Provided by: mold_1.4.2+dfsg-1_amd64 
NAME
mold — a modern linker
SYNOPSIS
mold [-options] objfile ...
DESCRIPTION
mold is a faster drop-in replacement for the default GNU ld(1).
How to use mold
See https://github.com/rui314/mold#how-to-use.
Compatibility
mold is designed to be a drop-in replacement for the GNU linkers for linking user-land
programs. If your user-land program cannot be built due to missing command-line options,
please file a bug at https://github.com/rui314/mold/issues.
mold supports a very limited set of linker script features, which is just sufficient to read
/usr/lib/x86_64-linux-gnu/libc.so on Linux systems (on Linux, that file is despite its name
not a shared library but an ASCII linker script that loads a real libc.so file.) Beyond
that, we have no plan to support any linker script features. The linker script is an ad-
hoc, over-designed, complex language which we believe needs to be disrupted by a simpler
mechanism. We have a plan to add a replacement for the linker script to mold instead.
Archive symbol resolution
Traditionally, Unix linkers are sensitive to the order in which input files appear on
command line. They process input files from the first (left-most) file to the last (right-
most) file one-by-one. While reading input files, they maintain sets of defined and
undefined symbols. When visiting an archive file (.a files), they pull out object files to
resolve as many undefined symbols as possible and go on to the next input file. Object
files that weren't pulled out will never have a chance for a second look.
Due to this semantics, you usually have to add archive files at the end of a command line,
so that when a linker reaches archive files, it knows what symbols are remain undefined. If
you put archive files at the beginning of a command line, a linker doesn't have any
undefined symbol, and thus no object files will be pulled out from archives.
You can change the processing order by --start-group and --end-group options, though they
make a linker slower.
mold as well as LLVM lld(1) linker take a different approach. They memorize what symbols
can be resolved from archive files instead of forgetting it after processing each archive.
Therefore, mold and lld(1) can "go back" in a command line to pull out object files from
archives, if they are needed to resolve remaining undefined symbols. They are not sensitive
to the input file order.
--start-group and --end-group are still accepted by mold and lld(1) for compatibility with
traditional linkers, but they are silently ignored.
Dynamic symbol resolution
Some Unix linker features are unable to be understood without understanding the semantics of
dynamic symbol resolution. Therefore, even though that's not specific to mold, we'll
explain it here.
We use "ELF module" or just "module" as a collective term to refer an executable or a shared
library file in the ELF format.
An ELF module may have lists of imported symbols and exported symbols, as well as a list of
shared library names from which imported symbols should be imported. The point is that
imported symbols are not bound to any specific shared library until runtime.
Here is how the Unix dynamic linker resolves dynamic symbols. Upon the start of an ELF
program, the dynamic linker construct a list of ELF modules which as a whole consist of a
complete program. The executable file is always at the beginning of the list followed by
its depending shared libraries. An imported symbol is searched from the beginning of the
list to the end. If two or more modules define the same symbol, the one that appears first
in the list takes precedence over the others.
This Unix semantics are contrary to systems such as Windows that have the two-level
namespace for dynamic symbols. On Windows, for example, dynamic symbols are represented as
a tuple of (symbol-name, shared-library-name), so that each dynamic symbol is guaranteed to
be resolved from some specific library.
Typically, an ELF module that exports a symbol also imports the same symbol. Such a symbol
is usually resolved to itself, but that's not the case if a module that appears before in
the symbol search list provides another definition of the same symbol.
Let me take malloc(3) as an example. Assume that you define your version of malloc(3) in
your main executable file. Then, all malloc calls from any module are resolved to your
function instead of that in libc, because the executable is always at the beginning of the
dynamic symbol search list. Note that even malloc(3) calls within libc are resolved to your
definition since libc exports and imports malloc. Therefore, by defining malloc yourself,
you can overwrite a library function, and the malloc(3) in libc becomes dead code.
These Unix semantics are tricky and sometimes considered harmful. For example, assume that
you accidentally define atoi(3) as a global function in your executable that behaves
completely differently from the one in the C standard. Then, all atoi function calls from
any modules (even function calls within libc) are redirected to your function instead of the
one in libc which obviously causes a problem. That is a somewhat surprising consequence for
an accidental name conflict. On the other hand, this semantic is sometimes considered
useful because it allows users to overwrite library functions without recompiling modules
containing them. Whether good or bad, you should keep this semantic in mind to understand
the Unix linkers behaviors.
Build reproducibility
mold's output is deterministic. That is, if you pass the same object files and the same
command-line options to the same version of mold, it is guaranteed to always produce the
same output. The linker's internal randomness, such as the timing of thread scheduling or
iteration orders of hash tables, doesn't affect the output.
mold does not have any host-specific default settings. This is contrary to the GNU linkers
to which some configurable values, such as system-dependent library search paths, are hard-
coded. mold depends only on its command-line arguments.
OPTIONS
--help Report usage information to stdout and exit.
-v, --version
Report version information to stdout.
-V Report version and target information to stdout.
-C dir, --directory dir
Change to dir before doing anything.
-E, --export-dynamic
--no-export-dynamic
When creating an executable, using the -E option causes all global symbols to be put
into the dynamic symbol table, so that the symbols are visible from other ELF
modules at runtime.
By default, or if --no-export-dynamic is given, only symbols that are referenced by
DSOs at link-time are exported from an executable.
-F libname, --filter=libname
Set the DT_FILTER dynamic section field to libname.
-Ifile, --dynamic-linker=file
--no-dynamic-linker
Set the dynamic linker path to file. If no -I option is given, or if
--no-dynamic-linker is given, no dynamic linker path is set to an output file. This
is contrary to the GNU linkers which sets a default dynamic linker path in that
case. However, this difference doesn't usually make any difference because the
compiler driver always passes -I to a linker.
-Ldir, --library-path=dir
Add dir to the list of library search paths from which mold searches libraries for
the -l option.
Unlike the GNU linkers, mold does not have the default search paths. This
difference doesn't usually make any difference because the compiler driver always
passes all necessary search paths to a linker.
-M, --print-map
Write a map file to stdout.
-N, --omagic
--no-omagic
Force mold to emit an output file with an old-fashioned memory layout. First, it
makes the first data segment to not be aligned to a page boundary. Second, text
segments are marked as writable if the option is given.
-S, --strip-debug
Omit .debug_* sections from the output file.
-T file, --script=file
Read linker script from file.
-X, --discard-locals
Discard temporary local symbols to reduce the sizes of the symbol table and the
string table. Temporary local symbols are local symbols starting with .L.
Compilers usually generate such symbols for unnamed program elements such as string
literals or floating-point literals.
-e symbol, --entry=symbol
Use symbol as the entry point symbol instead of the default entry point symbol
_start.
-f shlib, --auxiliary=shlib
Set the DT_AUXILIARY dynamic section field to shlib.
-h libname, --soname=libname
Set the DT_SONAME dynamic section field to libname. This option is used when
creating a shared object file. Typically, when you create SyXXX lib foo.so, you
want to pass --soname=foo to a linker.
-llibname
Search for Sylib libname.so or Sylib libname.a from library search paths.
-m [target]
Choose a target.
-o file, --output=file
Use file as the output file name instead of the default name a.out.
-r, --relocatable
Instead of generating an executable or a shared object file, combine input object
files to generate another object file that can be used as an input to a linker.
-s, --strip-all
Omit .symtab section from the output file.
-u symbol, --undefined=symbol
If symbol remains as an undefined symbol after reading all object files, and if
there is an static archive that contains an object file defining symbol, pull out
the object file and link it so that the output file contains a definition of symbol.
--Bdynamic
Link against shared libraries.
--Bstatic
Do not link against shared libraries.
--Bsymbolic
When creating a shared library, make global symbols export-only (i.e. do not import
the same symbol). As a result, references within a shared library is always
resolved locally, negating symbol override at runtime. See Dynamic symbol
resolution for more information about symbol imports and exports.
--Bsymbolic-functions
Have the same effect as --Bsymbolic but works only for function symbols. Data
symbols remains being both imported and exported.
--Bno-symbolic
Cancel --Bsymbolic and --Bsymbolic-functions.
--Map=file
Write map file to file.
--Tbss=address
Alias for --section-start=.bss=address.
--Tdata=address
Alias for --section-start=.data=address.
--Ttext=address
Alias for --section-start=.text=address.
--allow-multiple-definition
Normally, the linker reports an error if there are more than one definition of a
symbol. This option changes the default behavior so that it doesn't report an error
for duplicate definitions and instead use the first definition.
--as-needed
--no-as-needed
By default, shared libraries given to a linker are unconditionally added to the list
of required libraries in an output file. However, shared libraries after
--as-needed are added to the list only when at least one symbol is actually used by
an object file. In other words, shared libraries after --as-needed are not added to
the list of needed libraries if they are not needed by a program.
The --no-as-needed option restores the default behavior for subsequent files.
--build-id
--build-id=[none | md5 | sha1 | sha256 | uuid | 0xhexstring]
--no-build-id
Create a .note.gnu.build-id section containing a byte string to uniquely identify an
output file. --build-id and --build-id=sha256 compute a 256-bit cryptographic hash
of an output file and set it to build-id. md5 and sha1 compute the same hash but
truncate it to 128 and 160 bits, respectively, before setting it to build-id. uuid
sets a random 128-bit UUID. 0xhexstring sets hexstring.
--chroot=dir
Set dir to root directory.
--color-diagnostics=[auto | always | never]
--color-diagnostics
--no-color-diagnostics
Show diagnostics messages in color using ANSI escape sequences. auto means that
mold prints out messages in color only if the standard output is connected to a TTY.
Default is auto.
--defsym=symbol=value
--compress-debug-sections=[none | zlib | zlib-gabi | zlib-gnu]
Compress DWARF debug info (.debug_* sections) using the zlib compression algorithm.
--defsym=symbol=value
Define symbol as an alias for value.
value is either an integer (in decimal or hexadecimal with ‘0x’ prefix) or a symbol
name. If an integer is given as a value, symbol is defined as an absolute symbol
with the given value.
--default-symver
Use soname as a symbol version and append that version to all symbols.
--demangle
--no-demangle
Demangle C++ symbols in log messages.
--dependency-file=file
Write a dependency file to file. The contents of the written file is readable by
make, which defines only one rule with the linker's output file as a target and all
input fiels as its prerequisite. Users are expected to include the generated
dependency file into a Makefile to automate the dependency management. This option
is analogous to the compiler's -MM -MF options.
--dynamic-list=file
Read a list of dynamic symbols from file. Same as --export-dynamic-symbol-list,
except that it implies --Bsymbolic.
--eh-frame-hdr
--no-eh-frame-hdr
Create .eh_frame_hdr section.
--emit-relocs
A linker usually "consumes" relocation sections. That is, a linker applies
relocations to other sections, and relocation sections themselves are discarded.
The --emit-relocs instructs the linker to leave relocation sections in the output
file. Some post-link binary analysis or optimization tools such as LLVM Bolt need
them.
mold always creates RELA-type relocation sections even if the native ELF format is
REL-type so that it is easy to read addends.
--enable-new-dtags
--disable-new-dtags
By default, mold emits DT_RUNPATH for --rpath. If you pass --disable-new-dtags,
mold emits DT_RPATH for --rpath instead.
--exclude-libs=libraries...
Mark all symbols in the given libraries hidden.
--export-dynamic-symbol=sym
Put symbols matching sym in the dynamic symbol table. sym may be a glob, with the
same syntax as the globs used in --export-dynamic-symbol-list or --version-script.
--export-dynamic-symbol-list=file
Read a list of dynamic symbols from file.
--fatal-warnings
--no-fatal-warnings
Treat warnings as errors.
--fini=symbol
Call symbol at unload-time.
--fork
--no-fork
Spawn a child process and let it do the actual linking. When linking a large
program, the OS kernel can take a few hundred milliseconds to terminate a mold
process. --fork hides that latency.
--gc-sections
--no-gc-sections
Remove unreferenced sections.
--gdb-index
Create a .gdb_index section to speed up GNU debugger. To use this, you need to
compile source files with the --ggnu-pubnames compiler flag.
--hash-style=[sysv | gnu | both]
Set hash style.
--icf=[none | safe | all]
--no-icf
It is not uncommon for a program to contain many identical functions that differ
only in name. For example, a C++ template std::vector is very likely to be
instantiated to the identical code for std::vector<int> and std::vector<unsigned>
because the container cares only about the size of the parameter type. Identical
Code Folding (ICF) is a size optimization to identify and merge such identical
functions.
If --icf=all is given, mold tries to merge all identical functions. This reduces the
size of the output most, but it is not “safe” optimization. It is guaranteed in C
and C++ that two pointeres pointing two different functions will never be equal, but
--icf=all breaks that assumption as two functions have the same address after
merging. So a care must be taken when you use that flag that your program does not
depend on the function pointer uniqueness.
--icf=safe is a flag to merge functions only when it is safe to do so. That is, if a
program does not take an address of a function, it is safe to merge that function
with other function, as you cannot compare a function pointer with something else
without taking an address of a function. needs to be used with a compiler that
supports .llvm_addrsig section which contains the information as to what symbols are
address-taken. LLVM/Clang supports that section by default. Since GCC does not
support it yet, you cannot use --icf=safe with GCC (it doesn't do any harm but can't
optimize at all.)
--icf=none and --no-icf disables ICF.
--ignore-data-address-equality
Make ICF to merge not only functions but also data. This option should be used in
combination with --icf=all.
--image-base=addr
Set the base address to addr.
--init=symbol
Call symbol at load-time.
--no-undefined
Report undefined symbols (even with --shared).
--noinhibit-exec
Create an output file even if errors occur.
--pack-dyn-relocs=[none | relr]
If relr is specified, all R_*_RELATIVE relocations are put into .relr.dyn section
instead of .rel.dyn or .rela.dyn section. Since .relr.dyn section uses a space-
efficient encoding scheme, specifying this flag can reduce the size of the output.
This is typically most effective for position-independent executable.
Note that a runtime loader has to support .relr.dyn to run executables or shared
libraries linked with --pack-dyn-relocs=relr, and only ChromeOS, Android and Fuchsia
support it as of now in 2022.
--package-metadata=string
Embed string to a .note.package section. This option in intended to be used by a
package management command such as rpm to embed metadata regarding a package to each
executable file.
--perf Print performance statistics.
--pie, --pic-executable
--no-pie, --no-pic-executable
Create a position-independent executable.
--preload
Preload object files.
--print-gc-sections
--no-print-gc-sections
Print removed unreferenced sections.
--print-icf-sections
--no-print-icf-sections
Print folded identical sections.
--push-state
--pop-state
--push-state saves the current values of --as-needed, --whole-archive, --static, and
--start-lib. The saved values can be restored by --pop-state.
--push-state and --pop-state pairs can nest.
These options are useful when you want to construct linker command line options
programmatically. For example, if you want to link libfoo.so by as-needed basis but
don't want to change the global state of --as-needed, you can append "--push-state
--as-needed -lfoo --pop-state" to the linker command line options.
--quick-exit
--no-quick-exit
Use quick_exit to exit.
--relax
--no-relax
Rewrite machine instructions with more efficient ones for some relocations. The
feature is enabled by default.
--require-defined=symbol
Like --undefined, except the new symbol must be defined by the end of the link.
--repro
Embed input files into .repro section.
--retain-symbols-file=file
Keep only symbols listed in file.
file is a text file containing a symbol name on each line. mold discards all local
symbols as well as global sybmol that are not in file. Note that this option
removes symbols only from .symtab section and does not affect .dynsym section, which
is used for dynamic linking.
--reverse-sections
Reverses the order of input sections before assigning them the offsets in the output
file.
--rpath=dir
Add dir to runtime search path.
--run command arg file ...
Run command with mold as /usr/bin/ld.
--section-start=section=address
Set address to section. address is a hexadecimal number that may start with an
optional ‘0x’.
--shared, --Bshareable
Create a share library.
--shuffle-sections
--shuffle-sections=number
Randomizes the output by shuffleing the order of input sections before assigning
them the offsets in the output file. If number is given, it's used as a seed for the
random number generator, so that the linker produces the same output as for the same
seed. If no seed is given, it uses a random number as a seed.
--spare-dynamic-tags=number
Reserve given number of tags in .dynamic section.
--start-lib
--end-lib
Handle object files between --start-lib and --end-lib as if they were in an archive
file. That means object files between them are linked only when they are needed to
resolve undefined symbols. The options are useful if you want to link object files
only when they are needed but want to avoid the overhead of running ar(3).
--static
Do not link against shared libraries.
--stats
Print input statistics.
--sysroot=dir
Set target system root directory to dir.
--thread-count=count
Use count number of threads.
--threads
--no-threads
Use multiple threads. By default, mold uses as many threads as the number of cores
or 32, whichever is the smallest. The reason why it is capped to 32 is because mold
doesn't scale well beyond that point. To use only one thread, pass --no-threads or
--thread-count=1.
--trace
Print name of each input file.
--unique=pattern
Don't merge input sections that match pattern.
--unresolved-symbols=[report-all | ignore-all | ignore-in-object-files |
ignore-in-shared-libs]
How to handle undefined symbols.
--version-script=file
Read version script from file.
--warn-common
--no-warn-common
Warn about common symbols.
--warn-once
Only warn once for each undefined symbol instead of warn for each relocation
referring an undefined symbol.
--warn-unresolved-symbols
--error-unresolved-symbols
Normally, the linker reports an error for unresolved symbols.
--warn-unresolved-symbols option turns it into a warning.
--error-unresolved-symbols option restores the default behavior.
--whole-archive
--no-whole-archive
When archive files (.a files) are given to a linker, only object files that are
needed to resolve undefined symbols are extracted from them and linked to an output
file. --whole-archive changes that behavior for subsequent archives so that a
linker extracts all object files and link them to an output. For example, if you
are creating a shared object file and you want to include all archive members to the
output, you should pass --whole-archive. --no-whole-archive restores the default
behavior for subsequent archives.
--wrap=symbol
Make symbol to be resolved to __wrap_symbol. The original symbol can be resolved as
__real_symbol. This option is typically used for wrapping an existing function.
-z cet-report=[none | warning | error]
Intel Control-flow Enforcement Technology (CET) is a new x86 feature available since
Tiger Lake which is released in 2020. It defines new instructions to harden
security to protect programs from control hijacking attacks. You can tell compiler
to use the feature by specifying the -fcf-protection flag.
-z cet-report flag is used to make sure that all object files were compiled with a
correct -fcf-protection flag. If warning or error are given, mold prints out a
warning or an error message if an object file was not compiled with the compiler
flag.
mold looks for GNU_PROPERTY_X86_FEATURE_1_IBT bit and
GNU_PROPERTY_X86_FEATURE_1_SHSTK bit in .note.gnu.property section to determine
whether or not an object file was compiled with -fcf-protection.
-z now
-z lazy
By default, functions referring other ELF modules are resolved by the dynamic linker
when they are called for the first time. -z now marks an executable or a shared
library file so that all dynamic symbols are loaded when a file is loaded to memory.
-z lazy restores the default behavior.
-z origin
Mark object requiring immediate $ORIGIN processing at runtime.
-z ibt Turn on GNU_PROPERTY_X86_FEATURE_1_IBT bit in .note.gnu.property section to indicate
that the output uses IBT-enabled PLT. This option implies -z ibtplt.
-z ibtplt
Generate Intel Branch Tracking (IBT)-enabled PLT which is the default on x86-64.
-z execstack
-z noexecstack
By default, the pages for the stack area (i.e. the pages where local variables
reside) are not executable for security reasons. -z execstack makes it executable.
-z noexecstack restores the default behavior.
-z keep-text-section-prefix
-z nokeep-text-section-prefix
Keep .text.hot, .text.unknown, .text.unlikely, .text.startup and .text.exit as
separate sections in the final binary.
-z relro
-z norelro
Some sections such as .dynamic have to be writable only during an executable or a
shared library file is being loaded to memory. Once the dynamic linker finishes its
job, such sections won't be mutated by anyone. As a security mitigation, it is
preferred to make such segments read-only during program execution.
-z relro puts such sections into a special segment called relro. The dynamic linker
make a relro segment read-only after it finishes its job.
By default, mold generates a relro segment. -z norelro disables the feature.
-z separate-loadable-segments
-z separate-code
-z noseparate-code
If one memory page contains multiple segments, the page protection bits are set in
such a way that needed attributes (writable or executable) are satisifed for all
segments. This usually happens at a boundary of two segments with two different
attributes.
separate-loadable-segments adds paddings between segments with different attributes
so that they do not share the same page. This is the default.
separate-code adds paddings only between executable and non-executable segments.
noseparate-code does not add any paddings between segments.
-z defs
-z nodefs
Report undefined symbols (even with --shared).
-z shstk
Enforce shadow stack by turning GNU_PROPERTY_X86_FEATURE_1_SHSTK bit in
.note.gnu.property output section. Shadow stack is part of Intel Control-flow
Enforcement Technology (CET), which is available since Tiger Lake (2020).
-z text
-z notext, -z textoff
mold by default reports an error if dynamic relocations are created in read-only
sections. If -z notext or -z textoff are given, mold creates such dynamic
relocations without reporting an error. -z text restores the default behavior.
-z max-page-size
Some CPU ISAs support multiple different memory page sizes. This option specifies
the maximum page size that an output binary can run on. If you specify a large
value, the output can run on both large and small page systems, but it wastes a bit
of memory at page boundaries on systems with small pages.
The default value is 4 KiB for i386, x86-64 and RISC-V, and 64 KiB for ARM64.
-z nodefaultlib
Make the dynamic loader to ignore default search paths.
-z nodelete
Mark DSO non-deletable at runtime.
-z nodlopen
Mark DSO not available to dlopen(3).
-z nodump
Mark DSO not available to dldump(3).
-z nocopyreloc
Do not create copy relocations.
-z initfirst
Mark DSO to be initialized first at runtime.
-z interpose
Mark object to interpose all DSOs but executable.
SEE ALSO
gold(1), ld(1), elf(5) ld.so(8)
AUTHORS
Rui Ueyama <ruiu@cs.stanford.edu>
BUGS
Report bugs to https://github.com/rui314/mold/issues.