Static Library and -weak-lSystem

Question

I build a static library that links against other frameworks, particularly CoreLocation. I want to use features provided by iOS 5 but be compatible with 4.3.

My app crash at launch when I start it on iOS devices in 4.3 with this error :

Date/Time:       2012-08-22 16:44:53.900 +0200
OS Version:      iPhone OS 4.3.3 (8J3)
Report Version:  104

Exception Type:  EXC_BREAKPOINT (SIGTRAP)
Exception Codes: 0x00000001, 0xe7ffdefe
Crashed Thread:  0

Dyld Error Message:
  Symbol not found: _UIKeyboardDidChangeFrameNotification

The problem I encounter is described in this post : iOS 4 app crashes at startup on iOS 3.1.3: Symbol not found: __NSConcreteStackBlock.

But how do you deal with that when building a static library ?? I can't compile when I set the -weak-lSystem flag. Here is a trace :

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool: -dynamic not specified, -all_load invalid
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool: -dynamic not specified the following flags are invalid: -ObjC 
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool: can't locate file for: -weak-lSystem
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool: file: -weak-lSystem is not an object file (not allowed in a library)
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool: file: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS5.1.sdk/usr/lib/libxml2.2.dylib is a dynamic library, not added to the static library
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool: file: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS5.1.sdk/usr/lib/libxml2.2.dylib is a dynamic library, not added to the static library
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool: file: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS5.1.sdk/usr/lib/libz.dylib is a dynamic library, not added to the static library
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool: file: /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS5.1.sdk/usr/lib/libz.dylib is a dynamic library, not added to the static library
Command /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/libtool failed with exit code 1

Resolved

See accepted answer below and do not forget to mark libraries used in different versions as Optional in Xcode. Ex: I use UIKit new notification for iOS 5 but my deployement target is 4.3 so I need to mark this library as Optional in order to make things work. It is the same for CoreLocation CLGeocoder new iOS 5 class.

Answer 1

The problem is that UIKeyboardDidChangeFrameNotification is not available on iOS 4 and therefore the dynamic loader (Dyld) fails.

From the perspective of the static library developer you don't have to do anything. The -weak-lSystem flag should be set in the Xcode project that uses the static library for an application (see the post mentioned in the question) - not in the project for the static library.