Undefined symbols for architecture x86_64 on OS X with fat library

Question

I have build libcrypto.a and libssl.a myself from source, specifying darwin64-x86_64-cc (for 64-bit) and darwin-i386-cc (for 32-bit) to OpenSSL's configure script.
Created the fat libraries with lipo and added them as a dependency in my Xcode project.

However, I'am getting an undefined symbol error:

undefined symbols for architecture x86_64:
  "_OPENSSL_ia32cap_P", referenced from:
      _AES_cbc_encrypt in libcrypto.a(aes-x86_64.o)
ld: symbol(s) not found for architecture x86_64

Note: Using the same technique works fine for iOS, though.

lipo -detailed_info libcrypto.a reveals:

Fat header in: libcrypto.a
fat_magic 0xcafebabe
nfat_arch 2
architecture i386
    cputype CPU_TYPE_I386
    cpusubtype CPU_SUBTYPE_I386_ALL
    offset 48
    size 2700624
    align 2^2 (4)
architecture x86_64
    cputype CPU_TYPE_X86_64
    cpusubtype CPU_SUBTYPE_X86_64_ALL
    offset 2700672
    size 3938432
    align 2^2 (4)

Answer 1

It looks to be a bug in the code generator for x64 in the static library case.

The easiest, non patch openssl change workaround is to add a reference to OPENSSL_cleanse somewhere in your code, even if it's not used. That will fix up the link-time reference.

What's actually happening is that the symbol is being referenced in some assembly code.

The assembly code simply says that _OPENSSL_ia32cap_P is an extern symbol, without making the cross-link to state that it needs to be linked in. This works for libcrypto.dylib because the reference is resolved when generating the .dylib file; however the reference is never resolved in the .a case because the only code that actually contains the symbol is x86_64cpuid.o, which only gets linked in if you use any of the routines provided by that .o.

Symbols in this file includes OPENSSL_cleanse, so if you reference this routine, the link works.