Examples: How DYLD_* Environmental Variables Break Mach-O Library Resolution

01-name-collision-example.sh

# DYLD_LIBRARY_PATH library variables break linking of standard system libraries by forcing dyld to ignore
# everything but the library's basename when performing what would otherwise be successful library
# resolution.
mkdir rust-lib
echo 'void my_libjpeg_api (void) { }' | clang -dynamiclib -x objective-c -o rust-lib/libjpeg.dylib -
echo 'extern void my_libjpeg_api(void); int main (int argc, char *argv[]) { my_libjpeg_api(); return 0; }' | clang -x objective-c -o example - -framework ImageIO -Lrust-lib -ljpeg
env DYLD_LIBRARY_PATH=`pwd`/rust-lib ./example

#dyld: Library not loaded: /System/Library/Frameworks/ImageIO.framework/Versions/A/Resources/libJPEG.dylib
#  Referenced from: /System/Library/Frameworks/ImageIO.framework/Versions/A/ImageIO
#  Reason: Incompatible library version: ImageIO requires version 1.0.0 or later, but libjpeg.dylib provides version 0.0.0

02-fallback-collision-example.sh

# DYLD_FALLBACK_LIBRARY_PATH breaks library path resolution when you have two applications that
# have a library name collision and use DYLD_* varibles to force dyld to find their libraries -- a
# use-case that would work without error when using proper install name/@rpath Mach-O linking.
mkdir rust-lib
mkdir mathematica-lib
echo 'void my_libjpeg_api (void) { }' | clang -dynamiclib -x objective-c -o rust-lib/libjpeg.dylib -install_name libjpeg.dylib -
clang -dynamiclib -x objective-c -o mathematica-lib/libjpeg.dylib -install_name libjpeg.dylib - < /dev/null

echo 'extern void my_libjpeg_api(void); int main (int argc, char *argv[]) { my_libjpeg_api(); return 0; }' | clang -x objective-c -o example - -framework ImageIO -Lrust-lib -ljpeg
env DYLD_FALLBACK_LIBRARY_PATH=`pwd`/mathematica-lib:`pwd`/rust-lib ./example

#dyld: lazy symbol binding failed: Symbol not found: _my_libjpeg_api
#  Referenced from: /private/tmp/./example
#  Expected in: /tmp/mathematica-lib/libjpeg.dylib
#
#dyld: Symbol not found: _my_libjpeg_api
#  Referenced from: /private/tmp/./example
#  Expected in: /tmp/mathematica-lib/libjpeg.dylib

03-fallback-weak-link-example.sh

# DYLD_FALLBACK_LIBRARY_PATH library variables can also break weak linking of libraries if 1) the weakly linked
# library is not found, and 2) a fallback path contains an incompatible library.
#
# Assume a library weakly linked by a third-party library. If the library is not found, but a similarly named library
# is found in DYLD_FALLBACK_LIBRARY_PATH, dyld will attempt to use that (incorrect) library.
mkdir rust-lib
mkdir third-party-lib

# Create a 'libthird_party.dylib' library that weakly links to a 'libcommon_libname.dylib'
echo 'void my_weak_api (void) { }' | clang -dynamiclib -x objective-c -o third-party-lib/libcommon_libname.dylib -
echo 'extern void my_weak_api (void); void my_weak_api_caller (void) { if (my_weak_api) my_weak_api(); }' | clang -dynamiclib -x objective-c -o third-party-lib/libthird_party.dylib -weak_library third-party-lib/libcommon_libname.dylib -

# Remove the weakly linked library; we're pretending that it's not available on this install, a possibility
# that led us to weakly link it in the first place.
rm third-party-lib/libcommon_libname.dylib

# Create a conflictingly-named 'libcommon_libname.dylib' library
clang -dynamiclib -x objective-c -o rust-lib/libcommon_libname.dylib - < /dev/null

# Link our binary against libthird_party.dylib
echo 'extern void my_weak_api_caller(void); int main (int argc, char *argv[]) { my_weak_api_caller(); return 0; }' | clang -x objective-c -o example - -Lthird-party-lib -lthird_party

env DYLD_FALLBACK_LIBRARY_PATH=`pwd`/rust-lib ./example

#dyld: lazy symbol binding failed: Symbol not found: _my_weak_api
#  Referenced from: third-party-lib/libthird_party.dylib
#  Expected in: /tmp/rust-lib/libcommon_libname.dylib

#dyld: Symbol not found: _my_weak_api
#  Referenced from: third-party-lib/libthird_party.dylib
#  Expected in: /tmp/rust-lib/libcommon_libname.dylib