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

Question

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

It would help if you could post the actual error message. The "note" is just there to help you understand the error, but it's not the error message itself. — Ken Wayne VanderLinde, May 26 '15 at 01:15
That was to be expected. Actrually, gcc will also notify this, but it does require -Wformat which is included in -Wall. The latter is the least one should always enable (-Wextra recommended). — too honest for this site, May 26 '15 at 01:29
Note that a function without arguments shall be noted `f(void)`. — too honest for this site, May 26 '15 at 01:34

score 3 · Accepted Answer · edited May 23 '17 at 11:43

3

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).

edited May 23 '17 at 11:43

Community

1
1

answered May 26 '15 at 01:15

Thomas Dickey

51,086
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sizeof return size_t; leave it to the toolchain what that actually maps to. – too honest for this site May 26 '15 at 01:26
@Olaf it does not return anything, it's an operator and it's evaluated at compile time. – Iharob Al Asimi May 26 '15 at 01:28
Thomas: Just reference the standard: n1570, 6.5.3.4/5 . And for an assignment - as for any other expression - default coercion rules are applied. However, gcc at least can (and should) are able to warn about some of those, too (-Wconversion e.g.). – too honest for this site May 26 '15 at 01:47
@iharob: how about a VLA? Anyway, every operator is actually a function, which takes argument(s) and returns a result. sizeof is not different in that from the unary '-' for instance. Note that the time of evaluation does not change that. The standard very well allows the compiler to evaluate a function at compile-time if it can prove it constant/pure. And it actually does: just think of expressions. Note that the stndard explicitly mentions it "yields" (6.5.3.4/2, also for VLAs). For C I take this as the same; but I will not discuss about the difference between "yield" and "return" here – too honest for this site May 26 '15 at 01:51

score 1 · Answer 2 · answered May 26 '15 at 01:27

1

The sizeof gives the value with size_t type, the right format specifier for size_t is "%zu".

answered May 26 '15 at 01:27

Iharob Al Asimi

52,653
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Expanded from macro 'num' while printing it using printf by Clang compiler

2 Answers2