How to ignore duplicate symbols error in a lib file? - objective-c

Question

We are trying to integrating a .a static lib file into a React Native project. During build time, we encountered an error where a symbol (constant) is found declared multiple times in the single .a file.
We also try to integrate it into a normal Xcode project while having largely the same build configuration. Xcode would only emit warnings about the duplicate symbols and the build was successful.
The screenshot below shows the linker flags we are using in the react-native project. We suspect it was the inherited linker flags that came from other required react-native pods that cause this issue. Is there a way to ignore this error or to build them separately?
Update #1
As I investigate further, I found that manually removing -ObjC from the OTHER_LDFLAGS in Pods-{TARGET_NAME}.xcconfig allows the project to build but crashes during runtime since the RCTRootView is depending on it.

Answer 1

The cause is likely to be function(s) defined in a header file which were intended to be inlined but weren't declared as such. When they were complied into .a, there would be a symbol foo in it and if the same header is included in your react-native project, there would be a duplicate symbol foo.
For eg:
in header.h
int foo() {
return(1);
}
instead of
inline int foo() {
return(1);
}
If header.h is included in a.cpp and compiled into liba.a and then we include header.h in b.cpp, compile b.cpp into b.o and link b.o with liba.a, we will get foo() as duplicate symbol.
The preferred way to address this is to fix the header.h file with the addition of the inline keyword.
If not, you could use some nasty hacks like renaming the symbols at the preprocessor level. (-DOldSymbol=NewSymbol flags to the ‘Other C Flags’ build setting). You will have to do this for all symbols for which you get conflicts.
See http://atastypixel.com/blog/avoiding-duplicate-symbol-issues-when-using-common-utilities-within-a-static-library/ for how one developer addressed this problem.
Another way is to remove the duplicate symbols manually. Essentially, use lipo to split the architecture specific libs into separate .a files, ar to unpack the .a file, remove offending .o files, repack the remaining .o files into a new .a file using libtool and then use that .a file. See http://atnan.com/blog/2012/01/12/avoiding-duplicate-symbol-errors-during-linking-by-removing-classes-from-static-libraries/ for how it is done.