os/mac/pkgconfig: use ${homebrew_sdkroot}/usr/lib for libffi

[kou]

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---

We need to use ${homebrew_sdkroot}/usr/lib/libffi.tbd not
/usr/lib/libffi.dylib to use libffi with
${homebrew_sdkroot}/usr/include/ffi/ffi.h. If we use
/usr/lib/libffi.dylib directly,
${homebrew_sdkroot}/usr/include/ffi/ffi.h and -lffi are inconsistent.

For example, ${homebrew_sdkroot}/usr/include/ffi/ffi.h for 10.14
doesn't provide ffi_prep_cif_var() but -lffi accept it.

a.c:

#include <ffi.h>

int
main(void)
{
  ffi_prep_cif_var(NULL, 0, 0, 0, 0, NULL);
  return 0;
}

No declaration error is expected, OK:

$ gcc $(PKG_CONFIG_PATH=$(brew --prefix)/Homebrew/Library/Homebrew/os/mac/pkgconfig/10.14 pkg-config --cflags --libs libffi) a.c
a.c:6:3: error: implicit declaration of function 'ffi_prep_cif_var' is invalid in C99
      [-Werror,-Wimplicit-function-declaration]
  ffi_prep_cif_var(NULL, 0, 0, 0, 0, NULL);
  ^
a.c:6:3: note: did you mean 'ffi_prep_cif'?
/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/ffi/ffi.h:312:1: note: 'ffi_prep_cif' declared here
ffi_prep_cif(
^
1 error generated.

Ignoring no declaration error. It must be a link error. NG:

$ gcc -Wno-implicit-function-declaration $(PKG_CONFIG_PATH=$(brew --prefix)/Homebrew/Library/Homebrew/os/mac/pkgconfig/10.14 pkg-config --cflags --libs libffi) a.c
(no error, unexpected)

With this change: Ignoring no declaration error. It must be a link error. OK:

$ gcc -Wno-implicit-function-declaration $(PKG_CONFIG_PATH=$(brew --prefix)/Homebrew/Library/Homebrew/os/mac/pkgconfig/10.14 pkg-config --cflags --libs libffi) a.c
Undefined symbols for architecture x86_64:
  "_ffi_prep_cif_var", referenced from:
      _main in a-ae3840.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Context: ruby/fiddle#52

[Bo98]

The change seems reasonable and technically more correct (if applied consistently across all libraries - not just libffi), but the real cause of the error you saw was because the wrong headers were being used for the system. 10.14 SDK headers should not be used on Catalina (10.15). That's why the mismatch between library and headers occur.

I suspect the user is using an old build of pkg-config and the problem will go away with a brew reinstall pkgconfig. There is plans to detect this issue and auto-prompt reinstalling pkg-config (I should probably open an issue detailing it).

[MikeMcQuaid]

I should probably open an issue detailing it

@Bo98 yup, that'd be great, thanks ❤️

[MikeMcQuaid]

I suspect the user is using an old build of pkg-config and the problem will go away with a brew reinstall pkgconfig. There is plans to detect this issue and auto-prompt reinstalling pkg-config (I should probably open an issue detailing it).

@kou could you try this?

[kou]

I'm sorry but I can't try it because I don't have an environment that reproduces this case.

@escogido Could you try the following command lines?

$ gem install fiddle:1.0.4 # This will be failed.
$ brew reinstall pkgconfig
$ gem install fiddle:1.0.4 # This will not be failed.

[MikeMcQuaid]

Thanks for the PR @kou but passing on this until we find someone who is affected by this who can test reproduce steps.