LD(1) General Commands Manual LD(1)
NAME
ld - Mach object file link editor
SYNOPSIS
ld [ option ... ] [ file ... ]
DESCRIPTION
The ld command combines several Mach-O (Mach object) files into one by combining like sections in like segments from all the object files,
resolving external references, and searching libraries. In the simplest case several object files are given, and ld combines them, produc-
ing an object file which can either be executed or become the input for a further ld run. (In the latter case, the -r option must be given
to preserve the relocation information.) Unless an output file is specified, ld produces a file named a.out. This file is made executable
only if no errors occurred during the link editing and there are no undefined symbols.
FAT FILE SUPPORT
The link editor accepts ``fat'' (multiple-architecture) input files, but always creates a ``thin'' (single-architecture), standard Mach-O
output file. The architecture is specified using the -arch arch_type option. If this option is not used, ld(1) attempts to determine the
output architecture by examining the first object file encountered on the command line. If it is a ``thin'' file, its architecture deter-
mines that of the output file. If the first input file is a ``fat'' file, the ``best'' architecture for the host is used. (See the expla-
nation of the -arch option, below.)
The compiler driver cc(1) handles creating fat executables by calling ld(1) multiple times and using lipo(1) to create a ``fat'' file from
the results of the ld(1) executions.
OUTPUT 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 appear in the output file last. Section names for sections created from files
are not allowed to overlap with a section name in the same segment as a section coming from an object file. Sections created from files
may be in a segment which has sections from object files and if so will be loaded at the end of the non-zero fill sections for that seg-
ment.
If the option -seglinkedit is specified, the segment it creates is the last segment in the output file.
The address of each segment can be specified with -segaddr, which takes the segment's name as an argument. The address of the first seg-
ment can alternatively be specified using -seg1addr, in which case a segment name is not used. Segments that do not have a specified
address will be assigned addresses in the order in which they appear in the output file. A segment's address will be assigned based on the
ending address of the previous segment. If the address of the first segment has not been specified by name, its assigned address will be
the specified (via -seg1addr) or default first segment address. If neither flag is used to specify the first segment's address, its
default address is zero for all formats except the demand-paged executable format (MH_EXECUTE), in which case the default first address is
the value of the segment alignment.
For demand-paged executable format (MH_EXECUTE) output files, if none of the segments' addresses covers address zero through the value of
the segment alignment, a segment with no access protection will be created to cover those addresses. This segment, named ``__PAGEZERO'',
is created so that any attempt to dereference a NULL pointer will cause a memory exception.
The entry point of the output file is the beginning of the first section in the first segment (unless the -e option is specified).
STATIC ARCHIVE LIBRARIES
ld supports two types of libraries: static archive libraries and dynamic shared libraries. Searching for undefined symbols is performed
differently for dynamic shared libraries than it is for static archive libraries. The searching of dynamic shared libraries is described
later.
When a static archive library is specified as an argument to ld, it is searched exactly once, at the point it is encountered in the argu-
ment list. Only those members defining an unresolved external reference, as defined by the static archive library's table of contents, are
loaded. To produce the table of contents, all static archive libraries must be processed by ranlib(1).
Generally, a static archive library does not have multiple members that define the same symbol. For these types of libraries, the order of
the members is not important, so the table of contents can be sorted for faster link editing using the -s option to ranlib(1). The first
member of the static archive library is named ``__.SYMDEF SORTED'', which is understood to be a sorted table of contents.
If the static archive library does have multiple members that define the same symbol, the table of contents that ranlib(1) produces can't
be sorted. Instead, it follows the order in which the members appear in the static archive library. The link editor searches the table of
contents iteratively, loading members until no further references are satisfied. In the unsorted case, the first member of the static ar-
chive library is named ``__.SYMDEF'', which is understood to be a table of contents in the order of the archive members.
Static archive library members can also be loaded in response to the -ObjC and -all_load flags. See their descriptions below.
DYNAMIC SHARED LIBRARIES
When a dynamic shared library or an object file that was linked against a dynamic shared library is specified as an argument to ld, that
library is placed in the dynamic shared library search list. The order of the search list is always the same order the libraries were
encountered on the command line. All dynamic libraries libraries that the dynamic libraries are dependent upon are added to the end of the
search list.
Once the search list is constructed, the static link editor checks for undefined symbols by simulating the way the dynamic linker will
search for undefined symbols at runtime. For each undefined symbol, the static link editor searches each library in the search list until
it finds a module that defines the symbol. With each undefined symbol, the search starts with the first library in the list. This is dif-
ferent than for static archive libraries, where each library is searched exactly once for all undefined symbols.
The static link editor simulates dynamic linking as if all the undefined symbols are to be bound at program launch time. The dynamic linker
actually binds undefined symbols as they are encountered during execution instead of at program launch. However, the static link editor
always produces the same linking as the dynamic linker as long as none of the dynamic shared libraries define the same symbol. Different
linking can occur only when there is more than one definition of a symbol and the library modules that contain the definitions for that
symbol do not define and reference exactly the same symbols. In this case, even different executions of the same program can produce dif-
ferent linking because the dynamic linker binds undefined functions as they are called, and this affects the order in which undefined sym-
bols are bound. Because it can produce different dynamic linking, using dynamic shared libraries that define the same symbols in the same
program is strongly discouraged.
If a static archive library appears after a dynamic shared library on the command line, the static library is placed in the dynamic library
search list and is searched as a dynamic library. In this way, when a dynamic library has undefined symbols, it will cause the appropriate
members of the static libraries to be loaded into the output. Searching static libraries as dynamic libraries can cause problems if the
dynamic library later changes to reference symbols from the static library that it did not previously reference. In this case when the pro-
gram runs, the dynamic linker will report these symbols as undefined because the members for these symbols were not loaded into the output.
TWO-LEVEL AND FLAT NAMESPACES
Two-level and flat namespaces refer to how references to symbols in dynamic libraries are resolved to a definition in specific dynamic
library. For two-level namespace that resolution is done at static link time when each image (program, bundle and shared library) is
built. When a program is using images built with two-level namespace there may be different global symbols with the same name being used
by different images in the program (this is now the default). When a program is using all flat namespace images then only one global sym-
bol for each global symbol name is used by all images of the program (this was the default in MacOS X 10.0).
When creating a output file with the static link editor that links against dynamic libraries, the references to symbols in those libraries
can be recorded at static link time to bind to a specific library definition (two-level namespace) or left to be bound at execution time to
the first library in the search order of the program (flat namespace). A program, its dynamic libraries and its bundles may each be either
two-level or flat namespace images. The dynamic linker will bind each image according to how it was built.
When creating a output file with the static link editor when -twolevel_namespace is in effect (now the default) all undefined references
must be satisfied at static link time. The flags to allow undefined references, -Usymbol_name, -undefined warning and -undefined suppress
can't be used. When the environment variable MACOSX_DEPLOYMENT_TARGET is set to 10.3 then -undefined dynamic_lookup can also be used. The
specific library definition recorded for each reference is the first library that has a definition as listed on the link line. Listing an
umbrella framework implies all of its sub-frameworks, sub-umbrellas and sub-libraries. For any reference to a definition found in an
umbrella framework's sub-framework, sub-umbrella or sub-library will be recorded as coming from the umbrella framework. Then at execution
time the dynamic linker will search that umbrella framework's sub-frameworks, sub-umbrellas and sub-libraries for those references. Also
when two-level namespace is in effect only those frameworks listed on the link line (and sub-frameworks, sub-umbrellas and sub-libraries of
umbrella frameworks) are searched. Other dependent libraries which are not sub-frameworks, sub-umbrellas or sub-libraries of umbrella
frameworks are not searched.
If a two-level namespace static link references a symbol from an indirectly referenced dynamic library not listed on the link line
the following error message will result:
ld: object_file illegal reference to symbol: symbol defined in indirectly referenced dynamic library: library
To correct the link error the user should add library to the link line in the order he wants the library to be searched.
When creating bundles (MH_BUNDLE outputs) with the static link editor when two-level namespace is in effect (now the default) and the bun-
dle has references to symbols expected to be defined in the program loading the bundle, then the -bundle_loader executable must be used.
When creating a output file with the static link editor when -flat_namespace is in effect (the MacOS X 10.0 default) all undefined refer-
ences must be satisfied at static link time when -undefined error (the default) is used. The static link editor checks the undefined ref-
erences by searching all the libraries listed on the link line then all dependent libraries. The undefined symbols in the created output
file are left to be resolved at execution time by the dynamic link editor in the dynamic libraries in the search order of the program.
MULTIPLY DEFINED SYMBOLS
If there are multiply defined symbols in the object files being linked into the output file being created this always results in a multiply
defined symbol error.
When the static link editor links symbols in from a dynamic library that result in multiply defined symbols the handling depends on the
type of name space of output file being created and possibly the type of name space of the dynamic library.
When the static link editor is creating a two-level namespace image and a there is a multiply defined symbol from dynamic library then that
generates a multiply defined symbol warning (by default), where the treatment of this warning can be changed with the -multiply_defined
flag.
When the static link editor is creating a flat namespace image and a there is a multiply defined symbol from dynamic library, if the
library is a flat namespace image then that generates a multiply defined symbol error. If the library is a two-level namespace image then
that generates a multiply defined symbol warning (by default), where the treatment of this warning can be changed with the -multi-
ply_defined flag.
USING THE DYNAMIC LINK EDITOR AND DYNAMIC SHARED LIBRARIES
The option -dynamic must be specified in order to use dynamic shared libraries (and any of the features used to implement them) and/or the
dynamic link editor. To make sure that the output is not using any features that would require the dynamic link editor, the flag -static
can be specified. Only one of these flags can be specified.
LINK EDITOR DEFINED SYMBOLS
There is a group of link editor defined symbols for the MH_EXECUTE, MH_DYLIB and MH_PRELOAD file types (see the header file <mach-
o/ldsyms.h>). Link editor symbols are reserved; it is an error if an input object file defines such a symbol. Only those link editor sym-
bols that are referenced by the object file appear in the output file's symbol table.
The link editor defined symbol `__mh_execute_header' (`_mh_execute_header' in C) is reserved when the output file format is MH_EXECUTE.
This symbol is the address of the Mach header in a Mach-O executable (a file of type MH_EXECUTE). It does not appear in any other Mach-O
file type. It can be used to get to the addresses and sizes of all the segments and sections in the executable. This can be done by pars-
ing the headers and load commands (see Mach-O(5)).
The link editor defined symbol `__mh_dylib_header' (`_mh_dylib_header' in C) is reserved when the output file format is MH_DYLIB. This
symbol is the address of the Mach header in a Mach-O dynamic shared library (a file of type MH_DYLIB) and is a private external symbol. It
does not appear in any other Mach-O file type. It can be used to get to the addresses and sizes of all the segments and sections in a
dynamic shared library. The addresses, however, must have the value _dyld_get_image_vmaddr_slide(3) added to them.
The MH_PRELOAD file type has link editor defined symbols for the beginning and ending of each segment, and for the beginning and ending of
each section within a segment. These names are provided for use in a Mach-O preloaded file, since it does not have its headers loaded as
part of the first segment. The names of the symbols for a segment's beginning and end have the form: __SEGNAME__begin and __SEGNAME__end,
where __SEGNAME is the name of the segment. Similarly, the symbols for a section have the form: __SEGNAME__sectname__begin and __SEG-
NAME__sectname__end, where __sectname is the name of the section in the segment __SEGNAME. These symbols' types are those of the section
that the names refer to. (A symbol that refers to the end of a section actually has, as its value, the beginning address of the next sec-
tion, but the symbol's type is still that of the section mentioned in the symbol's name.)
OPTIONS
Ld understands several options. Filenames and options that refer to libraries (such as -l and -framework), as well as options that create
symbols (such as -u and -i), are position-dependent: They define the load order and affect what gets loaded from libraries. Some ld
options overlap with compiler options. If the compiler driver cc(1) is used to invoke ld , it maybe necessary to pass the ld(1) options to
cc(1) using -Wl,-option,argument1,argument2.
The most common option is:
-o name
The output file is named name, instead of a.out.
The following flags are related to architectures:
-arch arch_type
Specifies the architecture, arch_type, for the output file. ``Fat'' input files that do not contain this specified architecture are
ignored. Only one -arch arch_type can be specified. See arch(3) for the currently known arch_types. If arch_type specifies a
certain implementation of an architecture (such as -arch m68040 or -arch i486 ), the resulting object file has that specific CPU
subtype, and it is an error if any input file has a CPU subtype that will not combine to the CPU subtype for arch_type.
The default output file architecture is determined by the first object file to be linked. If it is a ``thin'' (standard Mach-O)
file, or a ``fat'' file that contains only one architecture, the output file will have the same architecture. Otherwise, if it is a
``fat'' file containing an architecture that would execute on the host, then the ``best'' architecture is used, as defined by what
the kernel exec(2) would select. Otherwise, it is an error, and a -arch arch_type must be specified.
-arch_multiple
This flag is used by the cc(1) driver program when it is run with multiple -arch arch_type flags. It instructs programs like ld(1)
to precede any displayed message with a line stating the program name, in this case ld, and the architecture (from the -arch
arch_type flag). This helps distinguish which architecture the error messages refer to.
-force_cpusubtype_ALL
The -force_cpusubtype_ALL flag causes the CPU subtype to remain the ALL CPU subtype and not to be combined or changed. This flag
has precedence over any -arch arch_type flag for a specific implementation.
The following flags are related to using the dynamic link editor and/or dynamic shared libraries (and any of the features used to implement
them):
-dynamic
Allows use of the features associated with dynamic link editor. The default is -dynamic.
-static
Causes those features associated with dynamic link editor to be treated as an error. (The description for the options that will
cause an error if you use them in conjunction with -static are marked with the statement "when -dynamic
is used").
-read_only_relocs treatment
Specifies how relocation entries in read-only sections are to be treated when -dynamic is used. To get the best possible sharing,
the read-only sections should not have any relocation entries. If they do, the dynamic linker will write on the section. Having
relocation entries appear in read-only sections is normally avoided by compiling with the option -dynamic. But in such cases non-
converted assembly code or objects not compiled with -dynamic relocation entries will appear in read-only sections. The treatment
can be: error, warning, or suppress. Which cause the treatment of relocation entries in read-only sections as either, errors, warn-
ings, or suppressed messages. The default is to treat these as errors.
-sect_diff_relocs treatment
Specifies how section difference relocation enries are to be treated when -dynamic and -execute are used. To get the best possible
code generation the compiler should not generate code for executables (MH_EXECUTE format outputs) that have any section difference
relocation entries. The gcc(1) compiler has the -mdynamic-no-pic flag for generating code for executables. The default treatment
is suppress, where no message is printed. The other treatments are error or warning. This option can also be specified by setting
the environment variable LD_SECT_DIFF_RELOCS to the treatment values.
-weak_reference_mismatches treatment
Specifies how to treat mismatches of symbol references in the the object files being linked. Normally the all the undefined symbol
references of the object files being linked should be consistent for each undefined symbol. That is all undefined symbols should
either be weak or non-weak references. The default treatment is error, where the link fails with an error message. The other treat-
ments are weak or non-weak, which makes mismatched undefined symbol references either weak or non-weak respectfully in the output.
Care must be taken when using the treatment weak as the use of the non-weak symbol references in an object file may cause the pro-
gram to crash when the symbol is not present at execution time.
-prebind
Have the static linker, ld(1), prebind an executable's or dynamic shared library's undefined symbols to the addresses of the dynamic
libraries it is being linked with. This optimization can only be done if the libraries don't overlap and no symbols are overridden.
When the resulting program is run and the same libraries are used to run the program as when the program was linked, the dynamic
linker can use the prebound addresses. If not, the dynamic linker undoes the prebinding and binds normally. This option can also
be specified by setting the environment variable LD_PREBIND. If the environment variable LD_FORCE_NO_PREBIND is set both the option
-prebind LD_PREBIND environment variable are ignore and the output is not prebound. and the
-noprebind
Do not have the static linker, ld(1), prebind the output. If this is specified the environment variable LD_PREBIND is ignored.
-prebind_allow_overlap
Have the static linker, ld(1), prebind the output even if the addresses of the dynamic libraries it uses overlap. The resulting
output can then have redo_prebinding(1) run on it to fix up the prebinding after the overlapping dynamic libraries have been
rebuilt. This option can also be specified by setting the environment variable LD_PREBIND_ALLOW_OVERLAP.
-prebind_all_twolevel_modules
Have the static linker, ld(1), mark all modules from prebound two-level namespace dynamic libraries as used by the program even if
they are not statically referenced. This can provide improved launch time for programs like Objective-C programs that use symbols
indirectly through NIB files. This option can also be specified by setting the environment variable LD_PREBIND_ALL_TWOLEVEL_MODULES.
-noprebind_all_twolevel_modules
Don't have the static linker, ld(1), mark all modules from prebound two-level namespace dynamic libraries as used by the program.
This flag overrides the setting of the environment variable LD_PREBIND_ALL_TWOLEVEL_MODULES.
-nofixprebinding
Have the static linker, ld(1), mark the executable so that the dynamic linker will never notify the prebinding agent if this
launched and its prebinding is out of date. This is used when building the prebinding agent itself.
The following flags are related to libraries:
-lx This option is an abbreviation for the library name `libx.a', where x is a string. If -dynamic is specified the abbreviation for
the library name is first search as `libx.dylib' and then `libx.a' is searched for. ld searches for libraries first in any directo-
ries specified with -L options, then in the standard directories /lib, /usr/lib, and /usr/local/lib. A library is searched when its
name is encountered, so the placement of the -l flag is significant. If string x is of the form x.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
The is the same as the -lx but forces the library and all references to it to be marked as weak imports. Care must be taken when
using this as the use of the non-weak symbol references in an object file may cause the program to crash when the symbol or library
is not present at execution time.
-weak_library file_name_path_to_library
This is the same as listing a file name path to a library on the link line except that it but forces the library and all references
to it to be marked as weak imports. Care must be taken when using this as the use of the non-weak symbol references in an object
file may cause the program to crash when the symbol or library is not present at execution time.
-Ldir Add dir to the list of directories in which to search for libraries. Directories specified with -L are searched before the standard
directories.
-Z Do not search the standard directories when searching for libraries.
-search_paths_first
By default when the -dynamic flag is in effect, the -lx and -weak-lx options 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 changes it so that in each path `libx.dylib' is searched for then `libx.a' before the next path in the library search path is
searched.
-framework name[,suffix]
Specifies a framework to link against. Frameworks are dynamic shared libraries, but they are stored in different locations, and
therefore must be searched for differently. When this option is specified, ld searches for framework `name.framework/name' first in
any directories specified with the -F option, then in the standard framework directories /Library/Frameworks, /Net-
work/Library/Frameworks, and /System/Library/Frameworks. The placement of the -framework option is significant, as it determines
when and how the framework is searched. If the optional suffix is specified the framework is first searched for the name with the
suffix and then without.
-weak_framework name[,suffix]
The is the same as the -framework name[,suffix] but forces the framework and all references to it to be marked as weak imports.
Care must be taken when using this as the use of the non-weak symbol references in an object file may cause the program to crash
when the symbol or framework is not present at execution time.
-Fdir Add dir to the list of directories in which to search for frameworks. Directories specified with -F are searched before the stan-
dard framework directories.
-ObjC Loads all members of static archive libraries that define an Objective C class or a category. This option does not apply to dynamic
shared libraries.
-all_load
Loads all members of static archive libraries. This option does not apply to dynamic shared libraries.
-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 non-
default location, you would use this option: -dylib_file /lib/libsys_s.A.dylib:/me/lib/libsys_s.A.dylib.
-executable_path path_name
Specifies that path_name is is is used to replace @executable_path for dependent libraries.
The following options specify the output file format (the file type):
-execute
Produce a Mach-O demand-paged executable format file. The headers are placed in the first segment, and all segments are padded to
the segment alignment. This has a file type of MH_EXECUTE. This is the default. If no segment address is specified at address
zero, a segment with no protection (no read, write, or execute permission) is created at address zero. This segment, whose size is
that of the segment alignment, is named ``__PAGEZERO''. This option was previously named -Mach, which will continue to be recog-
nized.
-object
Produce a Mach-O file in the relocatable object file format that is intended for execution. This differs from using the -r option
in that it defines common symbols, does not allow undefined symbols and does not preserve relocation entries. This has a file type
of MH_OBJECT. In this format all sections are placed in one unnamed segment with all protections (read, write, execute) allowed on
that segment. This is intended for extremely small programs that would otherwise be large due to segment padding. In this format,
and all non-MH_EXECUTE formats, the link editor defined symbol ``__mh_execute_header'' is not defined since the headers are not part
of the segment. This format file can't be use with the dynamic linker.
-preload
Produce a Mach-O preloaded executable format file. The headers are not placed in any segment. All sections are placed in their
proper segments and they are padded to the segment alignment. This has a file type of MH_PRELOAD. This option was previously -p,
which will continue to be recognized.
-dylib Produce a Mach-O dynamically linked shared library format file. The headers are placed in the first segment. All sections are
placed in their proper segments and they are padded to the segment alignment. This has a file type of MH_DYLIB. This option is
used by libtool(1) when its -dynamic option is specified.
-bundle
Produce a Mach-O bundle format file. The headers are placed in the first segment. All sections are placed in their proper segments
and they are padded to the segment alignment. This has a file type of MH_BUNDLE.
-dylinker
Produces a Mach-O dynamic link editor format file. The headers are placed in the first segment. All sections are placed in their
proper segments, and they are padded to the segment alignment. This has a file type of MH_DYLINKER.
-fvmlib
Produce a Mach-O fixed VM shared library format file. The headers are placed in the first segment but the first section in that
segment will be placed on the next segment alignment boundary in that segment. All sections are placed in their proper segments and
they are padded to the segment alignment. This has a file type of MH_FVMLIB.
The following flags affect the contents of the output file:
-r Save the relocation information in the output file so that it can be the subject of another ld run. The resulting file type is a
Mach-O relocatable file (MH_OBJECT) if not otherwise specified. This flag also prevents final definitions from being given to com-
mon symbols, and suppresses the `undefined symbol' diagnostics.
-d Force definition of common storage even if the -r option is present. This option also forces link editor defined symbols to be
defined. This option is assumed when there is a dynamic link editor load command in the input and -r is not specified.
The following flags support segment specifications:
-segalign value
Specifies the segment alignment. value is a hexadecimal number that must be an integral power of 2. The default is the target
pagesize (currently 1000 hex for the PowerPC and i386).
-seg1addr addr
Specifies the starting address of the first segment in the output file. addr is a hexadecimal number and must be a multiple of the
segment alignment. This option can also be specified as -image_base.
-segaddr name addr
Specifies the starting address of the segment named name to be addr. The address must be a hexadecimal number that is a multiple of
the segment alignment.
-segs_read_only_addr addr
Specifies the starting address of the read-only segments in a dynamic shared library. When this option is used the dynamic shared
library is built such that the read-only and read-write segments are split into separate address ranges. By default the read-write
segments are 256meg (0x10000000) after the read-only segments. addr is a hexadecimal number and must be a multiple of the segment
alignment.
-segs_read_write_addr addr
Specifies the starting address of the read-write segments in a dynamic shared library. When this option is used the
-segs_read_only_addr must also be used (see above). addr is a hexadecimal number and must be a multiple of the segment alignment.
-seg_addr_table filename
For dynamic shared libraries the -seg1addr or the pair of -segs_read_only_addr and -segs_read_write_addr are specified by an entry
in the segment address table in filename that matches the install name of the library. The entries in the table are lines contain-
ing either a single hex address and an install name or two hex addresses and an install name. In the first form the single hex
address is used as the -seg1addr . In the second form the first address is used as the -segs_read_only_addr address and the second
address is used as the -segs_read_write_addr address. This option can also be specified by setting the environment variable
LD_SEG_ADDR_TABLE. If the environment variable is set then any -seg1addr, -segs_read_only_addr, -segs_read_write_addr and
-seg_addr_table options are ignored and a warning is printed.
-seg_addr_table_filename pathname
Use pathname instead of the install name of the library for matching an entry in the segment address table.
-segprot name max init
Specifies the maximum and initial virtual memory protection of the named segment, name, to be max and init respectfully. The values
for max and init are any combination of the characters `r' (for read), `w' (for write), `x' (for execute) and '-' (no access). The
default is `rwx' for the maximum protection for all segments. The default for the initial protection for all segments is `rw'
unless the segment contains a section which contains some machine instructions, in which case the default for the initial protection
is `rwx'. The default for the initial protection for the ``__TEXT'' segment is `rx' (not writable).
-seglinkedit
Create the link edit segment, named ``__LINKEDIT'' (this is the default). This segment contains all the link edit information
(relocation information, symbol table, string table, etc.) in the object file. If the segment protection for this segment is not
specified, the initial protection is not writable. This can only be specified when the output file type is not MH_OBJECT and
MH_PRELOAD output file types. To get at the contents of this section, the Mach header and load commands must be parsed from the
link editor defined symbols like `__mh_execute_header' (see Mach-O(5)).
-noseglinkedit
Do not create the link edit segment (see -seglinkedit above).
-pagezero_size value
Specifies the segment size of __PAGEZERO to be of size value, where value is a hexadecimal number rounded to the segment alignment.
The default is the target pagesize (currently, 1000 hexadecimal for the PowerPC and for i386).
-stack_addr value
Specifies the initial address of the stack pointer value, where value is a hexadecimal number rounded to the segment alignment. The
default segment alignment is the target pagesize (currently, 1000 hexadecimal for the PowerPC and for i386). If -stack_size is
specified and -stack_addr is not, a default stack address specific for the architecture being linked will be used and its value
printed as a warning message. This creates a segment named __UNIXSTACK. Note that the initial stack address will be either at the
high address of the segment or the low address of the segment depending on which direction the stack grows for the architecture
being linked.
-stack_size value
Specifies the size of the stack segment value, where value is a hexadecimal number rounded to the segment alignment. The default
segment alignment is the target pagesize (currently, 1000 hexadecimal for the PowerPC and for i386). If -stack_addr is specified
and -stack_size is not, a default stack size specific for the architecture being linked will be used and its value printed as a
warning message. This creates a segment named __UNIXSTACK .
The following flags support section specifications:
-sectcreate segname sectname file
The section sectname in the segment segname is created from the contents of file. The combination of segname and sectname must be
unique; there cannot already be a section (segname,sectname) in any input object file. This option was previously called -segcre-
ate, which will continue to be recognized.
-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. This can be used to set the alignment of a section created from a file, or to increase the align-
ment of a section from an object file, or to set the maximum alignment of the (__DATA,__common) section, where common symbols are
defined by the link editor. Setting the alignment of a literal section causes the individual literals to be aligned on that bound-
ary. If the section alignment is not specified by a section header in an object file or on the command line, it defaults to 10
(hex), indicating 16-byte alignment.
-sectorder segname sectname orderfile
The section sectname in the segment segname of the input files will be broken up into blocks associated with symbols in the section.
The output section will be created by ordering the blocks as specified by the lines in the orderfile. These blocks are aligned to
the output file's section alignment for this section. Any section can be ordered in the output file except symbol pointer and sym-
bol stub sections.
For non-literal sections, each line of the orderfile contains an object name and a symbol name, separated by a single colon (':').
If the object file is in an archive, the archive name, followed by a single colon, must precede the object file name. The object
file names and archive names should be exactly the names as seen by the link editor, but if not, the link editor attempts to match
up the names the best it can. For non-literal sections, the easiest way to generate an order file is with the ``-jonls +segname
sectname'' options to nm(1).
The format of the orderfile for literal sections is specific to each type of literal section. For C string literal sections, each
line of the order file contains one literal C string, which may include ANSI C escape sequences. For four-byte literal sections,
the order file format is one 32-bit hex number with a leading 0x per line, with the rest of the line treated as a comment. For
eight-byte literal sections, the order file has two 32-bit hex numbers per line; each number has a leading 0x, the two numbers are
separated by white space, and the rest of the line is treated as a comment. For literal pointer sections, the lines in the order
file represent pointers, one per line. A literal pointer is represented by the name of the segment that contains the literal being
pointed to, followed by the section name, followed by the literal. These three strings are separated by colons with no extra white
space. For all the literal sections, each line in the the order file is simply entered into the literal section and will appear in
the output file in the same order as in the order file. There is no check to see whether the literal is present in the loaded
objects. For literal sections, the easiest way to generate an order file is with the ``-X -v -s segname sectname'' options to
otool(1).
-sectorder_detail
When using the -sectorder option, any pairs of object file names and symbol names that are found in the loaded objects, but not
specified in the orderfile, are placed last in the output file's section. These pairs are ordered by object file (as the filenames
appear on the command line), with the different symbols from a given object file being ordered by increasing symbol address (that
is, the order in which the symbols occurred in the object file, not their order in the symbol table). By default, the link editor
displays a summary that simply shows the number of symbol names found in the loaded objects but not in the orderfile, as well as the
number of symbol names listed in the orderfile but not found in the loaded objects. (The summary is omitted if both values are
zero.) To instead produce a detailed list of these symbols, use the -sectorder_detail flag. If an object file-symbol name pair is
listed multiple times, a warning is generated, and the first occurrence is used.
-sectobjectsymbols segname sectname
This causes the link editor to generate local symbols in the section sectname in the segment segname. Each object file that has one
of these sections will have a local symbol created whose name is that of the object file, or of the member of the archive. The sym-
bol's value will be the first address where that object file's section was loaded. The symbol has the type N_SECT and its section
number is the the same as that of the section (segname,sectname) in the output file. This symbol will placed in the symbol table
just before all other local symbols for the object file. This feature is typically used where the section is (__TEXT,__text), in
order to help the debugger debug object files produced by old versions of the compiler or by non-Apple compilers.
The following flags are related to name spaces:
-twolevel_namespace
Specifies the output to be built as a two-level namespace image. This option can also be specified by setting the environment vari-
able LD_TWOLEVEL_NAMESPACE. This is the default.
-flat_namespace
Specifies the output to be built as a flat namespace image. This is not the default (but was the default in MacOS X 10.0).
-force_flat_namespace
Specifies the executable output to be built and executed treating all its dynamic libraries as flat namespace images. This marks
the executable so that the dynamic link editor know to treat all dynamic libraries as flat namespace images when the program is exe-
cuted.
-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. If the bundle being
created with -twolevel_namespace in effect then the searching of the executable for symbols is based on the placement of the -bun-
dle_loader flag relative to the dynamic libraries. If the the bundle being created with -flat_namespace then the searching of the
executable is done before all dynamic libraries.
-private_bundle
This allows symbols defined in the output to also be defined in executable in the -bundle_loader argument when -flat_namespace is in
effect. This implies that the bundle output file being created is going to be loaded by the executable with the NSLINKMOD-
ULE_OPTION_PRIVATE option to NSLinkModule(3).
-twolevel_namespace_hints
Specifies to create the output with the two-level namespace hints table to be used by the dynamic linker. This is the default
except when the -bundle flag is specified. If this is used when the -bundle flag is specified the bundle will fail to load on a
MacOS X 10.0 system with a malformed object error.
NSLINKMODULE_OPTION_PRIVATE option to NSLinkModule(3) and that the symbols in the executable are not to cause multiply defined sym-
bol errors.
-multiply_defined treatment
Specifies how multiply defined symbols in dynamic libraries when -twolevel_namespace is in effect are to be treated. treatment can
be: error, warning, or suppress. Which cause the treatment of multiply defined symbols in dynamic libraries as either, errors,
warnings, or suppresses the checking of multiply symbols from dynamic libraries when -twolevel_namespace is in effect. The default
is to treat multiply defined symbols in dynamic libraries as warnings when -twolevel_namespace is in effect.
-multiply_defined_unused treatment
Specifies how unused multiply defined symbols in dynamic libraries when -twolevel_namespace is in effect are to be treated. An
unused multiply defined symbol is one when there is a symbol defined in the output that is also defined in the dynamic libraries the
output is linked with but the symbol in the dynamic library is not used by any reference in the output. treatment can be: error,
warning, or suppress. The default for unused multiply defined symbols is to suppress these messages.
-nomultidefs
specifying this flag marks the umbrella being created such that the dynamic linker is guaranteed that no multiple definitions of
symbols in the umbrella's sub-images will ever exist. This allows the dynamic linker to always use the two-level namespace lookup
hints even if the timestamps of the sub-images do not match. This flag implies -multiply_defined error.
The following flags are related to symbols. These flags' arguments are external symbols whose names have `_' prepended to the C, FORTRAN,
or Pascal variable name.
-ysym Display each file in which sym appears, its type, and whether the file defines or references it. Any multiply defined symbols are
automatically traced. Like most of the other symbol-related flags, -y takes only one argument; the flag may be specified more than
once in the command line to trace more than one symbol.
-Y number
For the first number undefined symbols, displays each file in which the symbol appears, its type and whether the file defines or
references it (that is, the same style of output produced by the -y option). To keep the output manageable, this option displays at
most number references.
-keep_private_externs
Don't turn private external symbols into static symbols, but rather leave them as private external in the resulting output file.
-m Don't treat multiply defined symbols from the linked objects as a hard error; instead, simply print a warning. The first linked
object defining such a symbol is used for linking; its value is used for the symbol in the symbol table. The code and data for all
such symbols are copied into the output. The duplicate symbols other than the first symbol may still end up being used in the
resulting output file through local references. This can still produce a resulting output file that is in error. This flag's use
is strongly discouraged!
-whyload
Indicate why each member of a library is loaded. In other words, indicate which currently undefined symbol is being resolved, caus-
ing that member to be loaded. This in combination with the above -ysym flag can help determine exactly why a link edit is failing
due to multiply defined symbols.
-u sym Enter the argument sym into the symbol table as an undefined symbol. This is useful for loading wholly from a library, since ini-
tially the symbol table is empty and an unresolved reference is needed to force the loading of the first object file.
-e sym The argument sym is taken to be the symbol name of the entry point of the resulting file. By default, the entry point is the
address of the first section in the first segment.
-idefinition:indirect
Create an indirect symbol for the symbol name definition which is defined to be the same as the symbol name indirect (which is taken
to be undefined). When a definition of the symbol named indirect is linked, both symbols will take on the defined type and value.
This option overlaps with a compiler option. If you use the compiler driver cc(1) to invoke ld, invoke this option in this way:
-Wl,-idefinition:indirect.
-undefined treatment
Specifies how undefined symbols are to be treated. treatment can be: error, warning, or suppress. Which cause the treatment of
undefined symbols as either, errors, warnings, or suppresses the checking of undefined symbols. The default is to treat undefined
symbols as errors. When the environment variable MACOSX_DEPLOYMENT_TARGET is set to 10.3 then -undefined dynamic_lookup can also be
used to allow any undefined symbols to be looked up dynamically at runtime. Use of a binary built with this flag requires a system
with a dynamic linker from Mac OS X 10.3 or later. The flag -undefined define_a_way can also be used to cause the static linker to
create a private definition for all undefined symbols. This flag should only be used if it is known that the undefined symbols are
not referenced as any use of them may cause a crash.
-U sym Allow the symbol sym to be undefined, even if the -r flag is not given. Produce an executable file if the only undefined symbols
are those specified with -U.
This option overlaps with a compiler option. If you use the compiler driver cc(1) to invoke ld, invoke this option in this way:
-Wl,-U,sym.
-bind_at_load
Causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or
launched.
The following flags are related to stripping link edit information. This information can also be removed by strip(1), which uses the same
options. (The exception is the -s flag below, but this is the same as strip(1) with no arguments.) The following flags are listed in
decreasing level of stripping.
-s Completely strip the output; that is, remove the symbol table and relocation information.
-x Strips the non-global symbols; only saves external symbols.
This option overlaps with a compiler option. If you use the compiler driver cc(1) to invoke ld, invoke this option in this way:
-Wl,-x.
-S Strip debugging symbols; only save local and global symbols.
-X Strip local symbols whose names begin with `L'; save all other symbols. (The compiler and assembler currently strip these inter-
nally-generated labels by default, so they generally do not appear in object files seen by the link editor.)
-Si Strip duplicate debugging symbols from include files. This is the default.
-b Strip the base file's symbols from the output file. (The base file is given as the argument to the -A option.)
This option overlaps with a compiler option. If you use the compiler driver cc(1) to invoke ld, invoke this option in this way:
-Wl,-b.
-Sn Don't strip any symbols.
-exported_symbols_list filename
The specified filename contains lists 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__ 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.
-unexported_symbols_list filename
The specified filename contains lists 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__ 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.
The remaining options are infrequently used:
-w Suppresses all warning messages.
-no_arch_warnings
Suppresses warning messages about files that have the wrong architecture for the -arch flag.
-arch_errors_fatal
Cause the errors having to do with files that have the wrong architecture to be fatal and stop the link editor.
-M Produce a load map, listing all the segments and sections. The list includes the address where each input file's section appears in
the output file, as well as the section's size.
This option overlaps with a compiler option. If you use the compiler driver cc(1) to invoke ld, invoke this option in this way:
-Wl,-M.
-whatsloaded
Display a single line listing each object file that is loaded. Names of objects in archives have the form libfoo.a(bar.o).
-filelist listfile[,dirname]
Specifies that the linker should link the files listed in listfile . 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.
-headerpad value
Specifies the minimum amount of space ("padding") following the headers for the MH_EXECUTE format and all output file types with the
dynamic linker. value is a hexadecimal number. When a segment's size is rounded up to the segment alignment, there is extra space
left over, which is placed between the headers and the sections, rather than at the end of the segment. The headerpad option speci-
fies the minimum size of this padding, which can be useful if the headers will be altered later. The default value is the larger of
2 * sizeof(struct section) so the program /usr/bin/objcunique can always add two section headers, or if the output is an MH_EXECUTE
filetype and -prebind is specified 3 times the size of the LC_PREBOUND_DYLIB load commands. The actual amount of pad will be as
large as the amount of the first segment's round-off. (That is, take the total size of the first segments' headers and non-zerofill
sections, round this size up to the segment alignment, and use the difference between the rounded and unrounded sizes as the minimum
amount of padding.)
-headerpad_max_install_names
Add to the header padding enough space to allow changing all dynamic shared library paths recorded in the output file to be changed
to MAXPATHLEN in length.
-t Trace the progress of the link editor; display the name of each file that is loaded as it is processed in the first and second pass
of the link editor.
-A basefile
Incremental loading: linking is to be done in a manner that lets the resulting object be read into an already executing program, the
basefile. basefile is the name of a file whose symbol table will be taken as a basis on which to define additional symbols. Only
newly linked material will be entered into the a.out file, but the new symbol table will reflect every symbol defined in the base
file and the newly linked files. Option(s) to specify the addresses of the segments are typically needed, since the default
addresses tend to overlap with the basefile. The default format of the object file is MH_OBJECT. Note: It is strongly recommended
that this option NOT be used, because the dyld package described in dyld(3) is a much easier alternative.
-dylib_install_name name
For dynamic shared library files, specifies the name of the file the library will be installed in for programs that use it. If this
is not specified, the name specified in the -o name option will be used. This option is used as the libtool(1) -install_name name
option when its -dynamic option is specified.
-umbrella framework_name
Specifies this is a subframework where framework_name is the name of the umbrella framework this subframework is a part of. Where
framework_name is the same as the argument to the -framework framework_name option. This subframework can then only be linked into
the umbrella framework with the same framework_name or another subframework with the same umbrella framework name. Any other
attempt to statically link this subframework directly will result in an error stating to link with the umbrella framework instead.
When building the umbrella framework that uses this subframework no additional options are required. However the install name of
the umbrella framework, required to be specified with -dylib_install_name, must have the proper format for an install name of a
framework for the framework_name of the umbrella framework to be determined.
-allowable_client client_name
Specifies that for this subframework the client_name can link with this subframework without error even though it is not part of the
umbrella framework that this subframework is part of. The client_name can be another framework name or a name used by bundles (see
the -client_name client_name option below).
-client_name client_name
Specifies the client_name of a bundle for checking of allowable clients of subframeworks (see the -allowable_client client_name
option above).
-sub_umbrella framework_name
Specifies that the framework_name being linked by a dynamic library is to be treated as it one of the subframeworks with respect to
twolevel namespace.
-sub_library library_name
Specifies that the library_name being linked by a dynamic library is to be treated as it one of the sublibraries with respect to
twolevel namespace. For example the library_name for /usr/lib/libobjc_profile.A.dylib would be libobjc.
-init sym
The argument sym is taken to be the symbol name of the dynamic shared library initialization routine. If any module is used from
the dynamic library the library initialization routine is called before any symbol is used from the library including C++ static
initializers (and #pragma CALL_ON_LOAD routines).
-run_init_lazily
This option is obsolete.
-dylib_compatibility_version number
For dynamic shared library files, this specifies the compatibility version number of the library. When a library is used by a pro-
gram, 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 used as the libtool(1) -compatibility_version number
option when its -dynamic option is set.
-dylib_current_version number
For dynamic shared library files, 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 for-
mat 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 used as the libtool(1) -current_version number option when its -dynamic option is set.
-single_module
When building a dynamic library build the library so that it contains only one module.
-multi_module
When building a dynamic library build the library so that it contains one module for each object file linked in. This is the
default.
-dylinker_install_name name
For dynamic link editor files, specifies the name of the file the dynamic link editor will be installed in for programs that use it.
The following environment variable is used to control the use of incompatible features in the output with respect to Mac OS X releases.
MACOSX_DEPLOYMENT_TARGET
This is set to indicate the oldest Mac OS X version that that the output is to be used on. When this is set to a release that is
older than the current release features that are incompatible with that release will be disabled. If a feature is seen in the input
that can't be in the output due to this setting a warning is issued. The current allowable values for this are 10.1, 10.2 and 10.3
with the default being 10.1.
The following environment variables are used by Apple's Build and Integration team:
RC_TRACE_ARCHIVES
When this is set it causes a message of the form ``[Logging for Build & Integration] Used static archive: filename'' for each static
archive that has members linked into the output.
RC_TRACE_DYLIBS
When this is set it causes a message of the form ``[Logging for Build & Integration] Used dynamic library: filename'' for each
dynamic library linked into the output.
RC_TRACE_PREBINDING_DISABLED
When this is set it causes a message of the form ``[Logging for Build & Integration] prebinding disabled for filename because rea-
son''. Where filename is the value of the -final_output argument if specified or the value of the -o argument.
-final_output filename
The argument filename is used in the above message when RC_TRACE_PREBINDING_DISABLED is set.
Options available in early versions of the Mach-O link editor may no longer be supported.
FILES
/lib/lib*.{a,dylib} libraries
/usr/lib/lib*.{a,dylib}
/usr/local/lib/lib*.{a,dylib}
/Library/Frameworks/*.framework/* framework libraries
/Network/Library/Frameworks/*.framework/* framework libraries
/System/Library/Frameworks/*.framework/* framework libraries
a.out output file
SEE ALSO
as(1), ar(1), cc(1), libtool(1), ranlib(1), atom(1), nm(1), otool(1) lipo(1), arch(3), dyld(3), Mach-O(5), strip(1), redo_prebinding(1)
Apple Computer, Inc. September 10, 2003 LD(1)