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DYLD(1) 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_FALL-BACK_LIBRARY_PATH DYLD_FALLBACK_LIBRARY_PATH
BACK_LIBRARY_PATH in turn.
Use the -L option to otool(1). to discover the frameworks and shared libraries that the exe-cutable executable
cutable 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:/Sys-tem/Library/Frameworks /Library/Frameworks:/Network/Library/Frameworks:/System/Library/Frameworks
tem/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 is 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 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_FALL-BACK_FRAMEWORK_PATH DYLD_FALLBACK_FRAMEWORK_PATH
BACK_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 exe-cutable executable
cutable 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 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_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 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 sym-bols. symbols.
bols.
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_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 conjunc-tion conjunction
tion 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/libSys-tem.B.dylib). /usr/lib/libSystem.B.dylib).
tem.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 frame-work framework
work 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 con-taining containing
taining 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 differ-ent. different.
ent. 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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