CMAKE_OSX_SYSROOT is set to incorrect SDK location

[sryze]

Output of brew config

HOMEBREW_VERSION: 4.1.14-dirty
ORIGIN: https://github.com/Homebrew/brew
HEAD: affc4c01aada2c973b63e084e7696e896edf2b7b
Last commit: 7 days ago
Core tap origin: https://github.com/Homebrew/homebrew-core
Core tap HEAD: b07104c2c05c94537309a91b274e7f7387a0f725
Core tap last commit: 31 hours ago
Core tap branch: master
Core tap JSON: 08 Oct 19:49 UTC
HOMEBREW_PREFIX: /usr/local
HOMEBREW_CASK_OPTS: []
HOMEBREW_DISPLAY: /private/tmp/com.apple.launchd.I3B5EKAEmx/org.xquartz:0
HOMEBREW_EDITOR: vim
HOMEBREW_MAKE_JOBS: 4
Homebrew Ruby: 2.6.10 => /usr/local/Homebrew/Library/Homebrew/vendor/portable-ruby/2.6.10_1/bin/ruby
CPU: quad-core 64-bit skylake
Clang: 12.0.0 build 1200
Git: 2.42.0 => /usr/local/bin/git
Curl: 7.64.1 => /usr/bin/curl
macOS: 10.15.7-x86_64
CLT: 12.0.0.32.28
Xcode: 12.4

Output of brew doctor

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Description of issue

I'm using macOS Catalina (can't upgrade).

When I install/update folly, or more generally when I update any package that depends on folly, compilation fails with the following error:

/tmp/folly-20231009-69505-k2af9s/folly-2023.10.02.00/folly/io/async/fdsock/AsyncFdSocket.h:236:25: error: non-type template argument is not a constant expression
    std::array<uint8_t, CMSG_SPACE(sizeof(int) * 253 /*kMaxFdsPerSocketMsg*/)>

After some digging I found that CMake does not use the latest installed Mac OS SDK version because it's being told by Homebrew to use an older one that matches the system version (10.15) via -DCMAKE_OSX_SYSROOT.

I have the following SDKs installed:

ls -l /Library/Developer/CommandLineTools/SDKs/
total 0
lrwxr-xr-x  1 root  wheel   14 Dec 31  2020 MacOSX.sdk -> MacOSX11.1.sdk
drwxr-xr-x  8 root  wheel  256 Dec 31  2020 MacOSX10.15.sdk
drwxr-xr-x  7 root  wheel  224 Nov 30  2020 MacOSX11.1.sdk

Homebrew uses the 10.15 SDK instead of the 11.1 SDK which is a more recent version (because of this PR from 3 years ago). By default CMake uses 11.1, and I'm able to build folly successfully from git by myself.

There is a bug in the 10.15 SDK that causes a C++ compilation error when using the macro __DARWIN_ALIGN or its derivatives. This bug has been fixed in the 11.1 SDK, which comes with Xcode 12.4 (the most recent Xcode release that runs on Catalina).

Here is an example:

#include <i386/_types.h>
#include <i386/_param.h>
#include <array>

int main() {
    std::array<int, __DARWIN_ALIGN32(1)> a;
}

10.15 SDK:

clang++ test.cpp -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk
test.cpp:12:21: error: non-type template argument of type 'unsigned long' is not an integral constant expression
    std::array<int, __DARWIN_ALIGN32(1)> a;
                    ^~~~~~~~~~~~~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/i386/_param.h:43:41: note: expanded from macro '__DARWIN_ALIGN32'
#define       __DARWIN_ALIGN32(p)       ((__darwin_size_t)((char *)(__darwin_size_t)(p) + __DARWIN_ALIGNBYTES32) &~ __DARWIN_ALIGNBYTES32)
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test.cpp:12:21: note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/i386/_param.h:43:42: note: expanded from macro '__DARWIN_ALIGN32'
#define       __DARWIN_ALIGN32(p)       ((__darwin_size_t)((char *)(__darwin_size_t)(p) + __DARWIN_ALIGNBYTES32) &~ __DARWIN_ALIGNBYTES32)
                                         ^
1 error generated.

11.1 SDK:

clang++ test.cpp -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX11.1.sdk
// Success

[sryze]

Workaround:

Edit /Library/Developer/CommandLineTools/SDKs/MacOSX11.1.sdk/usr/include/i386/_param.h and remove the casts to (char *).

[Bo98]

This is correct behaviour because if you link to things like libcurl using a newer SDK than the system, you could get runtime crashes like Homebrew/homebrew-core#46466. It is always safest to use the SDK matching the system you are running, unless you restrict yourself to frameworks that support deployment target based builds.

Officially, folly only supports the last 2 macOS versions but we try test the last 3 where possible. MacPorts encountered multiple problems with building folly and had to revert multiple upstream changes to get it fully working on 10.15: macports/macports-ports@671ce78.