Warning "Use of GNU statement expression extension"

Question

I have this Objective-C istruction:

NSRange range = NSMakeRange(i, MIN(a, b));

where a and bare NSUIntegers.

MIN() is the macro defined in the standard NSObjCRuntime.hheader file as:

#if !defined(MIN)
   #define MIN(A,B) ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a < __b ? __a : __b; })
#endif

During the compilation, the LLVM Compiler 4.1 highlights my instruction showing the warning: "Use of GNU statement expression extension".

What does this mean? Is it my fault? If yes, how can I fix it? If not, how can I remove the compiler warning?

score 11 · Answer 1 · answered Oct 26 '12 at 12:44

11

"Statement expressions" is an extension of the GNU C compiler and allows you to execute a group of statements, returning the value of the last statement:

x = ({
    statement1;
    statement2;
    statement3;
});

In the above example, x will have the value returned by statement3.

It is a convenient feature that enables you to have multi-statement macros that can be nested easily into other expressions. It is not, however, defined by any C standard.

answered Oct 26 '12 at 12:44

Blagovest Buyukliev

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Thank you, very interesting. Since the `MIN()` macro is defined in the Apple headers (which I can't obviously change), there is a way to suppress this warning in my code? – Dev Oct 26 '12 at 12:55
@Dev You should just be able to do *`#undef MIN`*. That goes for all *`#define`* s. – Pryftan Jun 10 '19 at 14:41

score 11 · Accepted Answer · answered Dec 16 '13 at 21:28

It’s a late answer, I know, but you can avoid this message by adding -Wno-gnu to your compiler flags.

(In Xcode 5 I believe you can change this by going to your project’s Build Settings and adding -Wno-gnu to the “Other C Flags”, which are in the “Apple LLVM 5.0 – Custom Compiler Flags” section.)

score 10 · Answer 3 · edited Aug 27 '18 at 18:59

10

Don't use -Wno-gnu, that shuts down too many warnings. Instead, use:

-Wno-gnu-statement-expression

edited Aug 27 '18 at 18:59

Felipe Augusto

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answered Aug 27 '18 at 15:57

Eric

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the pragma statement would be: `#pragma GCC diagnostic ignored "-Wgnu-statement-expression" ` – Ugo Sep 19 '20 at 07:48
@Ugo With that, I get `error: unknown option after ‘#pragma GCC diagnostic’ kind [-Werror=pragmas]`. I'm compiling with `-Wall -Wextra -pedantic -Werror`. Any idea? – someonewithpc Apr 07 '23 at 00:20

score 2 · Answer 4 · answered Jan 03 '15 at 00:52

Statement expressions have been declared.

You can selectively ignore the warning by using pragma codes without changing project settings.

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wgnu"

NSRange range = NSMakeRange(i, MIN(a, b));

#pragma GCC diagnostic pop

Warning "Use of GNU statement expression extension"

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