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man page for dyld

DYLD(1) 						      General Commands Manual							   DYLD(1)

NAME
dyld - the dynamic link editor SYNOPSIS
DYLD_FRAMEWORK_PATH DYLD_FALLBACK_FRAMEWORK_PATH DYLD_VERSIONED_FRAMEWORK_PATH DYLD_LIBRARY_PATH DYLD_FALLBACK_LIBRARY_PATH DYLD_VERSIONED_LIBRARY_PATH DYLD_ROOT_PATH DYLD_SHARED_REGION DYLD_INSERT_LIBRARIES DYLD_FORCE_FLAT_NAMESPACE DYLD_IMAGE_SUFFIX DYLD_PRINT_OPTS DYLD_PRINT_ENV DYLD_PRINT_LIBRARIES DYLD_PRINT_LIBRARIES_POST_LAUNCH DYLD_BIND_AT_LAUNCH DYLD_NO_FIX_PREBINDING 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 DYLD_SHARED_CACHE_DONT_VALIDATE DESCRIPTION
The dynamic linker uses the following environment variables. They affect any program that uses the dynamic linker. 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/YYY/XXX or XXX.framework/XXX, where XXX and YYY 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 the directories and can't find the framework, it searches the directories in DYLD_LIBRARY_PATH in turn. If it still can't find the framework, it then searches DYLD_FALLBACK_FRAMEWORK_PATH and DYLD_FALLBACK_LIBRARY_PATH in turn. 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. It is used as the default location for frameworks not found in their install path. By default, it is set to /Library/Frameworks:/Network/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 direc- tories 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 overrid- ing, it only overrides is the supplied framework is newer. Note: dyld does not check the framework's Info.plist to find its ver- sion. 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 library that a program uses, the dynamic linker looks for it in each directory in DYLD_LIBRARY_PATH in turn. If it still can't find the library, it then searches DYLD_FALLBACK_FRAMEWORK_PATH and DYLD_FALLBACK_LIBRARY_PATH in turn. 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. It is used as the default location for libraries not found in their install path. By default, it is set to $(HOME)/lib:/usr/local/lib:/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 directo- ries 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 overrid- ing, it only overrides is the supplied library is newer. DYLD_ROOT_PATH This is a colon separated list of directories. The dynamic linker will prepend each of this directory paths to every image access until a file is found. 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 mod- ules 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 pro- gram is using. This is useful to make sure that the use of DYLD_LIBRARY_PATH is getting what you want. DYLD_PRINT_LIBRARIES_POST_LAUNCH This does the same as DYLD_PRINT_LIBRARIES but the printing starts after the program gets to its entry point. 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 can 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_NO_FIX_PREBINDING Normally, dyld will trigger the dyld shared cache to be regenerated if it notices the cache is out of date while launching a process. If this environment variable is set, dyld will not trigger a cache rebuild. This is useful to set while installing a large set of OS dylibs, to ensure the cache is not regenerated until the install is complete. DYLD_DISABLE_DOFS Causes dyld not register dtrace static probes with the kernel. DYLD_PRINT_INITIALIZERS Causes dyld to print out a line when running each initializers in every image. Initializers run by dyld included 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 and DYLD_SHARED_CACHE_DONT_VALIDATE to run a process with an alternate shared cache. DYLD_SHARED_CACHE_DONT_VALIDATE Causes dyld to not check that the inode and mod-time of files in the shared cache match the requested dylib on disk. Thus a program can be made to run with the dylib in the shared cache even though the real dylib has been updated on disk. DYNAMIC LIBRARY LOADING 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 frame- work 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 sys- tem 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 rela- tive 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 dif- ferent. 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. SEE ALSO
libtool(1), ld(1), otool(1) Apple Inc. December 14, 2009 DYLD(1)

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