ld(1) BSD General Commands Manual ld(1)NAMEld — linker
SYNOPSISld files... [options] [-o outputfile]
DESCRIPTION
The ld command combines several object files and libraries, resolves ref‐
erences, and produces an ouput 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 gcc(1) 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. Sections created from files with
the -sectcreate option will be laid out at after sections from .o files.
The use of the -order_file option will alter the layout rules above, and
move the symbols specified to start of their section.
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 sym‐
bol 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 com‐
mand 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 informa‐
tion 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 indi‐
rect 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 defini‐
tion. 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.
-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.
-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.
-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 ref‐
erences to it to be marked as weak imports.
-reexport-lx
This is the same as the -lx but specifies that the all sym‐
bols in library x should be available to clients linking to
the library being created. This was previously done with a
separate -sub_library option.
-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 sepa‐
rate -sub_library option.
-lazy-lx This is the same as the -lx but it is only for shared
libraries and the linker will construct glue code so that the
shared library is not loaded until the first function in it
is called.
-lazy_library path_to_library
This is the same as listing a file name path to a shared
library on the link line except that the linker will con‐
struct glue code so that the shared library is not loaded
until the first function in it is called.
-upward-lx This is the same as the -lx but specifies that the dylib is
an upward dependency.
-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 depen‐
dency.
-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.frame‐
work/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.frame‐
work/name_suffix' first, if not there try `name.frame‐
work/name').
-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.
-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.
-lazy_framework name[,suffix]
This is the same as the -framework name[,suffix] except that
the linker will construct glue code so that the framework is
not loaded until the first function in it is called. You
cannot directly access data or Objective-C classes in a
framework linked this way.
-upward_framework name[,suffix]
This is the same as the -framework name[,suffix] but also
specifies that the framework is an upward dependency.
-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 or category.
-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.
Options that control additional content
-sectcreate segname sectname file
The section sectname in the segment segname is created from
the contents of file file. The combination of segname and
sectname must be unique � there cannot already be a section
(segname,sectname) from any other input.
-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 sepa‐
rated 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 sec‐
tion 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. Lit‐
eral 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.
-no_order_data
By default the linker reorders global data in the __DATA seg‐
ment so that all global variables that dyld will need to
adjust at launch time will early in the __DATA segment. This
reduces the number of dirty pages at launch time. This
option disables that optimization.
-macosx_version_min version
This is set to indicate the oldest Mac OS X 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 out‐
put file. The format of version is a Mac OS X version number
such as 10.4 or 10.5
-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 perferred 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. This option is also called
-seg1addr for compatibility.
-no_implicit_dylibs
When creating a two-level namespace final linked image, nor‐
mally the linker will hoist up public dylibs that are implic‐
itly 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.
-exported_symbols_order file
When targeting Mac OS X 10.6 or later, the format of the
exported symbol information can be optimized to make lookups
of popular symbols faster. This option is used to pass a
file containing a list of the symbols most frequently used by
clients of the dynamic library being built. Not all exported
symbols need to be listed.
-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 sec‐
tions but all are within the branch range.
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 compati‐
bility.
-mark_dead_strippable_dylib
Specifies that the dylib being built can be dead strip by any
client. That is, the dylib has no initialization side
effects. So if a client links against the dylib, but never
uses any symbol from it, the linker can optimize away the use
of the dylib.
-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_compatibil‐
ity_version for compatibility.
-current_version number
Specifies the current version number of the library. The cur‐
rent 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 -mdy‐
namic-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 executa‐
bles.
-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 seg‐
ment. By default on 32-bit architectures the page zero size
is 4KB. On 64-bit architectures, the default size is 4GB.
The ppc64 architecture has some special cases. Since Mac OS X
10.4 did not support 4GB page zero programs, the default page
zero size for ppc64 will be 4KB unless -macosx_version_min is
10.5 or later. Also, the -mdynamic-no-pic codegen model for
ppc64 will only work if the code is placed in the lower 2GB
of the address space, so the if the linker detects any such
code, the page zero size is set to 4KB and then a new unread‐
able trailing segment is created after the code, filling up
the lower 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 lead‐
ing 0x. The size should be an even multiple of 4KB, that is
the last three hexadecimal digits should be zero.
-allow_stack_execute
Marks executable so that all stacks in the task will be given
stack execution privilege. This includes pthread stacks.
-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 exter‐
nal in the resulting object file.
-d Force definition of common symbols. That is, transform ten‐
tative definitions into real definitions.
-rename_section fromSegment fromSection toSegment toSection
Renames section fromSegment/fromSection to toSegment/toSec‐
tion.
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 file‐
name 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 charac‐
ter. [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.
-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 __pri‐
vate_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 visibil‐
ity. 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 defini‐
tion. 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.
-rpath path
Add path to the runpath search path list for image being cre‐
ated. 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,
use_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 con‐
flicts. The dylibs option means the linker should check
linked dylibs for definitions and use them to replace tenta‐
tive definitions from object files. The error option means
the linker should issue an error whenever a tentative defini‐
tion 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 compat‐
ibility.
-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.
-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.
-whatsloaded
Logs just object files the linker loads.
-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 controling 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 ta‐
ble. 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 ta‐
ble. All other symbol names will be removed from the output
file's symbol table. See -exported_symbols_list for syntax
and use of wildcards.
Rarely used Options
-v Prints the version of the linker.
-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.
-fatal_warnings
Causes the linker to exit with a non-zero value if any warn‐
ings 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.
-objc_gc_compaction
Marks the Objective-C image info in the final linked image
with the bit that says that the code was built to work the
compacting garbage collection.
-objc_gc Verifies all code was compiled with -fobjc-gc or -fobjc-gc-
only.
-objc_gc_only
Verifies all code was compiled with -fobjc-gc-only.
-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 architec‐
tures 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.
-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 initial‐
izer. 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 -allow‐
able_client name would be "foo". If a client is Foo.frame‐
work 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 com‐
mands 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 executa‐
bles.
-sectalign segname sectname value
The section named sectname in the segment segname will have
its alignment set to value, where value is a hexadecimal num‐
ber 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 bound‐
ary.
-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 char‐
acters `r' (for read), `w' (for write), `x' (for execute) and
`-' (no access).
-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 speci‐
fied 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 speci‐
fied 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/lib‐
sys_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 per‐
formance.
-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 mod‐
ify (copy-on-write) read-only pages. The compiler will nor‐
mally 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 con‐
tains 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 gcc driver when it is invoked
with multiple -arch arguments.
-arch_multiple
Specifes that the linker should augment error and warning
messages with the architecture name. This option is used by
gcc 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 opti‐
mization because the compiler put a bad stab symbol inside a
BINCL/EINCL range.
-warn_commons
Print a warning whenever the 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 seg‐
ment 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 inter‐
posed) 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_version_load_command
By default the linker creates a load command in final linked
images that contains the -macosx_version_min. 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.
-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.
-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 use‐
ful 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).
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 sup‐
ported. This option is obsolete.
-preload Preload executables (MH_PRELOAD) are no longer supported.
-sectobjectsymbols segname sectname
Adding a local label at a section start is no longer sup‐
ported. 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 run‐
time since Mac OS X 10.4.0. This option is obsolete.
-prebind_all_twolevel_modules
Multi-modules in dynamic libraries have been ignored at run‐
time 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 pre‐
binding, 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 run‐
time 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 obso‐
lete.
-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 previ‐
ously 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 sym‐
bol 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 sym‐
bol 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 sym‐
bols 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 defi‐
nition 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.
SEE ALSOas(1), ar(1), cc(1), nm(1), otool(1)lipo(1), arch(3), dyld(3), Mach-
O(5), strip(1), rebase(1)Darwin March 7, 2011 Darwin
