Linking to c++ libraries in babel code block

Question

The title says it all, and I've included the relevant sections of my file below. I'm trying to create a src block that uses some functionality of the boost libraries, the first src block sets up the compiler, including the libraries to link to. The second src block is just a test function, but it quits with a linker error.

#+begin_src elisp :exports results
   (setq org-babel-C++-compiler
       (concat "g++ -std=c++0x"
               " -lboost_system -lboost_filesystem"
               " -I"
               (expand-file-name ".")
       )
   )
#+end_src


#+begin_src C++ :exports both :includes (list "\"boost/filesystem.hpp\"" "<stdio.h>")
 printf("testing\n");
#+end_src

The error:

/tmp/ccvz9bRM.o: In function __static_initialization_and_destruction_0(int, int)': C-src-2374gem.cpp:(.text+0x51): undefined reference toboost::system::generic_category()' C-src-2374gem.cpp:(.text+0x5d): undefined reference to boost::system::generic_category()' C-src-2374gem.cpp:(.text+0x69): undefined reference toboost::system::system_category()' collect2: error: ld returned 1 exit status /bin/bash: /tmp/babel-2374l5F/C-bin-2374tos: No such file or directory

Answer 1

Use :libs to specify libraries.

((or `c `cpp)
 (org-babel-eval
  (format "%s -o %s %s %s %s"
    (pcase org-babel-c-variant
      (`c org-babel-C-compiler)
      (`cpp org-babel-C++-compiler))
    tmp-bin-file
    flags
    (org-babel-process-file-name tmp-src-file)
    libs)
   ""))

As we can see, org-babel-C-execute will inserts :flags in front of the source file and :libs behind it.

Answer 2

First off, it's not necessary to use the elisp code included. The flags property lets compiler flags be passed,

:flags "-lboost_system -std=c++0x"

Unfortunately, due to the order the flags are passed in it's necessary to do some lisp hacking in order to allow linking.

1. Open the file with the definition of org-babel-C-execute: C-h f org-babel-C-execute, then click on the source file to open it.
2. Switch the last two arguments to format. It should be (format "%s -o %s %s %s" (cond ((equal org-babel-c-variant 'c) org-babel-C-compiler) ((equal org-babel-c-variant 'cpp) org-babel-C++-compiler)) (org-babel-process-file-name tmp-bin-file) (org-babel-process-file-name tmp-src-file) (mapconcat 'identity (if (listp flags) flags (list flags)) " ") ) ""))))
3. Load the new function with M-x compile-defun
4. If you had the file byte compiled, recompile the file with byte-compile-file.