Expanded from macro ‘num’ while printing it using printf by Clang compiler

cc-preprocessorclanggccmacros

Code:

char *color_name[] = {
    "red",
    "blue",
    "green"
};

#define color_num (sizeof(color_name)/sizeof(char*))

int main(){
    printf("size %d \n",color_num);
    return 0;
}

It works fine with GCC 4.8.2 on Centos 7.

But I got error running above program on mac which says:

note:expanded from macro 'color_num'

Compiler on my Mac: 

……include/c++/4.2.1
Apple LLVM version 6.1.0 (clang-602.0.49) (based on LLVM 3.6.0svn)
Target: x86_64-apple-darwin14.3.0
Thread model: posix

I've been told that GCC has been linked to Clang on Mac when it is used to compile program, am I right?

Qestion:

So why does Clang report that error? Is that concerning pre-processing?

And if I do this, it works fine:

int a = color_num;
printf("%d\n",a);

or:

printf("%d\n",sizeof(color_num)/sizeof(char*));

UPDATA=============

Crayon_277@Macintosh 20150525$ gcc -g -o ex11 ex1.c
ex1.c:16:21: warning: format specifies type 'int' but the argument has type 'unsigned long' [-Wformat]
        printf("size %d\n",color_num);
                     ~~    ^~~~~~~~~
                     %lu
ex1.c:14:19: note: expanded from macro 'color_num'
#define color_num (sizeof(color)/sizeof(char*))
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.

It seems no error but just that format warning.

I think it may concerning the extension I use for vim
scrooloose/syntastic

I got error from that:

enter image description here

Best Answer

It is probably complaining that the expression expanded from color_num is an unsigned (perhaps unsigned long) while the format in the printf is a signed integer.

sizeof gives size_t, which is always an unsigned type, as noted in is size_t always unsigned?, but the number of bits depends on the implementation. Compiler warnings may — and often do — refer to the mismatch in terms of the equivalent type rather than size_t as such. The C standard after all, does not specify the nature of diagnostic messages.

When you changed that to an assignment, it is less strict, since that is a different check.

The "note" lines are something that the compiler adds to a warning/error message to help you understand where the problem came from.

(As the comment notes, you should quote the entire warning message, to make the question understandable).

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