gcc, static library, external assembly function becomes undefined symbol

Question

Hello everybody, I have a problem with g++ building an application which links to a static library, where the latter shall contain some global functions written in external asm-files, compiled with yasm. So in the library, I have

#ifdef __cplusplus
extern "C" {
#endif

extern void __attribute__((cdecl)) interp1( char *pSrc );
extern void __attribute__((cdecl)) interp2( char *pSrc );

#ifdef __cplusplus
}
#endif

which I reference elsewhere inside the library. Then, there is the implementation in an asm-file, like this:

section .data
; (some data)
section .text
; (some text)

global _interp1
_interp1:
    ; (code ...)
    ret

global _interp2
_interp2:
    ; (code ...)
    ret

Compiling and Linking work fine for the library, I do

yasm -f elf32 -O2 -o interp.o interp.asm

and then

ar -rc libInterp.a objs1.o [...] objsN.o interp.o 
ranlib libInterp.a

Now finally, to link the library to the main application, I do

g++ -O4 -ffast-math -DNDEBUG -fomit-frame-pointer -DARCH_X86 -fPIC -o ../bin/interp this.o that.o -lboost_thread -lpthread ./libInterp.a 

and I get the errors

undefined reference to `interp1'
undefined reference to `interp2'

What am I doing wrong here? any help is appreciated.

Answer 1

Depending on the target type, gcc will not prepend a leading underscore to external symbols. It appears that this is the case in your scenario.

The simple fix is probably to remove the underscores from the names in your assembly file.

A couple alternatives you might consder might be to use something like one of the following macros for your symbols in the assembly file:

• from http://svn.xiph.org/trunk/oggdsf/src/lib/codecs/webm/libvpx/src/vpx_ports/x86_abi_support.asm

  ; sym()
  ; Return the proper symbol name for the target ABI.
  ;
  ; Certain ABIs, notably MS COFF and Darwin MACH-O, require that symbols
  ; with C linkage be prefixed with an underscore.
  ;
  %ifidn   __OUTPUT_FORMAT__,elf32
  %define sym(x) x
  %elifidn __OUTPUT_FORMAT__,elf64
  %define sym(x) x
  %elifidn __OUTPUT_FORMAT__,x64
  %define sym(x) x
  %else
  %define sym(x) _ %+ x
  %endif
  

• from http://www.dcs.warwick.ac.uk/~peter/otherstuff.html

  %macro public_c_symbol 1
  GLOBAL %1,_%1
  %1:
  _%1:
  %endmacro
  
  
  public_c_symbol my_external_proc:
      ; ...
      RET