netfilter buglog - Jun 2013 - [Bug 826] New: libiptc/libip6tc.h doesn't specify C linkage

https://bugzilla.netfilter.org/show_bug.cgi?id=826

           Summary: libiptc/libip6tc.h doesn't specify C linkage
           Product: iptables
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: minor
          Priority: P5
         Component: libiptc
        AssignedTo: netfilter-buglog at lists.netfilter.org
        ReportedBy: quentin at armitage.org.uk
   Estimated Hours: 0.0


libiptc/libiptc.h specifies extern "C", and although libip6tc.h
includes a test
for __cplusplus at the beginning, it doesn't specify C linkage.

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Phil Oester <netfilter at linuxace.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |netfilter at linuxace.com

--- Comment #1 from Phil Oester <netfilter at linuxace.com> 2013-06-20
03:23:12 CEST ---
And this is a problem for you why, exactly?  Please explain why this deficiency
should be corrected.

Also, patches are welcomed.

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--- Comment #2 from Quentin Armitage <quentin at armitage.org.uk>
2013-06-20 10:57:57 CEST ---
If I want to include libiptc/libip6tc.h in a C++ program, I have to wrap the
#include in a extern "C" section, i.e.

extern "C" {
#include <libiptc/libip6tc.h>
}

On the other hand, this is not necessary for libiptc/libiptc.h since it already
includes the extern "C" section in the header file.

Since libiptc/libip6tc.h already includes a check for #ifdef __cplusplus, it is
clearly attempting to be compatible with C++, but without the extern
"C"
statement, it isn't fully. This mean that in my C++ code, which uses both
IPv4
and IPv6, the way I have to include the headers is inconsistent:

#include <libiptc/libiptc.h>
extern "C" {
#include <libiptc/libip6tc.h>
}

I have tested nested extern "C" sections, e.g.
extern "C" {
// Some code
extern "C" {
// some more code
}
}
and that compiles without a problem, so adding extern "C" to
libip6tc.h
shouldn't break any code that already includes includes it within an extern
"C"
section.

Finally, libiptc/libiptc.h and libiptc/libip6tc.h are so similar that it seems
inconsistent that one should have the external linkage statement and not the
other.

This isn't really a problem for me now, since I have added the necessary
externs to my code, but I thought that it would be helpful to point out the
inconsistency and may help others in the future.

I will attach a patch.

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--- Comment #3 from Quentin Armitage <quentin at armitage.org.uk>
2013-06-20 10:59:36 CEST ---
Created attachment 402
  --> https://bugzilla.netfilter.org/attachment.cgi?id=402
Patch to add extern "C" linkage section to libiptc/libip6tc.h

This patch makes libiptc/libip6tc.h consistent with libiptc/libiptc.h and
standardises inclusion of the two headers in C++ programs.

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Pablo Neira Ayuso <pablo at netfilter.org> changed:

           What    |Removed                     |Added
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                 CC|                            |pablo at netfilter.org

--- Comment #4 from Pablo Neira Ayuso <pablo at netfilter.org> 2013-06-20
12:15:09 CEST ---
I suggest you to keep a cached copy of that file (plus the changes to make it
compile with C++) in your source tree.

We do so in most of the existing netfilter utilities. Relying on the installed
Linux kernel headers is not a good idea, as your utility would break
compilation if they are not installed.

That library was not ever intended to be used for third party applications.

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--- Comment #5 from Phil Oester <netfilter at linuxace.com> 2013-06-21
23:35:54 CEST ---
Quentin - is Pablo's suggestion to maintain a copy of this header going to
work
for you?  It sounds as though this change is not going to be added to the
official header file.

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Phil Oester <netfilter at linuxace.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |WONTFIX

--- Comment #6 from Phil Oester <netfilter at linuxace.com> 2013-07-01
17:19:49 CEST ---
Closing, as it does not appear changes to support C++ compiling of headers is
going to be accepted.

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