did not change the behavior. Is there any other initialization that needs to be done?

[Christopher Lloyd]

Hm, I tried this out as-is in a small example (beginning of TextEditor example) and it seems fine, runs as expected.

Possibly something else is interacting here?

Chris

[andyvn22]

Perhaps it's a Linux-specific issue, then... I've no way to run Windows tests.