DYLD(1) General Commands Manual DYLD(1)

dyld - the dynamic linker

DYLD_FRAMEWORK_PATH
DYLD_FALLBACK_FRAMEWORK_PATH
DYLD_VERSIONED_FRAMEWORK_PATH
DYLD_LIBRARY_PATH
DYLD_FALLBACK_LIBRARY_PATH
DYLD_VERSIONED_LIBRARY_PATH
DYLD_PRINT_TO_FILE
DYLD_SHARED_REGION
DYLD_INSERT_LIBRARIES
DYLD_FORCE_FLAT_NAMESPACE
DYLD_IMAGE_SUFFIX
DYLD_PRINT_OPTS
DYLD_PRINT_ENV
DYLD_PRINT_LIBRARIES
DYLD_BIND_AT_LAUNCH
DYLD_DISABLE_DOFS
DYLD_PRINT_APIS
DYLD_PRINT_BINDINGS
DYLD_PRINT_INITIALIZERS
DYLD_PRINT_REBASINGS
DYLD_PRINT_SEGMENTS
DYLD_PRINT_STATISTICS
DYLD_PRINT_DOFS
DYLD_PRINT_RPATHS
DYLD_SHARED_CACHE_DIR

The dynamic linker checks the following environment variables during the launch of each process.
Note: If System Integrity Protection is enabled, these environment variables are ignored when executing binaries protected by System Integrity Protection.
DYLD_FRAMEWORK_PATH
This is a colon separated list of directories that contain frameworks. The dynamic linker searches these directories before it searches for the framework by its install name. It allows you to test new versions of existing frameworks. (A framework is a library install name that ends in the form XXX.framework/Versions/A/XXX or XXX.framework/XXX, where XXX and A are any name.)
For each framework that a program uses, the dynamic linker looks for the framework in each directory in DYLD_FRAMEWORK_PATH in turn. If it looks in all those directories and can't find the framework, it uses whatever it would have loaded if DYLD_FRAMEWORK_PATH had not been set.
Use the -L option to otool(1) to discover the frameworks and shared libraries that the executable is linked against.
DYLD_FALLBACK_FRAMEWORK_PATH
This is a colon separated list of directories that contain frameworks. If a framework is not found at its install path, dyld uses this as a list of directories to search for the framework.

By default, it is set to /Library/Frameworks:/System/Library/Frameworks

DYLD_VERSIONED_FRAMEWORK_PATH
This is a colon separated list of directories that contain potential override frameworks. The dynamic linker searches these directories for frameworks. For each framework found dyld looks at its LC_ID_DYLIB and gets the current_version and install name. Dyld then looks for the framework at the install name path. Whichever has the larger current_version value will be used in the process whenever a framework with that install name is required. This is similar to DYLD_FRAMEWORK_PATH except instead of always overriding, it only overrides if the supplied framework is newer. Note: dyld does not check the framework's Info.plist to find its version. Dyld only checks the -currrent_version number supplied when the framework was created.
DYLD_LIBRARY_PATH
This is a colon separated list of directories that contain libraries. The dynamic linker searches these directories before it searches the default locations for libraries. It allows you to test new versions of existing libraries.
For each dylib that a program uses, the dynamic linker looks for its leaf name in each directory in DYLD_LIBRARY_PATH.
Use the -L option to otool(1) to discover the frameworks and shared libraries that the executable is linked against.
DYLD_FALLBACK_LIBRARY_PATH
This is a colon separated list of directories that contain libraries. If a dylib is not found at its install path, dyld uses this as a list of directories to search for the dylib. By default, it is set to /usr/local/lib:/usr/lib.
DYLD_VERSIONED_LIBRARY_PATH
This is a colon separated list of directories that contain potential override libraries. The dynamic linker searches these directories for dynamic libraries. For each library found dyld looks at its LC_ID_DYLIB and gets the current_version and install name. Dyld then looks for the library at the install name path. Whichever has the larger current_version value will be used in the process whenever a dylib with that install name is required. This is similar to DYLD_LIBRARY_PATH except instead of always overriding, it only overrides is the supplied library is newer.
DYLD_PRINT_TO_FILE
This is a path to a (writable) file. Normally, the dynamic linker writes all logging output (triggered by DYLD_PRINT_* settings) to file descriptor 2 (which is usually stderr). But this setting causes the dynamic linker to write logging output to the specified file.
DYLD_SHARED_REGION
This can be "use" (the default), "avoid", or "private". Setting it to "avoid" tells dyld to not use the shared cache. All OS dylibs are loaded dynamically just like every other dylib. Setting it to "private" tells dyld to remove the shared region from the process address space and mmap() back in a private copy of the dyld shared cache in the shared region address range. This is only useful if the shared cache on disk has been updated and is different than the shared cache in use.
DYLD_INSERT_LIBRARIES
This is a colon separated list of dynamic libraries to load before the ones specified in the program. This lets you test new modules of existing dynamic shared libraries that are used in flat-namespace images by loading a temporary dynamic shared library with just the new modules. Note that this has no effect on images built a two-level namespace images using a dynamic shared library unless DYLD_FORCE_FLAT_NAMESPACE is also used.
DYLD_FORCE_FLAT_NAMESPACE
Force all images in the program to be linked as flat-namespace images and ignore any two-level namespace bindings. This may cause programs to fail to execute with a multiply defined symbol error if two-level namespace images are used to allow the images to have multiply defined symbols.
DYLD_IMAGE_SUFFIX
This is set to a string of a suffix to try to be used for all shared libraries used by the program. For libraries ending in ".dylib" the suffix is applied just before the ".dylib". For all other libraries the suffix is appended to the library name. This is useful for using conventional "_profile" and "_debug" libraries and frameworks.
DYLD_PRINT_OPTS
When this is set, the dynamic linker writes to file descriptor 2 (normally standard error) the command line options.
DYLD_PRINT_ENV
When this is set, the dynamic linker writes to file descriptor 2 (normally standard error) the environment variables.
DYLD_PRINT_LIBRARIES
When this is set, the dynamic linker writes to file descriptor 2 (normally standard error) the filenames of the libraries the program is using. This is useful to make sure that the use of DYLD_LIBRARY_PATH is getting what you want.
DYLD_BIND_AT_LAUNCH
When this is set, the dynamic linker binds all undefined symbols the program needs at launch time. This includes function symbols that are normally lazily bound at the time of their first call.
DYLD_PRINT_STATISTICS
Right before the process's main() is called, dyld prints out information about how dyld spent its time. Useful for analyzing launch performance.
DYLD_PRINT_STATISTICS_DETAILS
Right before the process's main() is called, dyld prints out detailed information about how dyld spent its time. Useful for analyzing launch performance.
DYLD_DISABLE_DOFS
Causes dyld to not register dtrace static probes with the kernel.
DYLD_PRINT_INITIALIZERS
Causes dyld to print out a line when running each initializer in every image. Initializers run by dyld include constructors for C++ statically allocated objects, functions marked with __attribute__((constructor)), and -init functions.
DYLD_PRINT_APIS
Causes dyld to print a line whenever a dyld API is called (e.g. NSAddImage()).
DYLD_PRINT_SEGMENTS
Causes dyld to print out a line containing the name and address range of each mach-o segment that dyld maps. In addition it prints information about if the image was from the dyld shared cache.
DYLD_PRINT_BINDINGS
Causes dyld to print a line each time a symbolic name is bound.
DYLD_PRINT_DOFS
Causes dyld to print out information about dtrace static probes registered with the kernel.
DYLD_PRINT_RPATHS
Cause dyld to print a line each time it expands an @rpath variable and whether that expansion was successful or not.
DYLD_SHARED_CACHE_DIR
This is a directory containing dyld shared cache files. This variable can be used in conjunction with DYLD_SHARED_REGION=private to run a process with an alternate shared cache.

Unlike many other operating systems, Darwin does not locate dependent dynamic libraries via their leaf file name. Instead the full path to each dylib is used (e.g. /usr/lib/libSystem.B.dylib). But there are times when a full path is not appropriate; for instance, may want your binaries to be installable in anywhere on the disk. To support that, there are three @xxx/ variables that can be used as a path prefix. At runtime dyld substitutes a dynamically generated path for the @xxx/ prefix.
@executable_path/
This variable is replaced with the path to the directory containing the main executable for the process. This is useful for loading dylibs/frameworks embedded in a .app directory. If the main executable file is at /some/path/My.app/Contents/MacOS/My and a framework dylib file is at /some/path/My.app/Contents/Frameworks/Foo.framework/Versions/A/Foo, then the framework load path could be encoded as @executable_path/../Frameworks/Foo.framework/Versions/A/Foo and the .app directory could be moved around in the file system and dyld will still be able to load the embedded framework.
@loader_path/
This variable is replaced with the path to the directory containing the mach-o binary which contains the load command using @loader_path. Thus, in every binary, @loader_path resolves to a different path, whereas @executable_path always resolves to the same path. @loader_path is useful as the load path for a framework/dylib embedded in a plug-in, if the final file system location of the plugin-in unknown (so absolute paths cannot be used) or if the plug-in is used by multiple applications (so @executable_path cannot be used). If the plug-in mach-o file is at /some/path/Myfilter.plugin/Contents/MacOS/Myfilter and a framework dylib file is at /some/path/Myfilter.plugin/Contents/Frameworks/Foo.framework/Versions/A/Foo, then the framework load path could be encoded as @loader_path/../Frameworks/Foo.framework/Versions/A/Foo and the Myfilter.plugin directory could be moved around in the file system and dyld will still be able to load the embedded framework.
@rpath/
Dyld maintains a current stack of paths called the run path list. When @rpath is encountered it is substituted with each path in the run path list until a loadable dylib if found. The run path stack is built from the LC_RPATH load commands in the depencency chain that lead to the current dylib load. You can add an LC_RPATH load command to an image with the -rpath option to ld(1). You can even add a LC_RPATH load command path that starts with @loader_path/, and it will push a path on the run path stack that relative to the image containing the LC_RPATH. The use of @rpath is most useful when you have a complex directory structure of programs and dylibs which can be installed anywhere, but keep their relative positions. This scenario could be implemented using @loader_path, but every client of a dylib could need a different load path because its relative position in the file system is different. The use of @rpath introduces a level of indirection that simplies things. You pick a location in your directory structure as an anchor point. Each dylib then gets an install path that starts with @rpath and is the path to the dylib relative to the anchor point. Each main executable is linked with -rpath @loader_path/zzz, where zzz is the path from the executable to the anchor point. At runtime dyld sets it run path to be the anchor point, then each dylib is found relative to the anchor point.

dyldinfo(1), ld(1), otool(1)
June 1, 2020 Apple Inc.