Thanks for the response.

On my machine, I get the undefined references even with GCC 5 (gcc version
5.2.1 20151010 (Ubuntu 5.2.1-22ubuntu2)), if I put -lm before the source
file name.

As far as a patch, I can try but this would be my first time really playing
with org-mode code. As far as I can see, I would need to add the following
lines in ob-C.el:
L133: (libs (cdr (assoc :libs params)))
         (libs (mapconcat 'identity
               (if (listp libs) libs (list libs)) " "))
L142: (format "%s -o %s %s %s %s" ;; (added %s)
L147: libs

Am I missing anything?

Oz

On Fri, Mar 11, 2016 at 1:52 PM, Thierry Banel <tbanelwebmin@free.fr> wrote:

> To summarize this thread:
> It seems there was an issue long ago.
> Version 5 of GCC magically fixed it.
> But it is still here for older compilers.
>
> -------------------------
>
> To debug, execute the following piece of lisp code:
>
>     (defadvice org-babel-eval (before xxx (cmd body))
>       ""
>       (message "org-babel-eval : %s" cmd))
>     (ad-activate 'org-babel-eval)
>
> The actual compilation command will be displayed in the *Messages* buffer.
>
> With your example, we get
>
>    gcc -o /tmp/babel-8412zIw/C-bin-8412IZR -lm
> /tmp/babel-8412zIw/C-src-84127OL.c
>
> --------------------------
>
> You are right, Oz, this can be fixed in line 147.
> But :flags is intended for flags like -g or -O, which need to appear
> before the source file.
>
> Probably we are missing an additional flag, which could be named :libs
> Then your example would be:
>
>   #+BEGIN_SRC C :includes '(<math.h> <stdio.h>) :libs -lm
>   int i=9;
>   printf("%d\n",(int)sqrt(i));
>   #+END_SRC
>
> --------------------------
>
> Oz, do you think you would be able to provide a patch?
> The way to contribute is documented here:
> http://orgmode.org/worg/org-contribute.html#orgheadline1
>
> Thanks for reporting.
> Thierry
>
>
> Le 06/03/2016 23:44, Oz Ben-Ami a écrit :
> > Hi,
> >
> > I'm a new org user, and I'm sorry if I'm missing something obvious.
> > When executing a C code snippet with org-babel, that contains a math
> > function, I get the famous "undefined reference" errors. I tried
> > adding ":flags -lm", but that doesn't help. Looking at the code, it
> > seems the -lm flag is inserted in the wrong place, before the source
> > file. An easy change would be in line 147 of ob-C.el version
> > 8.3.4-634, moving "flags" to after the source file. This seems to
> > work, but I don't know if it would break anything else.
> >
> > A minimal working example, attached, includes the following snippet:
> >
> > #+BEGIN_SRC C :includes '(<math.h> <stdio.h>) :flags -lm
> > int i=9;
> > printf("%d\n",(int)sqrt(i));
> > #+END_SRC
> >
> > Note the issue disappears if constants are directly used rather than
> > variables, presumably because the function call is optimized away
> > entirely.
> >
> > Any thoughts are appreciated.
> >
> > Oz
>
>
>