MinGW64 and "conflicting declaration of C function int select(...)"

Question

I'm trying to build Botan on MinGW, which is really MinGW-w64 (x86_64-pc-msys). It is failing on MinGW as shown below. I think I have it reduced to an MCVE :

#include <sys/select.h>
#include <winsock2.h>
#include <windows.h>
#include <memory>

int main(int argc, char* argv[])
{
    fd_set fds;
    FD_ZERO(&fds);
    FD_SET(0, &fds);

    timeval tv;
    tv.tv_sec = 5;
    tv.tv_usec = 0;

    int rc = select(1, &fds, NULL, NULL, &tv);

    return 0;
}

It results in:

$ g++ -m64 -pthread -std=c++11 test.cxx -o test.exe
In file included from /usr/include/w32api/winsock2.h:56:0,
                 from test.cxx:2:
/usr/include/w32api/psdk_inc/_fd_types.h:100:2: warning: #warning "fd_set and associated macros have been defined in sys/types.      This can cause runtime problems with W32 sockets" [-Wcpp]
 #warning "fd_set and associated macros have been defined in sys/types.  \
  ^~~~~~~
In file included from test.cxx:2:0:
/usr/include/w32api/winsock2.h:995:34: error: conflicting declaration of C function ‘int select(int, _types_fd_set*, _types_fd_set*, _types_fd_set*, PTIMEVAL)’
   WINSOCK_API_LINKAGE int WSAAPI select(int nfds,fd_set *readfds,fd_set *writefds,fd_set *exceptfds,const PTIMEVAL timeout);
                                  ^~~~~~
In file included from test.cxx:1:0:
/usr/include/sys/select.h:73:5: note: previous declaration ‘int select(int, _types_fd_set*, _types_fd_set*, _types_fd_set*, timeval*)’
 int select __P ((int __n, fd_set *__readfds, fd_set *__writefds,
     ^~~~~~

Including <sys/select.h> first is important. Including it after the Windows headers does not witness the problem. Including <memory> is important. <memory> includes additional headers like <sys/select.h>, which has already been included.

The best I can tell, MinGW headers are causing the problem. They provide the same function with two different signatures:

/usr/include/w32api/winsock2.h:

int select(int, _types_fd_set*, _types_fd_set*, _types_fd_set*, PTIMEVAL)

/usr/include/sys/select.h

int select(int, _types_fd_set*, _types_fd_set*, _types_fd_set*, timeval*)

The problem appears to be the last argument. In one case it is PTIMEVAL and in the other case it is timeval*. This seems to be related from the mailing list, but I don't understand what I should be doing: Replace struct timeval usage with PTIMEVAL and define TIMEVAL differently on LP64.

There may be some pointer size problems as detailed at What do I have to look out for when porting applications to 64 bit Cygwin?, but it is MinGW code (and not Botan code).

What is the problem, and how do I fix it?

---

Here is Botan's src/lib/utils/socket/socket.cpp.

Here is the compile error:

g++ -fstack-protector -m64 -pthread   -std=c++11 -D_REENTRANT -O3 -momit-leaf-frame-pointer  \
-Wall -Wextra -Wpedantic -Wstrict-aliasing -Wcast-align -Wmissing-declarations  \
-Wpointer-arith -Wcast-qual -Wzero-as-null-pointer-constant -Wnon-virtual-dtor  \
-Ibuild/include -c src/lib/utils/socket/socket.cpp -o build/obj/lib/utils_socket.o
In file included from /usr/include/w32api/winsock2.h:56:0,
                 from src/lib/utils/socket/socket.cpp:35:
/usr/include/w32api/psdk_inc/_fd_types.h:100:2: warning: #warning "fd_set and associated macros have been defined in sys/types.      This can cause runtime problems with W32 sockets" [-Wcpp]
 #warning "fd_set and associated macros have been defined in sys/types.  \
  ^~~~~~~
In file included from src/lib/utils/socket/socket.cpp:35:0:
/usr/include/w32api/winsock2.h:995:34: error: conflicting declaration of C function ‘int select(int, _types_fd_set*, _types_fd_set*, _types_fd_set*, PTIMEVAL)’
   WINSOCK_API_LINKAGE int WSAAPI select(int nfds,fd_set *readfds,fd_set *writefds,fd_set *exceptfds,const PTIMEVAL timeout);
                                  ^~~~~~
In file included from /usr/include/sys/types.h:68:0,
                 from /usr/include/pthread.h:11,
                 from /usr/lib/gcc/x86_64-pc-msys/6.3.0/include/c++/x86_64-pc-msys/bits/gthr-default.h:35,
                 from /usr/lib/gcc/x86_64-pc-msys/6.3.0/include/c++/x86_64-pc-msys/bits/gthr.h:148,
                 from /usr/lib/gcc/x86_64-pc-msys/6.3.0/include/c++/ext/atomicity.h:35,
                 from /usr/lib/gcc/x86_64-pc-msys/6.3.0/include/c++/memory:73,
                 from build/include/botan/types.h:17,
                 from build/include/botan/internal/socket.h:11,
                 from src/lib/utils/socket/socket.cpp:8:
/usr/include/sys/select.h:73:5: note: previous declaration ‘int select(int, _types_fd_set*, _types_fd_set*, _types_fd_set*, timeval*)’
 int select __P ((int __n, fd_set *__readfds, fd_set *__writefds,
     ^~~~~~
make: *** [Makefile:1066: build/obj/lib/utils_socket.o] Error 1

Answer 1

I'm trying to build Botan on MinGW, which is really MinGW-w64 (x86_64-pc-msys)

I suspect that this is the wrong target--you are inadvertently building for the MSYS2 environment itself, which is indeed POSIXy. That is why e.g. <sys/select.h> is being pulled in and causing conflicts. At least, I severely doubt you would be trying to use <winsock2.h> or <windows.h> if it were your intent to build for use solely within the MSYS2 environment. You should either make sure that you invoked MSYS correctly from the command-line/used the right shortcut, so that you are configured to build for MINGW64, or, if you are, make sure you've actually installed the gcc toolchain for MINGW64. If you didn't, then even though you've opened MINGW64 properly, it will silently default back to the gcc toolchain for MSYS2, which will proceed to pull in includes specific to an MSYS2 build. GCC for MINGW64 can be installed (or reinstalled) by running

pacman -Sy mingw-w64-x86_64-gcc

You can also install or reinstall the Windows-specific headers with this:

pacman -Sy mingw-w64-x86_64-headers-git 

Once you've done this, run uname to confirm that you're in the correct build environment, which should yield

MINGW64_NT-10.0-18363

and then gcc -dumpmachine, which should yield

x86_64-w64-mingw64

For MSYS2, those values would be

MSYS_NT-10.0-18363

and

x86_64-pc-msys

respectively.

Why do I know this? Literally I stayed up all night with the same problem, and it was only after noticing that _WIN32 and _WIN64 were undefined, as well as where the include files were coming from (/usr/include, rather than /mingw64/x86_64-w64-mingw32/include), that I realized I was building for x86_64-pc-msys, rather than x86_64-w64-mingw32.

(Of course, all of the above is also applicable if you'd like to build for 32-bit Windows. Make sure you've invoked the MINGW32 environment properly, and that you've run pacman -Sy mingw-w64-i686-gcc mingw-w64-i686-headers-git, so you don't end up trying to build with a gcc that's configured for MSYS2.)

Anyway, I'm sure given this question is two years old, OP has since moved on. But it's an easy configuration mistake to make. So, regardless, I hope this is helpful! Don't make my mistake! And stay safe everyone!

Answer 2

<sys/select.h> is a Linux system header file, so it shouldn't come shipped in the MinGW toolchain. If you have it in your system, you might have gotten it in an indirect or unsupported manner. So remove the first line from your program. Also both <windows> and <memory> don't bring anything to this particular program.

MinGW uses the select function provided by the Windows Sockets (2) library, and as such, your program has to be linked to it, using the linker flag -lws2_32 in the build command.