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
