Re: [Mingw-users] Undefined reference to 'getaddrinfo'

[Brian D. <br...@de...>]

James Steward wrote:

> I've googled for this for a while now and I'm not sure what to do next.

Try searching the mailing list archives first.  This has come up before,
e.g. <http://sourceforge.net/mailarchive/message.php?msg_id=14887873>.

> When I print the value of _WIN32_WINNT I get 0x400 so obviously the
> function isn't declared.  That explains the implicit declarations
> warning.  But what about the undefined references?  The functions are in
> the library ws2_32.a.
> 
> What do I need to do to resolve this?  Is it a MinGW version or a
> Windows version that needs updating?  Or something completely different?

The problem, as I described before, is that getaddrinfo() only exists in
ws2_32 for Windows XP and higher.  If you're willing to generate a
binary that only runs on XP (and fails with a "Cannot find procedure
entry point <whatever>" popup box on all other versions) then just do:

#define WINVER WindowsXP

...before including any w32api headers.  This signals that you want the
feature set of XP, and you don't care about older versions.  Note that
WindowsXP is defined as 0x0501, and that _WIN32_WINNT gets set to WINVER
if not already defined, so this is the proper way of declaring this, not
"#define _WIN32_WINNT 0x0501".

Now, to complicate things, Microsoft has added support for a
getaddrinfo() wrapper in their platform SDK, which works only for IPv4,
as a way of supporting such code on older versions of Windows.  So if
you are using the platform SDK you can still call getaddrinfo() from
older versions, but you'll really be just calling the existing
underlying IPv4 function (gethostbyname() or whatever) and failing if
you try to use IPv6.

So, as I said in that reply above, your options are essentially:

1. Live with XP-only binary.
2. Write (and potentially contribute to the w32api project) a suitable
getaddrinfo() wrapper for older versions.
3. If you're just using IPv4 then call the old standard IPv4 functions
directly (gethostby*).

Note that the reason that you got a link error when you did not define
WINVER is because the getaddrinfo function (and most every Windows API
function) is declared as type WINAPI which is synonymous with the
stdcall calling convention, which dictates a function decoration of @nn
on the end.  But if you call a function without a prototype (as happens
if you don't set WINVER high enough) then the compiler assumes the
default cdecl calling convention which does not have that decoration,
and thus you're trying to call an undecorated function, and the name
lookup fails at link time.  Another very common way that this plays out
is that the function actually only exists as FooA and FooW for the ANSI
and UNICODE variants, but you're meant to call it as just Foo() and the
include header #defines it to the correct variant based on whether you
enabled a Unicode build.  Here you get a similar link error if you call
a function that was not declared in a header.

Brian