Apache log4cxx  Version 0.12.1

Frequently Asked Technical Questions

Table of contents

  1. How do I add a custom level to Apache log4cxx?
  2. My application on Windows crashes on shutdown?
  3. Does Apache log4cxx support Unicode?

How do I add a custom level to Apache log4cxx?

This is a common topic for all the Apache logging frameworks and typically motivated to try to categorize events by functionality or audience. An common request is to add an AUDIT level so that the user can configure AUDIT level messages to go to a specific appender. However, the logger name was designed explicitly to support routing of messages by topic or audience. The common pattern of using classnames for logger names obscures the more general capability of logger name to represent the topic or audience of the logging request. The easiest approach to solve the underlying issue is to use a logger names like "AUDIT.com.example.MyPackage.MyClass" that allow all AUDIT messages to be routed to a particular appender. If you attempted to use a level for that then you would lose the ability to distinguish between different significances within the audit messages.

My application on Windows crashes on shutdown?

Apache log4cxx API calls use C++ Standard Template Library string parameters. If the caller is using a different instance or type of the C Runtime Library that log4cxx, then it is very likely that some memory that was originally allocated by log4cxx would be freed by the caller. If log4cxx and the caller are using different C RTL's, the program will likely crash at the point. Use "Multithread DLL" with release builds of log4cxx and "Multithread DLL Debug" with debug builds.

Does Apache log4cxx support Unicode?

Yes. Apache log4cxx exposes API methods in multiple string flavors supporting differently encoded textual content, like char*, std::string, wchar_t*, std::wstring, CFStringRef et al. All provided texts will be converted to the LogString type before further processing, which is one of several supported Unicode representations selected by the --with-logchar option. If methods are used that take LogString as arguments, the macro LOG4CXX_STR() can be used to convert literals to the current LogString type. FileAppenders support an encoding property as well, which should be explicitly specified to UTF-8 or UTF-16 for e.g. XML files. The important point is to get the chain of input, internal processing and output correct and that might need some additional setup in the app using log4cxx:

According to the libc documentation, all programs start in the C locale by default, which is the same as ANSI_X3.4-1968 and what's commonly known as the encoding US-ASCII. That encoding supports a very limited set of characters only, so inputting Unicode with that encoding in effect to output characters can't work properly. For example, here is some Hebrew text which says "People with disabilities":

נשים עם מוגבלות

If you are to log this information, output on some console might be like the following, simply because the app uses US-ASCII by default and that can't map those characters:

loggername - ?????????? ???? ??????????????

The important thing to understand is that this is some always applied, backwards compatible default behaviour and even the case when the current environment sets a locale like en_US.UTF-8. One might need to explicitly tell the app at startup to use the locale of the environment and make things compatible with Unicode this way. See also some SO post on setting the default locale in C++.

std::setlocale( LC_ALL, "" ); /* Set locale for C functions */
std::locale::global(std::locale("")); /* set locale for C++ functions */

See LOGCXX-483 or GHPR #31 for additional details.