Re: Multiple declarations of objc_msgSend_stret
Re: Multiple declarations of objc_msgSend_stret
- Subject: Re: Multiple declarations of objc_msgSend_stret
- From: Jeff Kelley <email@hidden>
- Date: Thu, 3 Feb 2011 21:53:52 -0500
On Feb 3, 2011, at 9:38 PM, Greg Parker wrote:
> On Feb 3, 2011, at 6:18 PM, Jeff Kelley wrote:
>> I’m trying to use objc_msgSend_stret to call -bounds on a UIScreen. I have the following code:
>>
>>> SEL bounds = NSSelectorFromString(@"bounds");
>>> CGRect screenBounds;
>>> objc_msgSend_stret((void *)&screenBounds, screen, bounds);
>>
>> The declaration of objc_msgSend_stret in the Xcode code sense pop-up is as follows:
>>
>>> void objc_msgSend_stret(void * stretAddr, id self, SEL op, ...);
>>
>> But that’s not what message.h would leave me to believe (with some unrelated lines removed):
>>
>>> #if defined(__OBJC2__)
>>> OBJC_EXPORT void objc_msgSend_stret(id self, SEL op, ...);
>>> #else
>>> /* For compatibility with old objc-runtime.h header */
>>> OBJC_EXPORT void objc_msgSend_stret(void * stretAddr, id self, SEL op, ...);
>>> #endif
>>
>> So, if the declaration is actually the first (I’m compiling for iOS 4.2, so I’m pretty sure __OBJC2__ is defined), how do you use it and get the return value of the function?
>
> Both declarations are wrong: you cannot and must not call them directly. The declaration of objc_msgSend_stret() cannot be expressed in C.
>
> The correct way to call objc_msgSend_stret() or any other objc_msgSend() variant is to cast it to an appropriate function pointer type. Like this:
>
> CGRect (*msgSendFn)(id self, SEL _cmd);
> msgSendFn = (CGRect(*)(id, SEL))objc_msgSend_stret;
> CGRect screenBounds = msgSendFn(screen, @selector(bounds));
>
> If the method had parameters, you would need to add them to the declaration of the function pointer variable.
>
>
> --
> Greg Parker email@hidden Runtime Wrangler
That worked like a charm. Thanks, and don’t worry—this isn’t going in shipping code. :)
Jeff Kelley
email@hidden
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