Re: org-babel C math.h issue

[Nick Dokos <ndokos@gmail.com>]

Oz Ben-Ami <ozzieba@gmail.com> writes:

> 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.
>

I can't reproduce it either (with fairly recent emacs and bleeding-edge
org-mode, but the code in question has not changed since 8.2.7 or so).

The command that is executed looks like this:

  gcc -o /tmp/babel-212464kj/C-bin-21246R6L -lm /tmp/babel-212464kj/C-src-21246EwF.c

and that should work fine for gcc: you don't need to have the -lm after the
source file.

You can test that that's the case: put your program into a file, say foo.c, and
execute

  gcc -o foo.out -lm foo.c

Does that give you undefined references?

--
Nick