Experimental Physics and
Industrial Control System

Michael Davidsaver via Core-talk <core-talk at aps.anl.gov> · Mon, 13 Apr 2020 00:29:20 -0700

All,

I've run up against a strange behavior with MSVC.  I've reduced the problem somewhat,
but which I'm not sure how to proceed.  This might have something to do with the
MSVC pre-processor, implicit concatenation of string constants, or maybe vararg functions.

It has to do with how I'm trying to do logging.  At core I have a function:

> void _log_printf(unsigned lvl, const char* fmt, ...);

which internally calls errlogVprintf()

I'm calling it through two levels of macro.  eg.

> #define log_info_printf(LOGGER, ...)  log_printf(LOGGER, ::pvxs::Level::Info, __VA_ARGS__)

> #define log_printf(LOGGER, LVL, FMT, ...) do{ \
>     if((LOGGER).test(LVL)) \
>        ::pvxs::detail:: _log_printf(unsigned(LVL), "%s " FMT, "", __VA_ARGS__); \
> }while(0)

called like:

> log_info_printf(logerr, "Enter loop worker for %p\n", base.get());

The reason for the second level of macro is that I'd like to prefix each log line
with some extra information, including a timestamp, in a way which can't be split.
(as could happen if I called errlogPrintf() twice)

The above is already a reduced case.  The empty string constant "" after FMT will be
replaced with a char* returned by a function (a thread local buffer).  However, I can't
get that far.

The above, with the empty string constant, is somehow corrupting output and crashing.
eg.

> `b"l� Enter loop worker for 00007FFA6C34DCC0
> `b"l� Enter loop worker for 00007FFA6C34DCC0

If I change the log_printf macro as follows, then everything is fine (sans timestamp).

> #define log_printf(LOGGER, LVL, FMT, ...) do{ \
>     if((LOGGER).test(LVL)) \
>        ::pvxs::detail:: _log_printf(unsigned(LVL), FMT, __VA_ARGS__); \
> }while(0)

Does anyone have an idea of what is going on here?

I don't think I'm depending on any GCC-isms here.  (eg.  __VA_ARGS__ always has
at least one argument)  And '"%s " FMT' should be taking advantage of automatic
concatenation of string constants.

I'm seeing this on appveyor with the MSVC 2019, 2017, and 2015.
mingw builds work fine.

https://ci.appveyor.com/project/mdavidsaver/pvxs

the full version I'm starting from (and would like to end with).

https://github.com/mdavidsaver/pvxs/blob/e2a7e5e451a06e3865b8684e747dfaa6026a6e15/src/pvxs/log.h#L77-L80

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System