使用共享DLL进行日志记录的体系结构

Question

I've been writing logging code for our product, and have an architectural problem. We have two command-line executables, written in C++ for Window, call them Foo and Bar, that depend on a DLL which we'll name Core. I want to log from within Core.

The question is, where should those log entries end up? If I run Foo, I want to see them in Foo.log, and if I run Bar, they should be in Bar.log. What if I run both Foo and Bar at the same time, what then? (I think I already sorted the case when multiple copies of Foo or Bar are run, effectively locking the log file).

One thought is that Core can keep a list of "all the loggers I need to invoke when someone makes a logging request". This implies there is a whole new API to write and that logging in DLLs is written different to logging in exes or static libraries. That's not ideal. I may not even know where the code ends up if it's in a lib!

I've looked at log4cplus, and boost logging but can't get any traction on how this would work with those components either, so am a bit stuck for ideas. This is surely a solved problem though?!

Answer 1

Function pointers is the key, IMO.

I usually implement loggers on the very top level, in the .exe code, and on startup I pass the function pointer all the way down. Here's an example.

enum struct eLogLevel : uint8_t
{
    Error,
    Warning,
    Info,
    Debug
};
// C function pointer to write messages to arbitrary destination. Messages are better to be UTF8.
using pfnLogMessage = void( *)( eLogLevel lvl, const char* msg );

// DLL API, call this once, immediately after loading your DLL. Keep the level + function pointer in static variables. 
// Unless messing with DLL segments, static variables are per-process, i.e. two processes will have different copy of these variables.
// Good idea to also call on shutdown, passing nullptr, before closing the underlying implementation stream.
void initializeLogging( eLogLevel maxLevelToLog, pfnLogMessage fnLogMessage );

// Invoke that function pointer to actually log stuff.
void logMessage( eLogLevel lvl, const CStringA& msg );

// Convenience wrappers to format and write various messages.
// Moar examples: https://github.com/Const-me/vis_avs_dx/blob/master/avs_dx/DxVisuals/Utils/logger.h
inline void logMessageV( eLogLevel lvl, const char* pszFormat, va_list args )
{
    CStringA str;
    str.FormatV( pszFormat, args );
    logMessage( lvl, str );
}

#define LOG_MESSAGE_FORMAT( lvl ) va_list args; va_start( args, pszFormat ); logMessageV( lvl, pszFormat, args ); va_end( args );

inline void logError( const char* pszFormat, ... )
{
    LOG_MESSAGE_FORMAT( eLogLevel::Error );
}