Compile different code on whether a function is available or not

Question

Windows provides only GetTickCount up to Windows Vista and starting from that OS also GetTickCount64. How can I make a C program compile with calls to different functions?

How can I make a C compiler check whether a function is declared in the included header files and compile different portions of code depending on whether that particular function is available or not?

#if ??????????????????????????????
unsigned long long get_tick_count(void) { return GetTickCount64(); }
#else
unsigned long long get_tick_count(void) { return GetTickCount(); }
#endif

Looking for a working sample file not just hints.

Edit: I tried the following using gcc 3.4.5 from MinGW on a (64-bit) Windows 7 RC but it didn't help. If this is a MinGW problem, how can I work around this issue?

#include <windows.h>
#if (WINVER >= 0x0600)
unsigned long long get_tick_count(void) { return 600/*GetTickCount64()*/; }
#else
unsigned long long get_tick_count(void) { return 0/*GetTickCount()*/; }
#endif

Answer 1

Compile time selection of an API based on the target Windows version locks the built executable to that version and newer. This is a common technique for open source, *nix targeted projects where it is assumed that the user will configure the source kit for his platform and compile clean to install.

On Windows, this is not the usual technique because it isn't generally safe to assume that an end user will have a compiler at all, let alone want to deal with the intricacies of getting a project to build.

Often, just using the older API that is present in all versions of Windows is a sufficient answer. This is also simple: you just ignore the existence of a new API.

When that isn't sufficient, you use LoadLibrary() and GetProcAddress() to attempt to resolve the new symbol at run time. If it can't be resolved, then you fall back to the older API.

Here's a possible implementation. It detects the first call, and at attempts to load the library and resolve the name "GetTickCount64". In all calls, if the pointer to resolved symbol is non-null, it calls it and returns the result. Otherwise, it falls back on the older API, casting its return value to match the wrapper's type.

unsigned long long get_tick_count(void) {
    static int first = 1;
    static ULONGLONG WINAPI (*pGetTickCount64)(void);

    if (first) {
        HMODULE hlib = LoadLibraryA("KERNEL32.DLL");
        pGetTickCount64 = GetProcAddressA(hlib, "GetTickCount64");
        first = 0;
    }
    if (pGetTickCount64)
        return pGetTickCount64();
    return (unsigned long long)GetTickCount();
}

Note that I used the ...A flavors of the API functions since it is known that the library name and the symbol name will only be ASCII... if using this technique to load symbols from an installed DLL that might be in a folder named with non-ASCII characters, then you will need to worry about using a Unicode build.

This is untested, your mileage will vary, etc...

Answer 2

You can achieve it using preprocessor definitions in Windows headers.

unsigned long long
get_tick_count(void)
{
#if WINVER >= 0x0600
    return GetTickCount64();
#else
    return GetTickCount();
#endif
}

Answer 3

The right way to deal with this kind of problems is to check whether the function is available, but this cannot be done reliably during the project compilation. You should add a configuration stage, which details depend on your build tool, both cmake and scons, two cross platforms build tools, provide the facilities. Basically, it goes like this:

/* config.h */
#define HAVE_GETTICKSCOUNT64_FUNC

And then in your project, you do:

#include "config.h"

#ifdef HAVE_GETTICKSCOUNT64_FUNC
....
#else
...
#endif

Although it looks similar to the obvious way, it is much more maintainable in the long term. In particular, you should avoid as much as possible to depend on versions, and check for capabilities instead. Checking for versions quickly leads to complicated, interleaved conditionals, whereas with the technique above, everything is controlled from one config.h, hopefully generated automatically.

In scons and cmake, they will have tests which are run automatically to check whether the function is available, and define the variable in the config.h or not depending on the check. The fundamental idea is to decouple the capability detection/setting from your code.

Note that this can handle cases where you need to build binaries which run on different platforms (say run on XP even if built on Vista). It is just a matter of changing the config.h. If dones poperly, that's just a matter of changing the config.h (you could have a script which generate the config.h on any platform, and then gather config.h for windows xp, Vista, etc...). I don't think it is specific to unix at all.