ld(1) General Commands Manual ld(1)
NAME
ld – linker
SYNOPSIS
ld files... [options] [-o outputfile]
DESCRIPTION
The ld command combines several object files and libraries, resolves
references, and produces an output file. ld can produce a final linked
image (executable, dylib, or bundle), or with the -r option, produce
another object file. If the -o option is not used, the output file
produced is named "a.out".
Universal
The linker accepts universal (multiple-architecture) input files, but
always creates a "thin" (single-architecture), standard Mach-O output
file. The architecture for the output file is specified using the -arch
option. If this option is not used, ld attempts to determine the output
architecture by examining the object files in command line order. The
first "thin" architecture determines that of the output file. If no
input object file is a "thin" file, the native 32-bit architecture for
the host is used.
Usually, ld is not used directly. Instead the compiler driver invokes
ld. The compiler driver can be passed multiple -arch options and it will
create a universal final linked image by invoking ld multiple times and
then running lipo(1) merge the outputs into a universal file.
Layout
The object files are loaded in the order in which they are specified on
the command line. The segments and the sections in those segments will
appear in the output file in the order they are encountered in the object
files being linked. All zero fill sections will appear after all non-
zero fill sections in their segments.
Libraries
A static library (aka static archive) is a collection of .o files with a
table of contents that lists the global symbols in the .o files. ld will
only pull .o files out of a static library if needed to resolve some
symbol reference. Unlike traditional linkers, ld will continually search
a static library while linking. There is no need to specify a static
library multiple times on the command line.
A dynamic library (aka dylib or framework) is a final linked image.
Putting a dynamic library on the command line causes two things: 1) The
generated final linked image will have encoded that it depends on that
dynamic library. 2) Exported symbols from the dynamic library are used to
resolve references.
Both dynamic and static libraries are searched as they appear on the
command line.
Search paths
ld maintains a list of directories to search for a library or framework
to use. The default library search path is /usr/lib then /usr/local/lib.
The -L option will add a new library search path. The default framework
search path is /Library/Frameworks then /System/Library/Frameworks.
(Note: previously, /Network/Library/Frameworks was at the end of the
default path. If you need that functionality, you need to explicitly add
-F/Network/Library/Frameworks). The -F option will add a new framework
search path. The -Z option will remove the standard search paths. The
-syslibroot option will prepend a prefix to all search paths.
Two-level namespace
By default all references resolved to a dynamic library record the
library to which they were resolved. At runtime, dyld uses that
information to directly resolve symbols. The alternative is to use the
-flat_namespace option. With flat namespace, the library is not
recorded. At runtime, dyld will search each dynamic library in load
order when resolving symbols. This is slower, but more like how other
operating systems resolve symbols.
Indirect dynamic libraries
If the command line specifies to link against dylib A, and when dylib A
was built it linked against dylib B, then B is considered an indirect
dylib. When linking for two-level namespace, ld does not look at
indirect dylibs, except when re-exported by a direct dylibs. On the
other hand when linking for flat namespace, ld does load all indirect
dylibs and uses them to resolve references. Even though indirect dylibs
are specified via a full path, ld first uses the specified search paths
to locate each indirect dylib. If one cannot be found using the search
paths, the full path is used.
Dynamic libraries undefines
When linking for two-level namespace, ld does not verify that undefines
in dylibs actually exist. But when linking for flat namespace, ld does
check that all undefines from all loaded dylibs have a matching
definition. This is sometimes used to force selected functions to be
loaded from a static library.
OPTIONS
Options that control the kind of output
-execute
The default. Produce a mach-o main executable that has file type
MH_EXECUTE.
-dylib Produce a mach-o shared library that has file type MH_DYLIB.
-bundle
Produce a mach-o bundle that has file type MH_BUNDLE.
-r Merges object files to produce another mach-o object file with
file type MH_OBJECT.
-dylinker
Produce a mach-o dylinker that has file type MH_DYLINKER. Only
used when building dyld.
-dynamic
The default. Implied by -dylib, -bundle, or -execute
-static
Produces a mach-o file that does not use the dyld. Only used
building the kernel.
-preload
Produces a mach-o file in which the mach_header, load commands,
and symbol table are not in any segment. This output type is
used for firmware or embedded development where the segments are
copied out of the mach-o into ROM/Flash.
-arch arch_name
Specifies which architecture (e.g. ppc, ppc64, i386, x86_64) the
output file should be.
-o path
Specifies the name and location of the output file. If not
specified, `a.out' is used.
Options that control libraries
-lx This option tells the linker to search for libx.dylib or libx.a
in the library search path. If string x is of the form y.o, then
that file is searched for in the same places, but without
prepending `lib' or appending `.a' or `.dylib' to the filename.
-needed-lx
This is the same as the -lx but means to really link with the
dylib even if no symbols are used from it. Thus, it can be used
suppress warnings about unused dylibs.
-reexport-lx
This is the same as the -lx but specifies that the all symbols in
library x should be available to clients linking to the library
being created. This was previously done with a separate
-sub_library option.
-upward-lx
This is the same as the -lx but specifies that the dylib is an
upward dependency.
-hidden-lx
This is the same as the -lx for locating a static library, but
treats all global symbols from the static library as if they are
visibility hidden. Useful when building a dynamic library that
uses a static library but does not want to export anything from
that static library.
-weak-lx
This is the same as the -lx but forces the library and all
references to it to be marked as weak imports. That is, the
library is allowed to be missing at runtime.
-assert-weak-lx
This is the same as the -weak-l but verifies that all references
were marked as weak imports, instead of forcing it.
-delay-lx
This is the same as the -lx but specifies that the dylib should
be delay initialized.
-needed_library path_to_dylib
This is the same as placing path_to_dylib on the link line but
means to really link with the dylib even if no symbols are used
from it. Thus, it can be used suppress warnings about unused
dylibs.
-reexport_library path_to_library
This is the same as listing a file name path to a library on the
link line and it specifies that the all symbols in library path
should be available to clients linking to the library being
created. This was previously done with a separate -sub_library
option.
-upward_library path_to_library
This is the same as listing a file name path to a library on the
link line but also marks the dylib as an upward dependency.
-weak_library path_to_library
This is the same as listing a file name path to a library on the
link line except that it forces the library and all references to
it to be marked as weak imports.
-assert_weak_library path_to_library
This is the same as the -weak_library but verifies that all
references were marked as weak imports, instead of forcing it.
-delay_library path_to_library
This is the same as listing a file name path to a library on the
link line except that will mark the dylib to be delay
initialized.
-Ldir Add dir to the list of directories in which to search for
libraries. Directories specified with -L are searched in the
order they appear on the command line and before the default
search path. In Xcode4 and later, there can be a space between
the -L and directory.
-Z Do not search the standard directories when searching for
libraries and frameworks.
-syslibroot rootdir
Prepend rootdir to all search paths when searching for libraries
or frameworks.
-search_paths_first
This is now the default (in Xcode4 tools). When processing -lx
the linker now searches each directory in its library search
paths for `libx.dylib' then `libx.a' before the moving on to the
next path in the library search path.
-search_dylibs_first
Changes the searching behavior for libraries. The default is
that when processing -lx the linker searches each directory in
its library search paths for `libx.dylib' then `libx.a'. This
option changes the behavior to first search for a file of the
form `libx.dylib' in each directory in the library search path,
then a file of the form `libx.a' is searched for in the library
search paths. This option restores the search behavior of the
linker prior to Xcode4.
-framework name[,suffix]
This option tells the linker to search for `name.framework/name'
the framework search path. If the optional suffix is specified
the framework is first searched for the name with the suffix and
then without (e.g. look for `name.framework/name_suffix' first,
if not there try `name.framework/name').
-needed_framework name[,suffix]
This is the same as the -framework name[,suffix] but means to
really link with the framework even if no symbols are used from
it. Thus, it can be used suppress warnings about unused dylibs.
-weak_framework name[,suffix]
This is the same as the -framework name[,suffix] but forces the
framework and all references to it to be marked as weak imports.
Note: due to a clang optimizations, if functions are not marked
weak, the compiler will optimize out any checks if the function
address is NULL.
-assert_weak_framework name[,suffix]
This is the same as the -weak_framework but verifies that all
references were marked as weak imports, instead of forcing it.
-reexport_framework name[,suffix]
This is the same as the -framework name[,suffix] but also
specifies that the all symbols in that framework should be
available to clients linking to the library being created. This
was previously done with a separate -sub_umbrella option.
-upward_framework name[,suffix]
This is the same as the -framework name[,suffix] but also
specifies that the framework is an upward dependency.
-delay_framework name[,suffix]
This is the same as the -framework name[,suffix] but also
specifies that the framework should be delay initialized.
-Fdir Add dir to the list of directories in which to search for
frameworks. Directories specified with -F are searched in the
order they appear on the command line and before the default
search path. In Xcode4 and later, there can be a space between
the -F and directory.
-all_load
Loads all members of static archive libraries.
-ObjC Loads all members of static archive libraries that implement an
Objective-C class, category or a Swift struct, class or an
extesion.
-force_load path_to_archive
Loads all members of the specified static archive library. Note:
-all_load forces all members of all archives to be loaded. This
option allows you to target a specific archive.
-load_hidden path_to_archive
Uses specified static library as usual, but treats all global
symbols from the static library to as if they are visibility
hidden. Useful when building a dynamic library that uses a
static library but does not want to export anything from that
static library.
-image_suffix suffix
Search for libraries and frameworks with suffix and then without.
Options that control additional content
-sectcreate segname sectname file
The section sectname in the segment segname is created from the
contents of file file. If there's a section (segname,sectname)
from any other input, the linker will append the content from the
file to that section.
-add_empty_section segname sectname
An empty section named sectname in the segment segname. If any of
the inputs contains a section (segname,sectname), that section
will be included in the output, and this option will be ignored.
-add_ast_path file
The linker will add a N_AST stab symbol to the output file where
the string is the path pointed by file and its values is the
modification time of the file.
-filelist file[,dirname]
Specifies that the linker should link the files listed in file.
This is an alternative to listing the files on the command line.
The file names are listed one per line separated only by
newlines. (Spaces and tabs are assumed to be part of the file
name.) If the optional directory name, dirname is specified, it
is prepended to each name in the list file.
-dtrace file
Enables dtrace static probes when producing a final linked image.
The file file must be a DTrace script which declares the static
probes.
Options that control optimizations
-dead_strip
Remove functions and data that are unreachable by the entry point
or exported symbols.
-order_file file
Alters the order in which functions and data are laid out. For
each section in the output file, any symbol in that section that
are specified in the order file file is moved to the start of its
section and laid out in the same order as in the order file file.
Order files are text files with one symbol name per line. Lines
starting with a # are comments. A symbol name may be optionally
preceded with its object file leaf name and a colon (e.g.
foo.o:_foo). This is useful for static functions/data that occur
in multiple files. A symbol name may also be optionally preceded
with the architecture (e.g. ppc:_foo or ppc:foo.o:_foo). This
enables you to have one order file that works for multiple
architectures. Literal c-strings may be ordered by by quoting
the string (e.g. "Hello, world\n") in the order file.
-no_order_inits
When the -order_file option is not used, the linker lays out
functions in object file order and it moves all initializer
routines to the start of the __text section and terminator
routines to the end. Use this option to disable the automatic
rearrangement of initializers and terminators.
-platform_version platform min_version sdk_version
This is set to indicate the platform, oldest supported version of
that platform that output is to be used on, and the SDK that the
output was built against. platform is a numeric value as defined
in <mach-o/loader.h>, or it may be one of the following strings:
• macos
• ios
• tvos
• watchos
• bridgeos
• visionos
• xros
• mac-catalyst
• ios-simulator
• tvos-simulator
• watchos-simulator
• visionos-simulator
• xros-simulator
• driverkit
• firmware
• sepOS
Specifying a newer min or SDK version enables the linker to
assume features of that OS or SDK in the output file. The format
of min_version and sdk_version is a version number such as 10.13
or 10.14
-macos_version_min version
This is set to indicate the oldest macOS version that that the
output is to be used on. Specifying a later version enables the
linker to assumes features of that OS in the output file. The
format of version is a macOS version number such as 10.9 or 10.14
-ios_version_min version
This is set to indicate the oldest iOS version that that the
output is to be used on. Specifying a later version enables the
linker to assumes features of that OS in the output file. The
format of version is an iOS version number such as 3.1 or 4.0
-image_base address
Specifies the preferred load address for a dylib or bundle. The
argument address is a hexadecimal number with an optional leading
0x. By choosing non-overlapping address for all dylibs and
bundles that a program loads, launch time can be improved because
dyld will not need to "rebase" the image (that is, adjust
pointers within the image to work at the loaded address). It is
often easier to not use this option, but instead use the
rebase(1) tool, and give it a list of dylibs. It will then
choose non-overlapping addresses for the list and rebase them
all. When building a position independent executable, this option
will be ignored. This option is also called -seg1addr for
compatibility.
-no_implicit_dylibs
When creating a two-level namespace final linked image, normally
the linker will hoist up public dylibs that are implicitly linked
to make the two-level namespace encoding more efficient for dyld.
For example, Cocoa re-exports AppKit and AppKit re-exports
Foundation. If you link with -framework Cocoa and use a symbol
from Foundation, the linker will implicitly add a load command to
load Foundation and encode the symbol as coming from Foundation.
If you use this option, the linker will not add a load command
for Foundation and encode the symbol as coming from Cocoa. Then
at runtime dyld will have to search Cocoa and AppKit before
finding the symbol in Foundation.
-no_zero_fill_sections
By default the linker moves all zero fill sections to the end of
the __DATA segment and configures them to use no space on disk.
This option suppresses that optimization, so zero-filled data
occupies space on disk in a final linked image.
-merge_zero_fill_sections
Causes all zero-fill sections in the __DATA segment to be merged
into one __zerofill section.
-no_branch_islands
Disables linker creation of branch islands which allows images to
be created that are larger than the maximum branch distance.
Useful with -preload when code is in multiple sections but all
are within the branch range.
-O0 Disables certain optimizations and layout algorithms to optimize
build time. This option should be used with debug builds to speed
up incremental development. The exact implementation might change
to match the intent.
-reproducible
By default output content will be deterministic, but small
changes in input files such as a compilation time might affect
certain data structures in the linked binary. This option
instructs ld to create a reproducible output binary by ignoring
certain input properties or using alternative algorithms.
Options when creating a dynamic library (dylib)
-install_name name
Sets an internal "install path" (LC_ID_DYLIB) in a dynamic
library. Any clients linked against the library will record that
path as the way dyld should locate this library. If this option
is not specified, then the -o path will be used. This option is
also called -dylib_install_name for compatibility.
-compatibility_version number
Specifies the compatibility version number of the library. When
a library is loaded by dyld, the compatibility version is checked
and if the program's version is greater that the library's
version, it is an error. The format of number is X[.Y[.Z]] where
X must be a positive non-zero number less than or equal to 65535,
and .Y and .Z are optional and if present must be non-negative
numbers less than or equal to 255. If the compatibility version
number is not specified, it has a value of 0 and no checking is
done when the library is used. This option is also called
-dylib_compatibility_version for compatibility.
-current_version number
Specifies the current version number of the library. The current
version of the library can be obtained programmatically by the
user of the library so it can determine exactly which version of
the library it is using. The format of number is X[.Y[.Z]] where
X must be a positive non-zero number less than or equal to 65535,
and .Y and .Z are optional and if present must be non-negative
numbers less than or equal to 255. If the version number is not
specified, it has a value of 0. This option is also called
-dylib_current_version for compatibility.
Options when creating a main executable
-pie This makes a special kind of main executable that is position
independent (PIE). On Mac OS X 10.5 and later, the OS the OS
will load a PIE at a random address each time it is executed.
You cannot create a PIE from .o files compiled with -mdynamic-no-
pic. That means the codegen is less optimal, but the address
randomization adds some security. When targeting Mac OS X 10.7 or
later PIE is the default for main executables.
-no_pie
Do not make a position independent executable (PIE). This is the
default, when targeting 10.6 and earlier.
-pagezero_size size
By default the linker creates an unreadable segment starting at
address zero named __PAGEZERO. Its existence will cause a bus
error if a NULL pointer is dereferenced. The argument size is a
hexadecimal number with an optional leading 0x. If size is zero,
the linker will not generate a page zero segment. By default on
32-bit architectures the page zero size is 4KB. On 64-bit
architectures, the default size is 4GB.
-stack_size size
Specifies the maximum stack size for the main thread in a
program. Without this option a program has a 8MB stack. The
argument size is a hexadecimal number with an optional leading
0x. The size should be a multiple of the architecture's page size
(4KB or 16KB).
-allow_stack_execute
Marks executable so that all stacks in the task will be given
stack execution privilege. This includes pthread stacks. This
option is only valid when targeting architectures that support
stack execution (i.e. Intel).
-export_dynamic
Preserves all global symbols in main executables during LTO.
Without this option, Link Time Optimization is allowed to inline
and remove global functions. This option is used when a main
executable may load a plug-in which requires certain symbols from
the main executable.
Options when creating a bundle
-bundle_loader executable
This specifies the executable that will be loading the bundle
output file being linked. Undefined symbols from the bundle are
checked against the specified executable like it was one of the
dynamic libraries the bundle was linked with.
Options when creating an object file
-keep_private_externs
Don't turn private external (aka visibility=hidden) symbols into
static symbols, but rather leave them as private external in the
resulting object file.
-d Force definition of common symbols. That is, transform tentative
definitions into real definitions.
Options that control symbol resolution
-exported_symbols_list filename
The specified filename contains a list of global symbol names
that will remain as global symbols in the output file. All other
global symbols will be treated as if they were marked as
__private_extern__ (aka visibility=hidden) and will not be global
in the output file. The symbol names listed in filename must be
one per line. Leading and trailing white space are not part of
the symbol name. Lines starting with # are ignored, as are lines
with only white space. Some wildcards (similar to shell file
matching) are supported. The * matches zero or more characters.
The ? matches one character. [abc] matches one character which
must be an 'a', 'b', or 'c'. [a-z] matches any single lower case
letter from 'a' to 'z'.
-exported_symbol symbol
The specified symbol is added to the list of global symbols names
that will remain as global symbols in the output file. This
option can be used multiple times. For short lists, this can be
more convenient than creating a file and using
-exported_symbols_list.
-no_exported_symbols
Useful for main executable that don't have plugins and thus need
no symbol exports.
-unexported_symbols_list file
The specified filename contains a list of global symbol names
that will not remain as global symbols in the output file. The
symbols will be treated as if they were marked as
__private_extern__ (aka visibility=hidden) and will not be global
in the output file. The symbol names listed in filename must be
one per line. Leading and trailing white space are not part of
the symbol name. Lines starting with # are ignored, as are lines
with only white space. Some wildcards (similar to shell file
matching) are supported. The * matches zero or more characters.
The ? matches one character. [abc] matches one character which
must be an 'a', 'b', or 'c'. [a-z] matches any single lower case
letter from 'a' to 'z'.
-unexported_symbol symbol
The specified symbol is added to the list of global symbols names
that will not remain as global symbols in the output file. This
option can be used multiple times. For short lists, this can be
more convenient than creating a file and using
-unexported_symbols_list.
-reexported_symbols_list file
The specified filename contains a list of symbol names that are
implemented in a dependent dylib and should be re-exported
through the dylib being created.
-alias symbol_name alternate_symbol_name
Create an alias named alternate_symbol_name for the symbol
symbol_name. By default the alias symbol has global visibility.
This option was previous the -idef:indir option.
-alias_list filename
The specified filename contains a list of aliases. The symbol
name and its alias are on one line, separated by whitespace.
Lines starting with # are ignored.
-flat_namespace
Alters how symbols are resolved at build time and runtime. With
-two_levelnamespace (the default), the linker only searches
dylibs on the command line for symbols, and records in which
dylib they were found. With -flat_namespace, the linker searches
all dylibs on the command line and all dylibs those original
dylibs depend on. The linker does not record which dylib an
external symbol came from, so at runtime dyld again searches all
images and uses the first definition it finds. In addition, any
undefines in loaded flat_namespace dylibs must be resolvable at
build time.
-u symbol_name
Specified that symbol symbol_name must be defined for the link to
succeed. This is useful to force selected functions to be loaded
from a static library.
-U symbol_name
Specified that it is ok for symbol_name to have no definition.
With -two_levelnamespace, the resulting symbol will be marked
dynamic_lookup which means dyld will search all loaded images.
-undefined treatment
Specifies how undefined symbols are to be treated. Options are:
error, warning, suppress, or dynamic_lookup. The default is
error. Note: dynamic_lookup that depends on lazy binding will not
work with chained fixups.
-rpath path
Add path to the runpath search path list for image being created.
At runtime, dyld uses the runpath when searching for dylibs whose
load path begins with @rpath/.
-commons treatment
Specifies how commons (aka tentative definitions) are resolved
with respect to dylibs. Options are: ignore_dylibs, error. The
default is ignore_dylibs which means the linker will turn a
tentative definition in an object file into a real definition and
not even check dylibs for conflicts. The error option means the
linker should issue an error whenever a tentative definition in
an object file conflicts with an external symbol in a linked
dylib. See also -warn_commons.
Options for introspecting the linker
-why_load
Log why each object file in a static library is loaded. That is,
what symbol was needed. Also called -whyload for compatibility.
-why_live symbol_name
Logs a chain of references to symbol_name. Only applicable with
-dead_strip . It can help debug why something that you think
should be dead strip removed is not removed. See
-exported_symbols_list for syntax and use of wildcards.
-print_statistics
Logs information about the amount of memory and time the linker
used.
-t Logs each file (object, archive, or dylib) the linker loads.
Useful for debugging problems with search paths where the wrong
library is loaded.
-order_file_statistics
Logs information about the processing of a -order_file.
-map map_file_path
Writes a map file to the specified path which details all symbols
and their addresses in the output image.
Options for controlling symbol table optimizations
-S Do not put debug information (STABS or DWARF) in the output file.
-x Do not put non-global symbols in the output file's symbol table.
Non-global symbols are useful when debugging and getting symbol
names in back traces, but are not used at runtime. If -x is used
with -r non-global symbol names are not removed, but instead
replaced with a unique, dummy name that will be automatically
removed when linked into a final linked image. This allows dead
code stripping, which uses symbols to break up code and data, to
work properly and provides the security of having source symbol
names removed.
-non_global_symbols_strip_list filename
The specified filename contains a list of non-global symbol names
that should be removed from the output file's symbol table. All
other non-global symbol names will remain in the output files
symbol table. See -exported_symbols_list for syntax and use of
wildcards.
-non_global_symbols_no_strip_list filename
The specified filename contains a list of non-global symbol names
that should be remain in the output file's symbol table. All
other symbol names will be removed from the output file's symbol
table. See -exported_symbols_list for syntax and use of
wildcards.
-oso_prefix prefix-path
When generating the debug map, the linker will remove the
specified prefix-path from the path in OSO symbols. This can be
used so to help build servers generate identical binaries. If
'.' is passed as argument, the linker will expand the argument to
the current working directory.
Options for Bitcode build flow
-bitcode_bundle
Generates an embedded bitcode bundle in the output binary. The
bitcode bundle is embedded in __LLVM, __bundle section. This
option requires all the object files, static libraries and user
frameworks/dylibs contain bitcode. Note: not all the linker
options are supported to use together with -bitcode_bundle.
-bitcode_hide_symbols
Specifies this option together with -bitcode_bundle to hide all
non-exported symbols from output bitcode bundle. The hide symbol
process might not be reversible. To obtain a reverse mapping file
to recover all the symbols, use -bitcode_symbol_map option.
-bitcode_symbol_map path
Specifies the output for bitcode symbol reverse mapping
(.bcsymbolmap). If path is an existing directory,
UUID.bcsymbolmap will be written to that directory. Otherwise,
the reverse map will be written to a file at path.
Rarely used Options
@response_file_path
Inserts contents of file at response_file_path into arguments.
This allows for linker command line args to be store in a file.
Note: ld is normally invoked through clang, and clang also
interprets @file on the command line. To have clang ignore the
@file and pass it through to ld, use -Wl,@file.
-v Prints the version of the linker.
-adhoc_codesign
Directs the linker to add an ad-hoc codesignature to the output
file. The default for Apple Silicon binaries is to be ad-hoc
codesigned.
-no_adhoc_codesign
Directs the linker to not add ad-hoc codesignature to the output
file, even for Apple Silicon binaries.
-data_const
By default the linker moves some data sections into __DATA_CONST
if it knows the target OS version supports that. This option
option overrides the default behavior and forces the use of
__DATA_CONST.
-no_data_const
By default the linker moves some data sections into __DATA_CONST
if it knows the target OS version supports that. This option
option overrides the default behavior and forces the linker to
never move sections to __DATA_CONST.
-const_selrefs
By default the linker moves __objc_selrefs section into
__DATA_CONST if it knows the target OS version supports that.
This option option overrides the default behavior and forces
__objc_selrefs being in __DATA_CONST. Note this only applies if
the __DATA_CONST segment is enabled. See -data_const for more
information.
-no_const_selrefs
By default the linker moves __objc_selrefs section into
__DATA_CONST if it knows the target OS version supports that.
This option option overrides the default behavior and keeps the
__objc_selrefs section in __DATA.
-version_details
Prints the version info about the linker in JSON
-no_weak_imports
Error if any symbols are weak imports (i.e. allowed to be
unresolved (NULL) at runtime). Useful for config based projects
that assume they are built and run on the same OS version.
-no_deduplicate
Don't run deduplication pass in linker
-verbose_deduplicate
Prints names of functions that are eliminated by deduplication
and total code savings size.
-no_inits
Error if the output contains any static initializers
-no_warn_inits
Do not warn if the output contains any static initializers
-warn_duplicate_libraries
Warn if the input contains duplicate library options.
-no_warn_duplicate_libraries
Do not warn if the input contains duplicate library options.
-debug_variant
Do not warn about issues that are only problems for binaries
shipping to customers.
-unaligned_pointers treatment
Specifies how unaligned pointers in __DATA segments should be
handled. Options are: 'warning', 'error', or 'suppress'. The
default for arm64e is 'error' and for all other architectures it
is 'suppress'.
-dirty_data_list filename
Specifies a file containing the names of data symbols likely to
be dirtied. If the linker is creating a __DATA_DIRTY segment,
those symbols will be moved to that segment.
-max_default_common_align value
Any common symbols (aka tentative definitions, or uninitialized
(zeroed) variables) that have no explicit alignment are normally
aligned to their next power of two size (e.g. a 240 byte array is
256 aligned). This option lets you reduce the max alignment.
For instance, a value of 0x40 would reduce the alignment for a
240 byte array to 64 bytes (instead of 256). The value specified
must be a hexadecimal power of two If -max_default_common_align
is not used, the default alignment is already limited to 0x8
(2^3) bytes for -preload and 0x8000 (2^15) for all other output
types.
-move_to_rw_segment segment_name filename
Moves data symbols to another segment. The command line option
specifies the target segment name and a path to a file containing
a list of symbols to move. Comments can be added to the symbol
file by starting a line with a #. If there are multiple
instances of a symbol name (for instance a "static int foo=5;" in
multiple files) the symbol name in the symbol list file can be
prefixed with the object file name (e.g. "init.o:_foo") to move a
specific instance.
-move_to_ro_segment segment_name filename
Moves code symbols to another segment. The command line option
specifies the target segment name and a path to a file containing
a list of symbols to move. Comments can be added to the symbol
file by starting a line with a #. If there are multiple
instances of a symbol name (for instance a "static int foo() {}"
in multiple files) the symbol name in the symbol list file can be
prefixed with the object file name (e.g. "init.o:_foo") to move a
specific instance.
-rename_section orgSegment orgSection newSegment newSection
Renames section orgSegment/orgSection to newSegment/newSection.
-rename_segment orgSegment newSegment
Renames all sections with orgSegment segment name to have
newSegment segment name.
-trace_symbol_layout
For using in debugging -rename_section, -rename_segment,
-move_to_ro_segment, and -move_to_rw_segment. This option prints
out a line show where and why each symbol was moved. Note: These
options do chain. For each symbol, the linker first checks
-move_to_ro_segment and -move_to_rw_segment. Next it applies any
-rename_section options, and lastly and -rename_segment options.
-section_order segname colon_separated_section_list
Only for use with -preload, -dylinker, -static, or with
-platform_version "firmware"/"sepOS". Specifies the order in
which the linker should lay out the sections for the specified
segment. For example: "-section_order __ROM
__text:__const:__cstring". The specified sections are the placed
first in the segment. The order of the rest of the sections is
not guaranteed.
-segment_order colon_separated_segment_list
Only for use with -preload, -static, or with -platform_version
"firmware". Specifies the order in which the linker should lay
out the segments. For example: "-segment_order
__ROM:__ROM2:__RAM". All segments should be specified.
-allow_heap_execute
Normally i386 main executables will be marked so that the Mac OS
X 10.7 and later kernel will only allow pages with the x-bit to
execute instructions. This option overrides that behavior and
allows instructions on any page to be executed.
-application_extension
Specifies that the code is being linked for use in an application
extension. The linker will then validate that any dynamic
libraries linked against are safe for use in application
extensions.
-no_application_extension
Specifies that the code is being linked is not safe for use in an
application extension. For instance, can be used when creating a
framework that should not be used in an application extension.
-fatal_warnings
Causes the linker to exit with a non-zero value if any warnings
were emitted.
-no_eh_labels
Normally in -r mode, the linker produces .eh labels on all FDEs
in the __eh_frame section. This option suppresses those labels.
Those labels are not needed by the Mac OS X 10.6 linker but are
needed by earlier linker tools.
-warn_compact_unwind
When producing a final linked image, the linker processes the
__eh_frame section and produces an __unwind_info section. Most
FDE entries in the __eh_frame can be represented by a 32-bit
value in the __unwind_info section. The option issues a warning
for any function whose FDE cannot be expressed in the compact
unwind format.
-warn_weak_exports
Issue a warning if the resulting final linked image contains weak
external symbols. Such symbols require dyld to do extra work at
launch time to coalesce those symbols.
-no_weak_exports
Issue an erro if the resulting final linked image contains weak
external symbols. Such symbols require dyld to do extra work at
launch time to coalesce those symbols.
-warn_unused_dylibs
Warn about dylibs that are linked by no symbols are used from
them.
-no_warn_unused_dylibs
Don't warn about dylibs that are linked by no symbols are used
from them.
-dead_strip_dylibs
Remove dylibs that are unreachable by the entry point or exported
symbols. That is, suppresses the generation of load command
commands for dylibs which supplied no symbols during the link.
This option should not be used when linking against a dylib which
is required at runtime for some indirect reason such as the dylib
has an important initializer.
-allow_sub_type_mismatches
Normally the linker considers different cpu-subtype for ARM (e.g.
armv4t and armv6) to be different different architectures that
cannot be mixed at build time. This option relaxes that
requirement, allowing you to mix object files compiled for
different ARM subtypes.
-no_uuid
Do not generate an LC_UUID load command in the output file. Be
warned that binaries without UUIDs may cause the debugger and
crash reporting tools to be unable to track and inspect the
binary.
-random_uuid
Generate a random LC_UUID load command in the output file. By
default the linker generates the UUID of the output file based on
a hash of the output file's content. But for very large output
files, the hash can slow down the link. Using a hash based UUID
is important for reproducible builds, but if you are just doing
rapid debug builds, using -random_uuid may improve turn around
time.
-root_safe
Sets the MH_ROOT_SAFE bit in the mach header of the output file.
-setuid_safe
Sets the MH_SETUID_SAFE bit in the mach header of the output
file.
-interposable
Indirects access to all to exported symbols when creating a
dynamic library.
-init symbol_name
The specified symbol_name will be run as the first initializer.
Only used when creating a dynamic library.
-sub_library library_name
The specified dylib will be re-exported. For example the
library_name for /usr/lib/libobjc_profile.A.dylib would be
libobjc. Only used when creating a dynamic library.
-sub_umbrella framework_name
The specified framework will be re-exported. Only used when
creating a dynamic library.
-allowable_client name
Restricts what can link against the dynamic library being
created. By default any code can link against any dylib. But if
a dylib is supposed to be private to a small set of clients, you
can formalize that by adding a -allowable_client for each client.
If a client is libfoo.1.dylib its -allowable_client name would be
"foo". If a client is Foo.framework its -allowable_client name
would be "Foo". For the degenerate case where you want no one to
ever link against a dylib, you can set the -allowable_client to
"!".
-client_name name
Enables a bundle to link against a dylib that was built with
-allowable_client. The name specified must match one of the
-allowable_client names specified when the dylib was created.
-umbrella framework_name
Specifies that the dylib being linked is re-exported through an
umbrella framework of the specified name.
-headerpad size
Specifies the minimum space for future expansion of the load
commands. Only useful if intend to run install_name_tool to
alter the load commands later. Size is a hexadecimal number.
-headerpad_max_install_names
Automatically adds space for future expansion of load commands
such that all paths could expand to MAXPATHLEN. Only useful if
intend to run install_name_tool to alter the load commands later.
-bind_at_load
Sets a bit in the mach header of the resulting binary which tells
dyld to bind all symbols when the binary is loaded, rather than
lazily.
-force_flat_namespace
Sets a bit in the mach header of the resulting binary which tells
dyld to not only use flat namespace for the binary, but force
flat namespace binding on all dylibs and bundles loaded in the
process. Can only be used when linking main executables.
-sectalign segname sectname value
The section named sectname in the segment segname will have its
alignment set to value, where value is a hexadecimal number that
must be an integral power of 2.
-stack_addr address
Specifies the initial address of the stack pointer value, where
value is a hexadecimal number rounded to a page boundary.
-segprot segname max_prot init_prot
Specifies the maximum and initial virtual memory protection of
the named segment, name, to be max and init ,respectively. The
values for max and init are any combination of the characters `r'
(for read), `w' (for write), `x' (for execute) and `-' (no
access). Except for x86_64 architecture, max_prot will always be
set to the same value as init_prot.
-seg_addr_table filename
Specifies a file containing base addresses for dynamic libraries.
Each line of the file is a hexadecimal base address followed by
whitespace then the install name of the corresponding dylib. The
# character denotes a comment.
-segs_read_write_addr address
Allows a dynamic library to be built where the read-only and
read-write segments are not contiguous. The address specified is
a hexadecimal number that indicates the base address for the
read-write segments.
-segs_read_only_addr address
Allows a dynamic library to be built where the read-only and
read-write segments are not contiguous. The address specified is
a hexadecimal number that indicates the base address for the
read-only segments.
-segaddr name address
Specifies the starting address of the segment named name to be
address. The address must be a hexadecimal number that is a
multiple of 4K page size.
-seg_page_size name size
Specifies the page size used by the specified segment. By
default the page size is 4096 for all segments. The linker will
lay out segments such that size of a segment is always an even
multiple of its page size.
-dylib_file install_name:file_name
Specifies that a dynamic shared library is in a different
location than its standard location. Use this option when you
link with a library that is dependent on a dynamic library, and
the dynamic library is in a location other than its default
location. install_name specifies the path where the library
normally resides. file_name specifies the path of the library you
want to use instead. For example, if you link to a library that
depends upon the dynamic library libsys and you have libsys
installed in a nondefault location, you would use this option:
-dylib_file /lib/libsys_s.A.dylib:/me/lib/libsys_s.A.dylib.
-prebind
The created output file will be in the prebound format. This was
used in Mac OS X 10.3 and earlier to improve launch performance.
-weak_reference_mismatches treatment
Specifies what to do if a symbol is weak-imported in one object
file but not weak-imported in another. The valid treatments are:
error, weak, or non-weak. The default is non-weak.
-read_only_relocs treatment
Enables the use of relocations which will cause dyld to modify
(copy-on-write) read-only pages. The compiler will normally
never generate such code.
-force_cpusubtype_ALL
The is only applicable with -arch ppc. It tells the linker to
ignore the PowerPC cpu requirements (e.g. G3, G4 or G5) encoded
in the object files and mark the resulting binary as runnable on
any PowerPC cpu.
-dylinker_install_name path
Only used when building dyld.
-no_arch_warnings
Suppresses warning messages about files that have the wrong
architecture for the -arch flag
-arch_errors_fatal
Turns into errors, warnings about files that have the wrong
architecture for the -arch flag.
-e symbol_name
Specifies the entry point of a main executable. By default the
entry name is "start" which is found in crt1.o which contains the
glue code need to set up and call main().
-w Suppress all warning messages
-final_output name
Specifies the install name of a dylib if -install_name is not
used. This option is used by compiler driver when it is invoked
with multiple -arch arguments.
-arch_multiple
Specifies that the linker should augment error and warning
messages with the architecture name. This option is used by
compiler driver when it is invoked with multiple -arch arguments.
-twolevel_namespace_hints
Specifies that hints should be added to the resulting binary that
can help speed up runtime binding by dyld as long as the
libraries being linked against have not changed.
-dot path
Create a file at the specified path containing a graph of symbol
dependencies. The .dot file can be viewed in GraphViz.
-keep_relocs
Add section based relocation records to a final linked image.
These relocations are ignored at runtime by dyld.
-warn_stabs
Print a warning when the linker cannot do a BINCL/EINCL
optimization because the compiler put a bad stab symbol inside a
BINCL/EINCL range.
-warn_commons
Print a warning whenever a tentative definition in an object file
is found and a external symbol by the same name is also found in
a linked dylib. This often means that the extern keyword is
missing from a variable declaration in a header file.
-read_only_stubs
[i386 only] Makes the __IMPORT segment of a final linked images
read-only. This option makes a program slightly more secure in
that the JMP instructions in the i386 fast stubs cannot be easily
overwritten by malicious code. The downside is the dyld must use
mprotect() to temporarily make the segment writable while it is
binding the stubs.
-slow_stubs
[i386 only] Instead of using single JMP instruction stubs, the
linker creates code in the __TEXT segment which calls through a
lazy pointer in the __DATA segment.
-interposable_list filename
The specified filename contains a list of global symbol names
that should always be accessed indirectly. For instance, if
libSystem.dylib is linked such that _malloc is interposable, then
calls to malloc() from within libSystem will go through a dyld
stub and could potentially indirected to an alternate malloc. If
libSystem.dylib were built without making _malloc interposable
then if _malloc was interposed at runtime, calls to malloc from
with libSystem would be missed (not interposed) because they
would be direct calls.
-no_function_starts
By default the linker creates a compress table of function start
addresses in the LINKEDIT of final linked image. This option
disables that behavior.
-no_objc_category_merging
By default when producing final linked image, the linker will
optimize Objective-C classes by merging any categories on a class
into the class. Both the class and its categories must be
defined in the image being linked for the optimization to occur.
Using this option disables that behavior.
-objc_relative_method_lists
By default when producing final linked image, if targeting a new
enough OS version, the linker will rewrite ObjC method lists from
the tradition three pointers to use three read-only delta
pointers. This option allows you to force the use of relative
method lists even though the OS version is too low.
-no_objc_relative_method_lists
By default when producing final linked image, if targeting a new
enough OS version, the linker will rewrite ObjC method lists from
the tradition three pointers to use three read-only delta
pointers. This option allows you to force the use of traditional
three pointer method lists.
-object_path_lto filename
When performing Link Time Optimization (LTO) and a temporary
mach-o object file is needed, if this option is used, the
temporary file will be stored at the specified path and remain
after the link is complete. Without the option, the linker picks
a path and deletes the object file before the linker tool
completes, thus tools such as the debugger or dsymutil will not
be able to access the DWARF debug info in the temporary object
file.
-lto_library path
When performing Link Time Optimization (LTO), the linker normally
loads libLTO.dylib relative to the linker binary
(../lib/libLTO.dylib). This option allows the user to specify the
path to a specific libLTO.dylib to load instead.
-cache_path_lto path
When performing Incremental Link Time Optimization (LTO), use
this directory as a cache for incremental rebuild.
-prune_interval_lto seconds
When performing Incremental Link Time Optimization (LTO), the
cache will pruned after the specified interval. A value 0 will
force pruning to occur and a value of -1 will disable pruning.
-prune_after_lto seconds
When pruning the cache for Incremental Link Time Optimization
(LTO), the cache entries are removed after the specified
interval.
-max_relative_cache_size_lto percent
When performing Incremental Link Time Optimization (LTO), the
cache will be pruned to not go over this percentage of the free
space. I.e. a value of 100 would indicate that the cache may fill
the disk, and a value of 50 would indicate that the cache size
will be kept under the free disk space.
-fixup_chains_section
For use with -static or -preload when -pie is used. Tells the
linker to add a __TEXT,__chain_starts section which starts with a
dyld_chained_starts_offsets struct which specifies the pointer
format and the offsets to the start of every fixup chain.
-fixup_chains_section_vm
Same as -fixup_chains_section but fixes a bug. The offsets in
the __chain_starts section are vm-offsets from the __TEXT
segment, and the rebase targets in the chains are vm-offsets.
-threaded_starts_section
For arm64e only. For use with -static or -preload when -pie is
used. Tells the linker to add a __TEXT,__thread_starts section
which starts with a 32-bit flag field, followed by an array
32-bit values. Each value is the offset to the start of a fixup
chain. This option is deprecated.
-page_align_data_atoms
During development, this option can be used to space out all
global variables so each is on a separate page. This is useful
when analyzing dirty and resident pages. The information can
then be used to create an order file to cluster commonly
used/dirty globals onto the same page(s).
-not_for_dyld_shared_cache
Normally, the linker will add extra info to dylibs with
-install_name starting with /usr/lib or /System/Library/ that
allows the dylib to be placed into the dyld shared cache. Adding
this option tells the linker to not add that extra info.
-search_in_sparse_frameworks
For use when linking against versioned frameworks that do not
have a normal variant. By default when -framework Foo,_suffix is
used, the linker will follow Foo.framework/Foo if it is a
symbolic link, append _suffix and search for a file with that
path. When this option is used, the linker will also search for
Foo.framework/Versions/Current/Foo_suffix.
-ld_classic
Override the choice of linker, and force the use of ld-classic to
link the binary. This is incompatible with options such as
-merge*, used to build/merge libraries.
-ld_new
Override the choice of linker, and force the use of ld to link
the binary. This is incompatible with older architectures such as
armv7k and i386.
Mergeable Library Options
-make_mergeable
Adds additional metadata to a dylib which makes it a mergeable
library. It can still be used as a dylib, or can be merged into
other binaries when they link it with a -merge* option.
-merge-lx
This is the same as the -lx option but means to merge the
contents of the library x into this binary.
-merge_library path_to_library
This is the same as listing a file name path to a library on the
link line but also merges the contents of the library into this
binary.
-merge_framework name[,suffix]
This is the same as the -framework name[,suffix] but means that
the contents of the framework should be merged into this binary.
-no_merged_libraries_hook
When using mergeable libraries ld automatically adds a hook to
redirect bundle resource lookups from mergeable frameworks into
the merged binary. Use this option to disable the hook.
The hook requires a minimum deployment version of iOS 12, you can
use the option to disable the hook with a lower deployment target
if your frameworks don't require bundle resource lookups.
Disabling the hook might also improve launch time performance, so
it's good to disable it regardless of the deployment target if
it's not required.
Obsolete Options
-segalign value
All segments must be page aligned.
-seglinkedit
Object files (MH_OBJECT) with a LINKEDIT segment are no longer
supported. This option is obsolete.
-noseglinkedit
This is the default. This option is obsolete.
-fvmlib
Fixed VM shared libraries (MH_FVMLIB) are no longer supported.
This option is obsolete.
-sectobjectsymbols segname sectname
Adding a local label at a section start is no longer supported.
This option is obsolete.
-nofixprebinding
The MH_NOFIXPREBINDING bit of mach_headers has been ignored since
Mac OS X 10.3.9. This option is obsolete.
-noprebind_all_twolevel_modules
Multi-modules in dynamic libraries have been ignored at runtime
since Mac OS X 10.4.0. This option is obsolete.
-prebind_all_twolevel_modules
Multi-modules in dynamic libraries have been ignored at runtime
since Mac OS X 10.4.0. This option is obsolete.
-prebind_allow_overlap
When using -prebind, the linker allows overlapping by default, so
this option is obsolete.
-noprebind
LD_PREBIND is no longer supported as a way to force on
prebinding, so there no longer needs to be a command line way to
override LD_PREBIND. This option is obsolete.
-sect_diff_relocs treatment
This option was an attempt to warn about linking .o files
compiled without -mdynamic-no-pic into a main executable, but the
false positive rate generated too much noise to make the option
useful. This option is obsolete.
-run_init_lazily
This option was removed in Mac OS X 10.2.
-single_module
This is now the default so does not need to be specified.
-multi_module
Multi-modules in dynamic libraries have been ignored at runtime
since Mac OS X 10.4.0. This option is obsolete.
-no_dead_strip_inits_and_terms
The linker never dead strips initialization and termination
routines. They are considered "roots" of the dead strip graph.
-A basefile
Obsolete incremental load format. This option is obsolete.
-b Used with -A option to strip base file's symbols. This option is
obsolete. Obsolete option to produce a load map. Use -map
option instead.
-Sn Don't strip any symbols. This is the default. This option is
obsolete.
-Si Optimize stabs debug symbols to remove duplicates. This is the
default. This option is obsolete.
-Sp Write minimal stabs which causes the debugger to open and read
the original .o file for full stabs. This style of debugging is
obsolete in Mac OS X 10.5. This option is obsolete.
-X Strip local symbols that begin with 'L'. This is the default.
This option is obsolete.
-s Completely strip the output, including removing the symbol table.
This file format variant is no longer supported. This option is
obsolete.
-m Don't treat multiple definitions as an error. This is no longer
supported. This option is obsolete.
-ysymbol
Display each file in which symbol is used. This was previously
used to debug where an undefined symbol was used, but the linker
now automatically prints out all usages. The -why_live option
can also be used to display what kept a symbol from being dead
striped. This option is obsolete.
-Y number
Used to control how many occurrences of each symbol specified
with -y would be shown. This option is obsolete.
-nomultidefs
Only used when linking an umbrella framework. Sets the
MH_NOMULTIDEFS bit in the mach_header. The MH_NOMULTIDEFS bit
has been obsolete since Mac OS X 10.4. This option is obsolete.
-multiply_defined_unused treatment
Previously provided a way to warn or error if any of the symbol
definitions in the output file matched any definitions in dynamic
library being linked. This option is obsolete.
-multiply_defined treatment
Previously provided a way to warn or error if any of the symbols
used from a dynamic library were also available in another linked
dynamic library. This option is obsolete.
-private_bundle
Previously prevented errors when -flat_namespace, -bundle, and
-bundle_loader were used and the bundle contained a definition
that conflicted with a symbol in the main executable. The linker
no longer errors on such conflicts. This option is obsolete.
-noall_load
This is the default. This option is obsolete.
-seg_addr_table_filename path
Use path instead of the install name of the library for matching
an entry in the seg_addr_table. This option is obsolete.
-sectorder segname sectname orderfile
Replaced by more general -order_file option.
-sectorder_detail
Produced extra logging about which entries from a sectorder
entries were used. Replaced by -order_file_statistics. This
option is obsolete.
-poison_symbol symbolName
Causes any use of the named symbol to be an error. Useful to
make sure you are not using some API.
-poison_symbols_list filePath
The specified filePath contains a list of symbol names that are
to be poisoned.
SEE ALSO
ld-classic(1), as(1), ar(1), cc(1), dyld_info(1), nm(1), otool(1)
lipo(1), arch(3), dyld(3), Mach-O(5), strip(1), rebase(1)
Darwin June 21, 2023 Darwin