GCC: __attribute__ ((format (printf, x, y)) does not seem to work when function is called using a variadic macro

Question

GCC version ntoarm-gcc (GCC) 4.4.2

I've added 'printf' format attributes to all my functions that wrap printf() and co. They work perfectly fine except when calling the functions using a variadic macro.

class Log { [...]
    void log_fmt(LogLevel level, const std::string& funcName, const char_t * const logFormatStr, ...)  __attribute__ ((format (printf, 4, 5)));
[...] };

An incorrect direct call like

log.log_fmt(Info, "test", "wrong %u", "type");

yields warning:

format '%u' expects type 'unsigned int', but argument 5 has type 'const char*'

The same incorrect call using a macro yields no warning, however:

#define LOGI(MSG, ...) log.log_fmt(Info, __func__, (MSG), __VA_ARGS__)
LOGI("wrong %u", "type");

Can I get the warnings to show up in this case too? Have I made a mistake or is this intended behaviour?

Answer 1

This:

#include <iostream>
#include <cstdio>

struct log {
    static void logf(std::string, std::string, const char*, ...) __attribute__((format (printf, 3, 4))) {}
};

#define L(m, ...) log::logf("no", __func__, (m), __VA_ARGS__)

int main() {
    //log::logf("hi", "hi", "test %u", "hi");
    L("test %u", "hi");
}

works perfectly, in the sense that it gives the correct warning, like below:

main.cpp: In function 'int main()':
main.cpp:8:61: warning: format '%u' expects argument of type 'unsigned int', but argument 4 has type 'const char*' [-Wformat=]
 #define L(m, ...) log::logf("no", __func__, (m), __VA_ARGS__)
                                                             ^
main.cpp:12:5: note: in expansion of macro 'L'
     L("test %u", "hi");
     ^
At global scope:
cc1plus: warning: unrecognized command line option "-Wno-undefined-internal" [enabled by default]

So, I would guess that the problem is on the position parameters (you have put 4, 5 on your format attribute, when it seems that you should have put 3, 4)....