why adding extern(C) cause a runtime error?

[mzfhhhh via Digitalmars-d-learn]

win7   x86  dmd2.067.1 ok
ubuntu x64  dmd2.067.1 error
-------------------------------------
import std.stdio;
import std.socket;

extern(C)
void recv()
{
     writeln("recv...");
}

extern(C)
void send()
{
     writeln("send...");
}


int main(string[] argv)
{
     //copy from std.socket unittest

     immutable ubyte[] data = [1, 2, 3, 4];
     auto pair = socketPair();
     scope(exit) foreach (s; pair) s.close();

     pair[0].send(data);

     auto buf = new ubyte[data.length];
     pair[1].receive(buf);
     assert(buf == data);

     return 0;
}
----------------------------------
send...
recv...
core.exception.AssertError at a.d(27): Assertion failure
----------------
./a() [0x43d61f]
./a(_Dmain+0xcc) [0x43d1bc]
./a(_D2rt6dmain211_d_run_mainUiPPaPUAAaZiZ6runAllMFZ9__lambda1MFZv+0x1f) [0x4400fb]
./a(void rt.dmain2._d_run_main(int, char**, extern (C) int 
function(char[][])*).tryExec(scope void delegate())+0x2a) 
[0x44004e]
./a(void rt.dmain2._d_run_main(int, char**, extern (C) int 
function(char[][])*).runAll()+0x30) [0x4400b4]
./a(void rt.dmain2._d_run_main(int, char**, extern (C) int 
function(char[][])*).tryExec(scope void delegate())+0x2a) 
[0x44004e]
./a(_d_run_main+0x1dc) [0x43ffc8]
./a(main+0x17) [0x43d637]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf5) 
[0x7f5fabd8fec5]