C++ with extern "C" in namespace prefix resolution and optimization level dependency - c++

Question

I have a file "test.cxx" with
namespace net {
extern "C" {
#include <arpa/inet.h>
}
}
int main() {
htons(1024);
}
When compiling with -O1 or more everything's fine.
When compiling with -O0:
error: ‘htons’ was not declared in this scope
suggested alternative: ‘net::htons’
Then I change htons to net::htons.
When compiling with -O0 everything's fine.
When compiling with -O1 or more:
error: expected unqualified-id before ‘(’ token
Reproduced that on gcc-4.9.2 and clang-3.7.0.
Can someone explain why does it happen?

Answer 1

It happens because at -O0, call is compiled to htons function and your declaration for this function is inside namespace net. In optimized version, -O2 for example, call is replaced with a macro.
You can verify this by pre-compiling your program using gcc -O0 -E v/s gcc -O2 -E
When htons is used
At -O2, htons is translated to
int main() {
(__extension__ (
{
register unsigned short int __v, __x = (unsigned short int) (1024);
if (__builtin_constant_p (__x))
__v = ((unsigned short int) ((((__x) >> 8) & 0xff) | (((__x) & 0xff) << 8)));
else
__asm__ ("rorw $8, %w0" : "=r" (__v) : "0" (__x) : "cc");
__v;
}
));
}
Making the code not throw the resolution error.
error: ‘htons’ was not declared in this scope
When net::htons is used
When you replace ntohs with net::ntohs, ntohs define is used / exposed for optimization and your pre-processed code look as:
int main() {
net::(__extension__ ({... /* removed for Brevity */ ...}));
}
And hence the error
error: expected unqualified-id before ‘(’ token
Why does it happenhtons may be implemented as a function or a macro. If it is defined as macro, htons would work fine. But if it is defined as a function net::htons would work fine.
It appears at -O1 or higher, header files expose macro versions instead of function.
Possible solutions
using namespace net; // Not recommended
#ifndef htons // Recommended
using net::htnos;
#endif
extern "C" { // Add all declarations in global space
#include <arpa/inet.h>
}