Using Objective C classes from C code

Question

I know it is possible to use Objective C code using objc_msgSend to, I believe, manually run the Objective C runtime however when I run this code I get errors referencing NSString (even though I never use it) as well as other un-used classes.

Errors from xcode

enter image description here

I have the Objective C Code above it (commented out) that I am trying to 'mimic'.

#include <Foundation/Foundation.h> /*Added suggestion by answer, same errors*/
#include <AppKit/AppKit.h>

int main()
{
// convert objective c into c code
/*
    NSAlert *alert = [[NSAlert alloc] init];
    [alert setAlertStyle:NSInformationalAlertStyle];
    [alert setMessageText:@"Hello World"];
    [alert setInformativeText:@"Hello World"];
    [alert runModal];
*/
    id alert = objc_msgSend(objc_msgSend(objc_getClass("NSAlert"), sel_registerName("alloc")), sel_registerName("init"));
    objc_msgSend(alert, sel_getUid("setAlertStyle:"), NSInformationalAlertStyle);
    objc_msgSend(alert, sel_getUid("setMessageText:"), CFSTR("Hello World!"));
    objc_msgSend(alert, sel_getUid("setInformativeText:"), CFSTR("Hello World!"));
    objc_msgSend(alert, sel_getUid("runModal"));
}

Do not call objc_msgSend() without a cast to a function pointer type that matches the method's signature. — Greg Parker, Sep 09 '13 at 03:05

Gabriele Petronella · Accepted Answer · 2013-09-09T09:05:04.347

6

You are missing some imports.

objc_msgSend is declared in <objc/message.h>.

objc_getClass is declared in <objc/runtime.h>.

sel_getUid and sel_registerName are declared in <objc/objc.h>.

Now, given that <objc/objc.h> is already imported by <objc/runtime.h>, importing the latter along with <objc/message.h> should suffice.

I tested it with the following example and it works as expected

#include <CoreFoundation/CoreFoundation.h> // Needed for CFSTR
#include <objc/runtime.h>
#include <objc/message.h>

int main(int argc, char *argv[]) {
    id alert = (id (*)(id, SEL))objc_msgSend((id (*)(id, SEL))objc_msgSend(objc_getClass("NSAlert"), sel_registerName("alloc")), sel_registerName("init"));
    (void (*)(id, SEL, int))objc_msgSend(alert, sel_getUid("setAlertStyle:"), 1); // NSInformationalAlertStyle is defined in AppKit, so let's just use 1
    (void (*)(id, SEL, id))objc_msgSend(alert, sel_getUid("setMessageText:"), CFSTR("Hello World!"));
    (void (*)(id, SEL, id))objc_msgSend(alert, sel_getUid("setInformativeText:"), CFSTR("Hello World!"));
    (int (*)(id, SEL))objc_msgSend(alert, sel_getUid("runModal"));
}

Note

I added the explicit cast to objc_msgSend as suggested by Greg Parker in the comments.

edited Sep 09 '13 at 09:05

answered Sep 08 '13 at 22:38

Gabriele Petronella

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Didn't work same errors even with the line #include before importing AppKit – Zimm3r Sep 08 '13 at 22:39
Ok, I managed to do it. Check my example in the updated answer – Gabriele Petronella Sep 08 '13 at 22:45
Still doesn't work for me, I'll try creating a new xcode project. – Zimm3r Sep 08 '13 at 22:47
I created a Cocoa Application and placed the code above in the `main.m`. I'll add a screenshot with code and result. – Gabriele Petronella Sep 08 '13 at 22:50
Ok it worked but I think it does defeat the purpose of not using Objective C, oh well thanks for the help! – Zimm3r Sep 08 '13 at 22:54
Well the headers you are imported are C headers, why do you think it defeats it? – Gabriele Petronella Sep 08 '13 at 22:54
Oh, almost forgot. This is relevant: http://stackoverflow.com/questions/10289890/how-to-write-ios-app-purely-in-c – Gabriele Petronella Sep 08 '13 at 22:56
I was attempting to avoid using the objective c compiler (which I assume this is needed) so as to be able to compile asm (translating the c code to asm) code in nasm – Zimm3r Sep 08 '13 at 22:58
But oh well I'll see what I can do, I could very well be missing something obvious :P – Zimm3r Sep 08 '13 at 22:59
Ok I modified the example to make it fully C compatible. Provided that you link the `CoreFoundation` framework and an Objective-C runtime library, you can compile it with a C compiler like `clang` or `gcc`. – Gabriele Petronella Sep 08 '13 at 23:11
2

Do not call objc_msgSend() without a cast to a function pointer type that matches the method's signature. – Greg Parker Sep 09 '13 at 03:06
@GregParker thanks for the input! Can you elaborate on it a bit? – Gabriele Petronella Sep 09 '13 at 08:14
@GregParker If I get it right, it should be something like my last edit. I'm still trying to fully understand the implications. – Gabriele Petronella Sep 09 '13 at 09:05
@GabrielePetronella According to the C ABI, vararg vs. statically typed call sites cannot be used interchangeably everywhere. – bbum Sep 09 '13 at 15:53

Using Objective C classes from C code

1 Answers1

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