What happen if a static library (.a) contains duplicated object files (.o)? - ios

Question

AFAIK, older version of Xcode failed to link a static library if it has been linked multiple times. I think there's a kind of duplicated object-file or symbol issue.
Recently, I realized Xcode5 is not reporting related error anymore on duplicated link of a static library. Here's my test.
I wrote 3 Objective-C classes are named as s1, s2, s3 in separated projects.
Each of them is compiled into s1.o, s2.o, s3.o,
and finally archived into s1.a, s2.a, s3.a.
s2 linked (merged) with s1, and s3 is linked (merged) with s2 and s1.
Finally I got these .a files.
Eonil$ ar -tv libs1.a
rw-r--r-- 501/20 64 Aug 3 18:38 2014 __.SYMDEF SORTED
rw-r--r-- 501/20 5528 Aug 3 18:38 2014 s1.o
Eonil$ ar -tv libs2.a
rw-r--r-- 501/20 120 Aug 3 18:38 2014 __.SYMDEF SORTED
rw-r--r-- 501/20 5872 Aug 3 18:38 2014 s2.o
rw-r--r-- 501/20 5528 Aug 3 18:38 2014 s1.o
Eonil$ ar -tv libs3.a
rw-r--r-- 501/20 224 Aug 3 18:38 2014 __.SYMDEF
rw-r--r-- 501/20 5872 Aug 3 18:38 2014 s3.o
rw-r--r-- 501/20 5528 Aug 3 18:38 2014 s1.o
rw-r--r-- 501/20 5872 Aug 3 18:38 2014 s2.o
rw-r--r-- 501/20 5528 Aug 3 18:38 2014 s1.o
Those same named .o files are all duplicated.
I wrote a p1 project which is a command line executable, and linked it with all the s1.a, s2.a, s3.a. I expected some duplication error, but it just built and working well!
What's happening in Xcode for these duplicated linking of static libraries?

Answer 1

Confirmed; the linker stops looking for symbols once it finds it in the first library.
If you use the -all_load option, however it will fail:
$ clang -all_load -o prog main.o -L. -lone -ltwo -lthree
duplicate symbol _func in:
./libone.a(file.o)
./libtwo.a(file.o)
duplicate symbol _func in:
./libone.a(file.o)
./libthree.a(file.o)
ld: 2 duplicate symbols for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
This was tested using C, not that that matters:
$ cat main.c
extern void func();
int main(int argc, const char **argv)
{
func();
return 0;
}
$ cat file.c
#include <stdio.h>
void func()
{
printf("func()\n");
}