C: passing arguments from variadic function to variadic macro

Question

I have a standard logging API built into my C code, a nice simple logF(const char *pFormat, ...) thing which has always, so far, been mapped to vprintf(), i.e.:

void logF(const char *pFormat, ...)
{
    va_list args;
    va_start(args, pFormat);
    vprintf(pFormat, args);
    va_end(args);
}

The C code above this API must work on multiple embedded platforms. I've just reached a platform (Nordic NRF52840) where the underlying logging interface I have to work with is presented as a variadic macro of the form NRF_LOG_INFO(...).

QUESTION: how do I correctly pass fn(const char *pFormat, ...) into a BLAH(...) macro? My brain hurts....

This is with GCC 4.9.3, though it would be nice to have a solution that is relatively relaxed about C compiler version.

EDIT 1: noted that I could redefine my logF() function to be a variadic macro and map it there, the issue is that I would then have a platform-specific header file rather than a generic one, I would have to move it down into the platform code and have one for each. Not impossible but more messy.

EDIT 2: I was asked for the trail of how NRF_LOG_INFO() expands. Here's the relevant output of the pre-processor:

#define NRF_LOG_INFO(...) NRF_LOG_INTERNAL_INFO( __VA_ARGS__)

#define NRF_LOG_INTERNAL_INFO(...) NRF_LOG_INTERNAL_MODULE(NRF_LOG_SEVERITY_INFO, NRF_LOG_SEVERITY_INFO, __VA_ARGS__)

#define NRF_LOG_INTERNAL_MODULE(level,level_id,...) if (NRF_LOG_ENABLED && (NRF_LOG_LEVEL >= level) && (level <= NRF_LOG_DEFAULT_LEVEL)) { if (NRF_LOG_FILTER >= level) { LOG_INTERNAL(LOG_SEVERITY_MOD_ID(level_id), __VA_ARGS__); } }

#define LOG_INTERNAL(type,...) LOG_INTERNAL_X(NUM_VA_ARGS_LESS_1( __VA_ARGS__), type, __VA_ARGS__)

#define LOG_INTERNAL_X(N,...) CONCAT_2(LOG_INTERNAL_, N) (__VA_ARGS__)

Then depending on number of args, anything up to:

#define LOG_INTERNAL_6(type,str,arg0,arg1,arg2,arg3,arg4,arg5) nrf_log_frontend_std_6(type, str, (uint32_t)(arg0), (uint32_t)(arg1), (uint32_t)(arg2), (uint32_t)(arg3), (uint32_t)(arg4), (uint32_t)(arg5))

void nrf_log_frontend_std_6(uint32_t severity_mid,
                            char const * const p_str,
                            uint32_t val0,
                            uint32_t val1,
                            uint32_t val2,
                            uint32_t val3,
                            uint32_t val4,
                            uint32_t val5);

You could rename your variadic function and use a variadic wrapper macro which calls either your own function or the platform specific macro. — Bodo
– Bodo, Commented Mar 25, 2020 at 14:07
@MOehm: it's the other way around, I'm inside a function which has received const char pFormat, ..., how do I shove that down BLAH(...). — Rob
– Rob, Commented Mar 25, 2020 at 14:09
Have you looked at what the variadic macro does inside? Can you post that? — John Zwinck
– John Zwinck, Commented Mar 25, 2020 at 14:10
@Bodo, agreed, I was trying to avoid that though as the header file is part of the API and should be platform independent. To do that I'd need to move the header down into the platform code and have one for each platform. Not impossible, just more messy. — Rob
– Rob, Commented Mar 25, 2020 at 14:10

Bodo · Accepted Answer · 2020-03-25 14:30:55Z

4

It is not possible to pass the arguments from a variadic function to a variadic macro.

As you want to hide the platform specific macro call from the API header you can process the function arguments with vsnprintf instead of vprintf and call the logging macro with format "%s" and the resulting string buffer.

void logF(const char *pFormat, ...)
{
    va_list args;
    /* Choose a reasonable size or replace with dynamic allocation based on the return value of vsnprintf */
    /* This could also be a static variable or a global variable to avoid allocation of a big buffer on the stack. */
    char buffer[1024];

    va_start(args, pFormat);
    vsnprintf(buffer, sizeof(buffer), pFormat, args);

    NRF_LOG_INFO("%s", buffer);

    va_end(args);
}

Note that you may have to call NRF_LOG_FLUSH before the buffer goes out of scope or gets overwritten. See https://devzone.nordicsemi.com/f/nordic-q-a/22647/nrf_log_info-how-to-print-log-with-string-parameter

edited Mar 25, 2020 at 14:30

answered Mar 25, 2020 at 14:22

Bodo

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1 Comment

Rob Over a year ago

Good post. Only thing I have to ask myself now is if I can afford the buffer space. Probably will avoid malloc() as I don't have much memory to play with and it might fragment.

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