building using Autotools + Clang causes PF_PACKET support detection to fail

[robertkirkman]

Hello, after 99f24b1, there is now an instance of code like this being used to check for PF_PACKET support,

libdnet/configure

Lines 15565 to 15571 in 06ecd5d

	int
	main (void)
	{
	int foo() { return ETH_P_ALL; }
	;
	return 0;
	}

Attempting to compile code like this using Clang results in this error, because of a function definition being inside another function definition:

test.c:7:11: error: function definition is not allowed here
    7 | int foo() { return ETH_P_ALL; }
      |           ^
1 error generated.

After getting the code this way,

git clone -b devel https://github.com/ofalk/libdnet.git
cd libdnet/

This results in these two commands producing different outputs on a regular Gentoo GNU/Linux amd64 computer:

./configure CC=gcc

./configure CC=clang

Here is the relevant, problematic difference between the outputs they produce:

@@ -144,7 +144,7 @@
 checking for strsep... yes
 checking for Berkeley Packet Filter... no
 checking for Linux proc filesystem... yes
-checking for Linux PF_PACKET sockets... yes
+checking for Linux PF_PACKET sockets... no
 checking for SNMP MIB2 STREAMS... no
 checking for route(7) STREAMS... no
 checking for arp(7) ioctls... yes

Unfortunately, using autoreconf -fi before ./configure does not seem to make any difference here when I tried it.

[robertkirkman]

Some additional information is that, the problem appears to also be related to this line:

libdnet/m4/acinclude.m4

Line 141 in 06ecd5d

[int foo() { return ETH_P_ALL; }],

which was added in this commit: b4e3aa2

It is believed that if "int foo()" is removed from this line, and then aclocal --force and autoreconf -fi and ./configure CC=clang are run again, then the problem will stop.

[ofalk]

Thanks @robertkirkman for bringing up this issue and digging deeper into it. Mind sending me an MR?

[robertkirkman]

Sure! I have opened one.

I hadn't mentioned this anywhere yet, but just for full clarity, I've checked and this doesn't affect the CMake build, only the Autotools build.

~/code/libdnet-vanilla $ cmake . -DCMAKE_C_COMPILER=gcc 2>&1 | grep ETH_P_ALL
-- Looking for ETH_P_ALL
-- Looking for ETH_P_ALL - found
~/code/libdnet-vanilla $ cmake . -DCMAKE_C_COMPILER=clang 2>&1 | grep ETH_P_ALL
-- Looking for ETH_P_ALL
-- Looking for ETH_P_ALL - found
~/code/libdnet-vanilla $ 

[ofalk]

Should be addressed now. In case there's still an issue, please let me know. Again, appreciate the contribution!