How can I avoid "duplicate symbol" errors in xcode with shared static libraries?

Carl's answer is right, but for the wrong reasons: there's actually nothing wrong with linking static libraries together, as we can see using Carl's own sample. Set-up Carl's sample code and then do this: (I use libtool because that is what XCode uses)

neutron:libtest jamie$ libtool -o a2.a a.a c.aneutron:libtest jamie$ libtool -o b2.a b.a c.aneutron:libtest jamie$ gcc main.o a2.a b2.a -o app2neutron:libtest jamie$ ./app2acbcneutron:libtest jamie$ 

This links a2.a and b2.a with main.o. According to Carl, this is the source of OPs problem, and app2 shouldn't link. But of course it does. The linker is smart enough to ignore two instances of the same file. We can see that both a2.a and b2.a contain c.o:

neutron:libtest jamie$ ar -t a2.a__.SYMDEF SORTEDa.oc.oneutron:libtest jamie$ ar -t b2.a__.SYMDEF SORTEDb.oc.o

Yet it links fine.

The problem is, I believe, linked to Universal Binaries, either PPC/x86 universal binaries, or armv6/armv7 iPhone universal binaries. The problem here is that there is a bug with categories and the fix (add -all_load to the linker flags) is a fix that only works for single architectures. Using -all_load breaks the linkers ability to ignore symbols that are defined for multiple architectures, and you have your duplicate symbol error.

I wrote about it here including a better solution than using -all_load.

A alternative to using -all_load is to use -force_load "path_to_lib" just for the libraries where it is needed. For example, you can use something like: -force_load "$(PROJECT_DIR)/libname".

This avoids what you need to do for Jamie's solution which requires you to modify implementation files.

This is the solution adopted by the three20 project: http://groups.google.com/group/three20/browse_thread/thread/ec208be4ff8b4dcb/0dccf992a26850df

edit: as of Xcode 4.3 the need for -all_load and -force_load has been removed. Now only -ObjC is needed. See https://stackoverflow.com/a/2615407/211292 for more details.

This problem isn't necessarily Xcode or Objective-C related. Don't link/archive libraries into other libraries. A & B only depend on C at final link time, not when they're built. You want:

1. build A
2. build B
3. build C
4. build app & link

Here's an example project I made to demonstrate:

Makefile:

app: main.o a.a b.a c.a        gcc $^ -o $@%.o: %.c        gcc -Wall -c $^%.a: %.o        ar -r $@ $^clean:        rm -rf *.o *.a app

a.c:

#include <stdio.h>void c(void);void a(void){  printf("a\n");  c();}

b.c:

#include <stdio.h>void c(void);void b(void){  printf("b\n");  c();}

c.c:

#include <stdio.h>void c(void){  printf("c\n");}

main.c:

#include <stdio.h>void a(void);void b(void);int main(int argc, char *argv[]){  a();  b();  return 0;}

Build and run log:

$ makegcc -Wall -c main.cgcc -Wall -c a.car -r a.a a.oar: creating archive a.agcc -Wall -c b.car -r b.a b.oar: creating archive b.agcc -Wall -c c.car -r c.a c.oar: creating archive c.agcc main.o a.a b.a c.a -o apprm a.o b.o c.o$ ./app acbc