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Re: [O] org-babel C math.h issue

From: Thierry Banel
Subject: Re: [O] org-babel C math.h issue
Date: Fri, 11 Mar 2016 19:52:25 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.0

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


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;


Oz, do you think you would be able to provide a patch?
The way to contribute is documented here:

Thanks for reporting.

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));
> 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

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