Accepting "%@" in format string with __attribute__ format

Accepting "%@" in format string with __attribute__ format

[Manfred Schwind]

Hi,

I have a function like

void myLog(const char *format, ...);

Now I want to let the compiler check the passed parameters if they fit to the format string like printf.
I know this can be done easily with the following:

void myLog(const char *format, ...) __attribute__((format(printf, 1, 2)));

Now my problem is: I also want to accept the "%@" specifier that is accepted in Obj-C NSStrings or NSLog. The "%@" specifier accepts Obj-C objects (NSObject or id) or Core Foundation Object References (CFTypeRef).
I saw that NSLog is declared this way:

void NSLog(NSString *format, ...) __attribute__((format(__NSString__, 1, 2)));

Problem is that I don't have an NSString as format string parameter in my function. I have a plain old C-string (it should also work in C++ code, not only in Obj-C or Obj-C++ code). But it accepts the "%@" specifier (I am using Core Foundation in the Implementation under-the-hood).
When I use the __NSString__ declaration like NSLog, gcc gives me an error message that my format parameter is not an NSString. Sure, it is not. But it doesn't matter. I just want gcc to do the format parameter checking like that.

Is there any chance to get gcc check my passed parameters the "Objective-C way" with "%@" included but still having the format string passed as plain old C-string? It there any other types than __NSString__ I could try?

BTW, is there an official list what types are accepted by __attribute__((format(...))) ?

Thanks,
Mani
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Re: Accepting "%@" in format string with __attribute__ format

[Cedric Roux]

Manfred Schwind wrote:
> Is there any chance to get gcc check my passed parameters the
> "Objective-C way" with "%@" included but still having the format
> string passed as plain old C-string? It there any other types
> than __NSString__ I could try?
> 
> BTW, is there an official list what types are accepted by __attribute__((format(...))) ?

This NSString does not seem to be recognized by gcc at all.

I see that llvm handles it.
(http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaDeclAttr.cpp?revision=97103&view=markup)
Are you sure you use gcc?

Compiling:
#import <Foundation/Foundation.h>

void mlog(NSString *format, ...) __attribute__((format(__NSString__, 1, 2)));

leads to:
toto.m:3: warning: ‘__NSString__’ is an unrecognized format function type

with gcc 4.4 and GNUstep stuff.

Apart from that, the gcc documentation tells what types you can use in __attribute__ format
stuff. (Here when I do "info gcc", in the C Extensions -> Function Attributes it is
described and allowed stuff is:

`printf', `scanf', `strftime',
`gnu_printf', `gnu_scanf', `gnu_strftime' or `strfmon'.  (You can
also use `__printf__', `__scanf__', `__strftime__' or
`__strfmon__'.)  On MinGW targets, `ms_printf', `ms_scanf', and
`ms_strftime' are also present.
)

My .1 euros...

Re: Accepting "%@" in format string with __attribute__ format

[Ian Lance Taylor]

Manfred Schwind <lists@mani.de> writes:

> Is there any chance to get gcc check my passed parameters the
> "Objective-C way" with "%@" included but still having the format
> string passed as plain old C-string?

Unfortunately I don't think this is possible.  Over the years there
have been various proposals for extending the printf format checking,
but nothing has been widely accepted or implemented.

Ian