Adopting os_log APIs while keeping backward compatibility - ios

Question

I'm trying to add support for the new logging and activity tracing APIs to a library in a way that maintains backward compatibility for users of the library who haven't yet adopted the latest version of the OS (iOS or macOS). I'm defining custom logging macros for each level of logging, and then for older OSes, falling back to NSLog. I've gotten this working, with one problem.
The new APIs require you to mark any non-constant, non-scalar values as explicitly public if you want them to show up in log output. This is what an invocation of my macro looks like:
UZKLogInfo("Reading file %{public}# from archive", fileName);
This compiles fine with the SDK that includes os_log (e.g. iOS 10.0 or later), but when I compile with an earlier version so my macro falls back to NSLog, I get a compiler warning:
Using 'public' format specifier annotation outside of os_log()/os_trace()
And the log line printed looks like this:
Reading file <decode: missing data> from archive
This is a simplified version of my macro definition (only including the info definition and simplifying the conditional:
#if UNIFIED_LOGGING_SUPPORTED
#import os.log;
#define UZKLogInfo(format, ...) os_log_info(OS_LOG_DEFAULT, format, ##__VA_ARGS__);
#else // Fall back to regular NSLog
#define UZKLogInfo(format, ...) NSLog(#format, ##__VA_ARGS__);
#endif
Is there any way to strip the "{public}" text (some kind of string replacement?) from format in the fallback case? Or is there another way to support the old and new APIs without giving up the level of info I've always shown in the logs? I need to use a macro (according the last year's WWDC session on the topic, or else I lose the call site metadata.

Answer 1

I chose to do an NSString replace in the macro, and to suppress the compiler warnings as part of it, so it could be done for each line, rather than globally for the whole file or project. It looks like this:
#if UNIFIED_LOGGING_SUPPORTED
#import os.log;
#define UZKLogInfo(format, ...) os_log_info(OS_LOG_DEFAULT, format, ##__VA_ARGS__);
#else // Fall back to regular NSLog
#define _removeLogFormatTokens(format) [#format stringByReplacingOccurrencesOfString:#"{public}" withString:#""]
#define _stringify(a) #a
#define _nsLogWithoutWarnings(format, ...) \
_Pragma( _stringify( clang diagnostic push ) ) \
_Pragma( _stringify( clang diagnostic ignored "-Wformat-nonliteral" ) ) \
_Pragma( _stringify( clang diagnostic ignored "-Wformat-security" ) ) \
NSLog(_removeLogFormatTokens(format), ##__VA_ARGS__); \
_Pragma( _stringify( clang diagnostic pop ) )
#define UZKLogInfo(format, ...) _nsLogWithoutWarnings(format, ##__VA_ARGS__);
#endif
It's called like so:
UZKLogInfo("Message: %#", anObjectToLog);