How to Correctly use Variadic Macros to Call Other Macros

Question

I know this isn't a fix my build error site, but I've exhausted all my options. None of my coworkers can see anything wrong.

I have a set of macros in the format EventWriteQuic* and take variable number of arguments. For example:

#define EventWriteQuicLibraryInitialized(PartitionCount, DatapathFeatures) \
        MCGEN_EVENT_ENABLED(QuicLibraryInitialized) \
        ? McTemplateU0qq(&MICROSOFT_MSQUIC_PROVIDER_Context, &QuicLibraryInitialized, PartitionCount, DatapathFeatures) : 0

I am then trying to define my own macro like this:

#define QuicTraceEvent(Name, ...) EventWriteQuic##Name(__VA_ARGS__)

Finally, I invoke my macro like this:

    QuicTraceEvent(LibraryInitialized,
        MsQuicLib.PartitionCount,
        QuicDataPathGetSupportedFeatures(MsQuicLib.Datapath));

For some reason I get the following build warning:

.\core\library.c(192,1): warning C4003: not enough arguments for function-like macro invocation 'EventWriteQuicLibraryInitialized'

The final bit of info I have is that for EventWriteQuic* functions that take no arguments, the above code seems to work just fine, which makes me think it's some how ignoring my __VA_ARGS__.

EDIT:

The output generated when using the /P flag:

((((const unsigned char*)Microsoft_QuicEnableBits)[0 >> 3] & (1u << (0 & 7))) != 0) ? McTemplateU0qq_EventWriteTransfer(&MICROSOFT_MSQUIC_PROVIDER_Context, &QuicLibraryInitialized, MsQuicLib.PartitionCount, QuicDataPathGetSupportedFeatures(MsQuicLib.Datapath), ) : 0;

For some reason it's adding an extra comma at the end it seems. When I call the EventWriteQuicLibraryInitialized macro directly, this does no occur.

@jxh I'm unclear how that fixes my problem. I'm not taking two arguments in and then ... — Nick Banks, Dec 13 '19 at 20:28
@Nick can you rewrite your code so it doesn't use ayn functions that we can't see. When I replace the unknown functions with dummy functions I can't get the compilation error you are getting. — hko, Dec 13 '19 at 20:36
Here is a good example: https://godbolt.org/z/MFdhzp (updated) — Nick Banks, Dec 13 '19 at 20:37
Thanks @jxh that fixed it! Feel free to make an answer with it. — Nick Banks, Dec 13 '19 at 20:43
I found a duplicate, which describes a different/similar workaround. — jxh, Dec 13 '19 at 20:49
@Nick I also suggest to change the code in your question to the new smaller code you posted on godbolt. This would make the question more useful to others in the future. reproducible and minimal — hko, Dec 13 '19 at 21:50

jxh · Accepted Answer · 2019-12-13T21:30:59.580

Apparently, MSVC has a bug that does not allow it to directly pass __VA_ARGS__ successfully to another macro.

The sample that failed for me was:

#define A(X, ...) A_##X(__VA_ARGS__)
#define A_TEST(A, B) ((A)+(B))

printf("%d\n", A(TEST, 1, 2));

However, if I wrap the __VA_ARGS__ within parentheses, the contents are treated as a single argument. I can pass this to another macro to actually do the invocation, and it works.

#define A(X, ...) A_(X, (__VA_ARGS__))
#define A_(X, Y) A_##X Y
#define A_TEST(A, B) ((A)+(B))

How to Correctly use Variadic Macros to Call Other Macros

1 Answers1