Re: [PATCH 1/1] Fix detection of pwritev support for OSX.

On 21.01.2021 02:07, Tom Lane wrote:
> I now believe what is actually happening with the short command is
> that it's iterating through the available SDKs (according to some not
> very clear search path) and picking the first one it finds that
> matches the host system version. That matches the ktrace evidence
> that shows it reading the SDKSettings.plist file in each SDK
> directory.

Yes, you are right. After some more digging...

It searches the DEVELOPER_DIR first and then
/Library/Developer/CommandLineTools, which is hardcoded.

My DEVELOPER_DIR is
% xcode-select -p
/Applications/Xcode.app/Contents/Developer

(For more detail try "otool -tV /usr/lib/libxcselect.dylib -p
_xcselect_get_developer_dir_path".)

It reads ProductVersion from
/System/Library/CoreServices/SystemVersion.plist

% plutil -p /System/Library/CoreServices/SystemVersion.plist | grep
ProductVersion
"ProductVersion" => "10.15.7"

Strips anything after the second dot, and prepends "macosx" to it, which
gives "macosx10.15".

Then it scans through SDK dirs looking up CanonicalName from
SDKSettings.plist until it finds a match with "macosx10.15".

The overall callstack:

% sudo dtrace -n 'syscall::getdirentries64:entry { ustack() }' -c 'xcrun
--show-sdk-path'
dtrace: description 'syscall::getdirentries64:entry ' matched 1 probe
/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk
dtrace: pid 20183 has exited
CPU ID FUNCTION:NAME
0 846 getdirentries64:entry
libsystem_kernel.dylib`__getdirentries64+0xa
libsystem_c.dylib`readdir$INODE64+0x23
libsystem_c.dylib`scandir$INODE64+0x6c
libxcrun.dylib`cltools_lookup_sdk_by_key+0x5f
libxcrun.dylib`cltools_lookup_boot_system_sdk+0xda
libxcrun.dylib`xcinfocache_resolve_sdkroot+0xc0
libxcrun.dylib`xcrun_main2+0x57a
libxcrun.dylib`xcrun_main+0x9
libxcselect.dylib`xcselect_invoke_xcrun_via_library+0xc8
libxcselect.dylib`xcselect_invoke_xcrun+0x25a
xcrun`DYLD-STUB$$getprogname
libdyld.dylib`start+0x1
xcrun`0x2

0 846 getdirentries64:entry
libsystem_kernel.dylib`__getdirentries64+0xa
libsystem_c.dylib`readdir$INODE64+0x23
libsystem_c.dylib`scandir$INODE64+0x6c
libxcrun.dylib`cltools_lookup_sdk_by_key+0x5f
libxcrun.dylib`cltools_lookup_boot_system_sdk+0xf3
libxcrun.dylib`xcinfocache_resolve_sdkroot+0xc0
libxcrun.dylib`xcrun_main2+0x57a
libxcrun.dylib`xcrun_main+0x9
libxcselect.dylib`xcselect_invoke_xcrun_via_library+0xc8
libxcselect.dylib`xcselect_invoke_xcrun+0x25a
xcrun`DYLD-STUB$$getprogname
libdyld.dylib`start+0x1
xcrun`0x2

The SDK search path:

% sudo dtrace -n 'pid$target:::entry
/probefunc=="cltools_lookup_sdk_by_key"/ { trace(copyinstr(arg0));
trace(copyinstr(arg1)) }' -c 'xcrun --show-sdk-path'
dtrace: description 'pid$target:::entry ' matched 17293 probes
/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk
dtrace: pid 20191 has exited
CPU ID FUNCTION:NAME
8 398290 cltools_lookup_sdk_by_key:entry
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer
macosx10.15
9 398290 cltools_lookup_sdk_by_key:entry
/Library/Developer/CommandLineTools macosx10.15

The properties read from SDKSettings.plist:

% sudo dtrace -n 'pid$target:::entry
/probefunc=="_cltools_lookup_property_in_path"/ {
trace(copyinstr(arg0)); trace(copyinstr(arg1)); trace(copyinstr(arg2))
}' -c 'xcrun --show-sdk-path'
dtrace: description 'pid$target:::entry ' matched 17293 probes
/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk
dtrace: pid 20195 has exited
CPU ID FUNCTION:NAME
8 398288 _cltools_lookup_property_in_path:entry /
System/Library/CoreServices/SystemVersion.plist
ProductVersion
8 398288 _cltools_lookup_property_in_path:entry
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/DriverKit20.2.sdk
SDKSettings.plist IsBaseSDK
8 398288 _cltools_lookup_property_in_path:entry
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/DriverKit20.2.sdk
SDKSettings.plist CanonicalName
4 398288 _cltools_lookup_property_in_path:entry
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/DriverKit20.2.sdk
SDKSettings.plist CanonicalNameForBuildSettings
4 398288 _cltools_lookup_property_in_path:entry
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk
SDKSettings.plist IsBaseSDK
4 398288 _cltools_lookup_property_in_path:entry
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk
SDKSettings.plist CanonicalName
4 398288 _cltools_lookup_property_in_path:entry
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk
SDKSettings.plist CanonicalNameForBuildSettings
4 398288 _cltools_lookup_property_in_path:entry
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk
SDKSettings.plist PLATFORM_NAME
4 398288 _cltools_lookup_property_in_path:entry
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk
SDKSettings.plist IsBaseSDK
2 398288 _cltools_lookup_property_in_path:entry
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk
SDKSettings.plist CanonicalName
2 398288 _cltools_lookup_property_in_path:entry
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk
SDKSettings.plist CanonicalNameForBuildSettings
2 398288 _cltools_lookup_property_in_path:entry
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk
SDKSettings.plist PLATFORM_NAME
2 398288 _cltools_lookup_property_in_path:entry
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk SDKSettings.plist
IsBaseSDK
2 398288 _cltools_lookup_property_in_path:entry
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk SDKSettings.plist
CanonicalName
2 398288 _cltools_lookup_property_in_path:entry
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk SDKSettings.plist
CanonicalNameForBuildSettings
0 398288 _cltools_lookup_property_in_path:entry
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk SDKSettings.plist
PLATFORM_NAME
0 398288 _cltools_lookup_property_in_path:entry
/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk
SDKSettings.plist IsBaseSDK
0 398288 _cltools_lookup_property_in_path:entry
/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk
SDKSettings.plist CanonicalName

BTW, on my machine /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk
is skipped because it points to 11.0:

% ls -l /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk
lrwxr-xr-x 1 root wheel 14 Nov 17 02:21
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -> MacOSX11.0.sdk

For more detail try
% otool -tV
/Applications/Xcode.app/Contents/Developer/usr/lib/libxcrun.dylib -p
_cltools_lookup_boot_system_sdk