.TH DYLD 1 "November 25, 2008" "Apple Inc." .SH NAME dyld \- the dynamic link editor .SH SYNOPSIS DYLD_FRAMEWORK_PATH .br DYLD_FALLBACK_FRAMEWORK_PATH .br DYLD_LIBRARY_PATH .br DYLD_FALLBACK_LIBRARY_PATH .br DYLD_ROOT_PATH .br DYLD_SHARED_REGION .br DYLD_INSERT_LIBRARIES .br DYLD_FORCE_FLAT_NAMESPACE .br DYLD_IMAGE_SUFFIX .br DYLD_PRINT_OPTS .br DYLD_PRINT_ENV .br DYLD_PRINT_LIBRARIES .br DYLD_PRINT_LIBRARIES_POST_LAUNCH .br DYLD_BIND_AT_LAUNCH .br DYLD_NO_FIX_PREBINDING .br DYLD_DISABLE_DOFS .br DYLD_PRINT_APIS .br DYLD_PRINT_BINDINGS .br DYLD_PRINT_INITIALIZERS .br DYLD_PRINT_REBASINGS .br DYLD_PRINT_SEGMENTS .br DYLD_PRINT_STATISTICS .br DYLD_PRINT_DOFS .br DYLD_NO_PIE .br DYLD_SHARED_CACHE_DIR .br DYLD_SHARED_CACHE_DONT_VALIDATE .SH DESCRIPTION The dynamic linker uses the following environment variables. They affect any program that uses the dynamic linker. .TP .B 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.) .IP For each framework that a program uses, the dynamic linker looks for the framework in each directory in .SM DYLD_FRAMEWORK_PATH in turn. If it looks in all the directories and can't find the framework, it searches the directories in .SM DYLD_LIBRARY_PATH in turn. If it still can't find the framework, it then searches .SM DYLD_FALLBACK_FRAMEWORK_PATH and .SM DYLD_FALLBACK_LIBRARY_PATH in turn. .IP Use the .B \-L option to .IR otool (1). to discover the frameworks and shared libraries that the executable is linked against. .TP .B 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 .TP .B 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. .IP For each library that a program uses, the dynamic linker looks for it in each directory in .SM DYLD_LIBRARY_PATH in turn. If it still can't find the library, it then searches .SM DYLD_FALLBACK_FRAMEWORK_PATH and .SM DYLD_FALLBACK_LIBRARY_PATH in turn. .IP Use the .B \-L option to .IR otool (1). to discover the frameworks and shared libraries that the executable is linked against. .TP .B 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. .TP .B 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. .TP .B 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. .TP .B 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 .SM DYLD_FORCE_FLAT_NAMESPACE is also used. .TP .B 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. .TP .B 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. .TP .B DYLD_PRINT_OPTS When this is set, the dynamic linker writes to file descriptor 2 (normally standard error) the command line options. .TP .B DYLD_PRINT_ENV When this is set, the dynamic linker writes to file descriptor 2 (normally standard error) the environment variables. .TP .B 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 .SM DYLD_LIBRARY_PATH is getting what you want. .TP .B DYLD_PRINT_LIBRARIES_POST_LAUNCH This does the same as .SM DYLD_PRINT_LIBRARIES but the printing starts after the program gets to its entry point. .TP .B 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. .TP .B 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. .TP .B 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. .TP .B DYLD_DISABLE_DOFS Causes dyld not register dtrace static probes with the kernel. .TP .B 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. .TP .B DYLD_PRINT_APIS Causes dyld to print a line whenever a dyld API is called (e.g. NSAddImage()). .TP .B 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. .TP .B DYLD_PRINT_BINDINGS Causes dyld to print a line each time a symbolic name is bound. .TP .B DYLD_PRINT_DOFS Causes dyld to print out information about dtrace static probes registered with the kernel. .TP .B DYLD_NO_PIE Causes dyld to not randomize the load addresses of images in a process where the main executable was built position independent. This can be helpful when trying to reproduce and debug a problem in a PIE. .TP .B 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. .TP .B 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. .SH 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. .TP .B @executable_path/ This variable is replaced with the path to the directory containing the main executable for the process. This is useful for .app directories where the main executable is in a well known location inside the .app directory. A typical load path for an embedded framework would look like @executable_path/../Frameworks/Foo.framework/Versions/A/Foo. .TP .B @loader_path/ This variable is replaced with the path to the directory containing the mach-o binary which contains the load path. This is useful for a plug-in that has an embedded framework. @executable_path/ is not helpful because you may not know where the plugin-in will be installed relative to the main executable, or there may be multiple applications that load the plug-in. A typical load path for an embedded framework for reference a sibling framework would look like @loader_path/../../../Frameworks/Foo.framework/Versions/A/Foo. .TP .B @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. .SH "SEE ALSO" libtool(1), ld(1), otool(1)