FAQ
From Log4cpp
General
What is Log for C++?
Log for C++ is a library of C++ classes for flexible logging to files, syslog and other destinations. It is modeled after the Log for Java library (http://jakarta.apache.org/log4j/), staying as close to their API as is reasonable.
How is Log for C++ related to Log4j? Is it a straight port?
Log for C++ strives to supply a similar interface for logging in C++ as Log4j provides in Java. However the implementation is not a translation of the Java code. So in that sense it is not a 'port' of Log4j. Of course Log for C++ does owe many of its concepts to log4j.
What is the name of this project, 'Log for C++' or 'log4cpp'?
The 'official' long name of the project is 'Log for C++', but in practice, the short name, 'log4cpp' has proven more convinient.
Under which license is Log for C++ available?
As of version 0.2.1 Log for C++ is released under the GNU Lesser General Public License (LGPL). Versions before that have been released under the GPL. See the license discussion on the forum at SourceForge for the motivations behind switching from GPL to LGPL.
Our legal department doesn't like the LGPL, can you release Log for C++ under license XYZ?
No. Long answer: Technically it may be possible if every contributor agrees, which due to their growing number has become increasingly difficult. But even if that could be overcome it will not happen. Of course the LGPL does grant you the opportunity to choose the GPL instead of the LGPL, but I bet XYZ != GPL.
Usage
I've succesfully compiled log4cpp, now how do I use this stuff?
For some small examples using log4cpp, see the 'tests' subdirectory. Also see the documentation section for a pointer for API documentation and more usage information.
Problems and error messages
I get 'Naming collision for 'ERROR' detected. Please read the FAQ for a workaround.'
This is caused by the rudeness of some platforms, which mutilate the namespace with some blunt #defines. To be more precise, the Win32 API includes #defines of 'ERROR' and 'DEBUG'. Since the preprocessor is unaware of C++ naming scopes this results in reserving the words ERROR and DEBUG litterally everywhere. In particular this conflicts with log4cpp::Prioritiy::ERROR and log4cpp::Priority::DEBUG. These latter two names come from log4j, so they are not something we made up ourselves.
They Win32 authors should not have rudelessly claimed these generic names through the preprocessor. There are much better alternatives:
1. If they use it as an integer constant, declare it using a language construct. Either 'enum {ERROR=1};' or 'static const int ERROR=1;' would do fine.
2. Use a less generic name like WIN32API_ERROR to make naming conflicts less likely
3. In case they use it as a flag for conditional compilation, use '#define DEBUG DEBUG' and '#if defined(DEBUG)'. In that case the preprocessor would simply replace all occurrences of 'DEBUG' in the source code with 'DEBUG', in effect leaving everything intact.
Of course the proper solution would be if the offending party would use one of the above methods, but we could have to wait some time for this to actually happen. As an alternative log4cpp can workaround these #defines. The workaround code is enabled by doing #define LOG4CPP_FIX_ERROR_COLLISION 1 before #including any log4cpp header files and after #including all platform headers. For Win32 platforms this #define has already been included in log4cpp/config-win32.h.
Once log4cpp has been updated to the log4j 1.2 API we can get rid of this problem by adopting the new names for log levels.
I am trying to compile/link the log4cpp package using the SunWorkshop compiler (CC) on a Solaris 7 machine. The library builds, but the testmain code fails to link properly.
A proper solution for this problem has not been found yet (suggestions are welcome), but James Emery reported success with the following workaround:
- Disable building of the static library
- Change the linker from /usr/ucb/ld to CC and enable 'place independent code' (pic).
In short, configure with:
export LD="CC -Kpic" && ./configure --disable-static
./configure fails to detect 'snprintf()' on platform XYZ.
./configure does not just check for the presence of a snprintf() function but for C99 compliancy as well. In particular, snprintf() should strictly honour the 'string size' parameter to avoid potential buffer overflow problems. log4cpp falls back to an alternative snprintf() implementation if the OS does not provide a correct one.
