Printf-like function with optional format string argument and GCC format type-checking

Question

I have created a printf-like function which takes the following arguments:

One mandatory argument (custom error code)
Optional format string
Variable count of format arguments

The function prototype looks like:

int my_printf(int err_code, ...);

err_code also includes information whether the format string (and possible format tags) are given.

If they are given, I can extract the format string using the va_arg function and pass it to vfprintf together with the rest of the arguments.

The calls look like:

my_printf(-ERR_SIMPLE);
my_printf(-ERR_COMPLICATED, "Error: problem with %d", 123);

Unfortunately, I am unable to use the GCC attribute for format type-checking since it needs a string-index:

format (archetype, string-index, first-to-check)

Is it still possible to do the type-checking anyhow? Solutions using helper macros, helper functions, modifying the optional format string part etc are acceptable.

Update: I am unsure whether passing a modified va_list to vfprintf-like functions is OK. See comments. If possible, it would be better to avoid this and use macros or something else for the optional format string part.

How does your function even work at all? How do you strip the format string argument out of the valist? — Kerrek SB
– Kerrek SB, Commented Sep 12, 2012 at 13:01
@KerrekSB I get the format string with const char *fmt = va_arg(args, const char *);. Then I can simply call vfprintf(stdout, fmt, args); — LRipa
– LRipa, Commented Sep 12, 2012 at 13:13
Isn't that UB? I'm pretty sure you can't pass a valist to a function if you've already started monkeying with it... — Kerrek SB
– Kerrek SB, Commented Sep 12, 2012 at 13:20
@KerrekSB I am not sure. It seems to work with GCC 4.4.5 on x86. I guess I should avoid that. The question is still valid (and updated), however. — LRipa
– LRipa, Commented Sep 12, 2012 at 13:42
Well, if we're already in the territory or terrible hacks, I wouldn't worry too much about getting the attributes right :-) — Kerrek SB
– Kerrek SB, Commented Sep 12, 2012 at 13:43

LSerni · Accepted Answer · 2012-09-12 14:07:10Z

(Sorry, I hadn't seen that you had tried with __attribute__. My bad).

You might try experimenting with "argument counting" as described here:

http://locklessinc.com/articles/overloading/

I don't know if this can result in GCC selectively applying argument checking though; but I think it should.

UPDATE it appears to be working, with an added #define hack:

#include <stdio.h>

int printf_1(int err)
{
        printf("Got error %d\n", err);
        return 0;
}

int printf_2(int error, char *string)
{
        printf("Error %d and message %s\n", error, string);
        return 0;
}

int printf_3(int error, char *fmt, ...) __attribute__ ((format (printf, 2, 3)));

int printf_3(int error, char *fmt, ...)
{
        printf("Received full string err=%d, fmt=%s\n", error, fmt);
        return 0;
}

#define printf_4        printf_3
#define printf_5        printf_3
#define printf_6        printf_3
#define printf_7        printf_3
#define printf_8        printf_3
#define printf_9        printf_3
#define printf_10       printf_3


#define COUNT_PARMS2(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _, ...) _
#define COUNT_PARMS(...)\
        COUNT_PARMS2(__VA_ARGS__, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1)

#define CAT(A, B) CAT2(A, B)
#define CAT2(A, B) A ## B

#define myprintf(...)\
        CAT(printf_, COUNT_PARMS(__VA_ARGS__))(__VA_ARGS__)

int main()
{
        myprintf(19);
        myprintf(19, "Hello");
        myprintf(19, "Hello '%s'", "world");
        // Warning!
        myprintf(19, "Hello '%s'", "world", 42);
        myprintf(19, 42);
        return 0;
}

I correctly receive (gcc 4.6.2):

$ gcc -W -Wall -o test test.c

test.c: In function ‘main’:
test.c:48:2: warning: too many arguments for format [-Wformat-extra-args]
test.c:49:2: warning: passing argument 2 of ‘printf_2’ makes pointer from integer without a cast [enabled by default]
test.c:9:5: note: expected ‘char *’ but argument is of type ‘int’

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