c - Static library link issue with Mac OS X: symbol(s) not found for architecture x86_64

Question

I'm trying to generate a static library and link it with an execution binary.

This is a library function:

#include <stdio.h>

int hello() {
    return 10;
}

With these commands, I could get a static library.

gcc -c io.c 
ar -crv libio.a io.o

With lip -info, I checked it is x86_64 architecture.

ar> lipo -info libio.a 
input file libio.a is not a fat file
Non-fat file: libio.a is architecture: x86_64

This is the main function that uses the library.

#include <stdio.h>
extern int hello();

int main(int argc, char *argv[]) {
    printf("%d", hello());
}

However, when I link the object with the static library, I have errors.

gcc main.c -lio -o main -L.

Error messages are:

ld: warning: ignoring file ./libio.a, file was built for archive which is not the architecture being linked (x86_64): ./libio.a
Undefined symbols for architecture x86_64:
  "_hello", referenced from:
      _main in main-2c41a0.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I use the ar as in /bin/ar, and Mac OS X is 10.10.2 with clang-602.0.53.

ar> clang -v
Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
Target: x86_64-apple-darwin14.3.0
Thread model: posix

What might be wrong?

Answer 1

The library should have generated with libtool -static.

gcc -c io.c 
libtool -static -o libio.a io.o
gcc main.c -lio -o main -L.
main

Returns

10

ar> lipo -info libio.a 
input file libio.a is not a fat file
Non-fat file: libio.a is architecture: x86_64

ar> file libio.a 
libio.a: current ar archive

ar> nm libio.a 

io.o:
0000000000000000 T _hello

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