% sw_vers
ProductName: macOS
ProductVersion: 12.0
BuildVersion: 21A5506j
% man dyld
DYLD(1) General Commands Manual DYLD(1)
NAME
dyld - the dynamic linker
SYNOPSIS
DYLD_FRAMEWORK_PATH
DYLD_FALLBACK_FRAMEWORK_PATH
DYLD_VERSIONED_FRAMEWORK_PATH
DYLD_LIBRARY_PATH
DYLD_FALLBACK_LIBRARY_PATH
DYLD_VERSIONED_LIBRARY_PATH
DYLD_IMAGE_SUFFIX
DYLD_INSERT_LIBRARIES
DYLD_PRINT_TO_FILE
DYLD_PRINT_LIBRARIES
DYLD_PRINT_LOADERS
DYLD_PRINT_SEARCHING
DYLD_PRINT_APIS
DYLD_PRINT_BINDINGS
DYLD_PRINT_INITIALIZERS
DYLD_PRINT_SEGMENTS
DYLD_PRINT_ENV
DYLD_SHARED_REGION
DYLD_SHARED_CACHE_DIR
DESCRIPTION
The dynamic linker (dyld) 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 -current_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_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_INSERT_LIBRARIES
This is a colon separated list of additional dynamic libraries to
load before the ones specified in the program. If instead, your
goal is to substitute a library that would normally be loaded, use
DYLD_LIBRARY_PATH or DYLD_FRAMEWORK_PATH instead.
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_PRINT_ENV
If set, causes dyld to print a line of key=valule for each
enviroment variable in the process.
DYLD_PRINT_LIBRARIES
If set, causes dyld to print a line for each mach-o image loaded
into a process. This is useful to make sure that the use of
DYLD_LIBRARY_PATH is getting what you want.
DYLD_PRINT_LOADERS
If set, causes dyld to print a line whether each image is tracked
by a JustInTimeLoader or a PrebuiltLoader. Additionally, it
prints if a PrebuiltLoaderSet was used to launch the process or if
a PrebuiltLoader was written to make the next launch faster.
DYLD_PRINT_SEARCHING
If set, causes dyld to print a line about each file system path
checked when searching for an image to load.
DYLD_PRINT_INITIALIZERS
If set, 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
If set, causes dyld to print a line whenever a dyld API is called
(e.g. dlopen()).
DYLD_PRINT_SEGMENTS
If set, 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
If set, causes dyld to print a line each time a symbolic name is
bound.
DYLD_SHARED_REGION
This can be "use" (the default) or "private". 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_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.
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 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.
SEE ALSO
dyldinfo(1), ld(1), otool(1)
Apple Inc. June 1, 2020 DYLD(1)
