Re: gnulib fails for build on Mac OS X 10.4: get_ppid_of.c:36:22: error:

[Ken Cunningham]

I asked:
> What solution can you offer so that
>   - in < 10.5, we have no error,
>   - in >= 10.5, proc_pidinfo gets used?

Would this work?

int proc_pidinfo (int, int, uint64_t, void *, int) WEAK_IMPORT_ATTRIBUTE;

if (proc_pidinfo != NULL)
  {
    ... code that invokes proc_pidinfo ...
  }

Based on [1].

Bruno

[1] 
https://opensource.apple.com/source/dyld/dyld-44.17/unit-tests/test-cases/weak-symbol/

That is not usually how this is done, although it might work and is suitably simple. The problem I suppose is the function definition can change somewhat over time.

Usually, the SDK features automatically supply the weak import attribute features, based on your SDK and your deployment target.

What was traditionally done (before the @available() compiler feature came out) is something like this, using as an example the function “pthread_threadid_np()” which is defined in the MacOSX 10.6 SDK and available in MacOSX version 10.6 and later.

MAC_OS_X_VERSION_MAX_ALLOWED = the SDK you are building against

MAC_OS_X_VERSION_MIN_REQUIRED = the minimum system you are willing to support

==========

#if MAC_OS_X_VERSION_MAX_ALLOWED < 1060
    native_id = pthread_mach_thread_np(pthread_self());
#elif MAC_OS_X_VERSION_MIN_REQUIRED < 1060
    if (&pthread_threadid_np) {
        (void) pthread_threadid_np(NULL, &native_id);
    } else {
        native_id = pthread_mach_thread_np(pthread_self());
    }
#else
    (void) pthread_threadid_np(NULL, &native_id);
#endif

========

with this method, if the SDK does not have the function, the fallback is always used. 

If the SDK does have the function, but the application might be deployed to a system where the function is unavailable, the null test for the function address is used. The function is automatically weak-linked by the SDK features. The fallback will then be used if the function is unavailable.

If the minimum deployment target is a system that is known to have the function, then no test is done and the function is strongly linked and assumed available. An error will result at runtime if there is no such function available.

Ken