mold is a faster drop-in replacement for the default GNU ld(1).
How to use mold
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
--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.
--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.
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.
Set dir to root directory.
- --color-diagnostics=[auto | always | never]
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.
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. By default, it does fork.
Print performance statistics.
Archive input files as a tar file.
Reverses the order of input sections before assigning them the offsets in the output file.
This option is useful for finding a bug that depends on an initialization order of global objects. In C++, constructors of global objects in a single source file are guaranteed to be executed in the source order, but there's no such guarantee across compilation units. Usually, constructors are executed in the order given to the linker, but depending on it is a mistake.
By reversing the order of input sections using --reverse-sections, you can easily test that your program works in the reversed initialization order.
- --run command arg file ...
commandwith mold as
Randomizes the output by shuffling 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, a random number is used as a seed.
This option is useful for benchmarking. Modern CPUs are sensitive to program's memory layout. A seeming benign change in program layout (such as a small size increase of a function in the middle of a program) can affect program's performance. Therefore, even if you write new code and get a good benchmark result, it is hard to say whether or not the new code improves the programs performance. It is possible that the new memory layout happens to perform better.
By running a benchmark multiple time with shuffling memory layout using --shuffle-sections, you can isolate your program's real performance number from the randomness caused by memory layout changes.
Print input statistics.
Use count number of 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.
Use or do not use
Report usage information to stdout and exit.
- -v, --version
Report version information to stdout.
Report version and target information to stdout.
- -C dir, --directory dir
Change to dir before doing anything.
- -E, --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
DT_FILTERdynamic section field to libname.
- -Ifile, --dynamic-linker=file
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
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
.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
DT_AUXILIARYdynamic section field to shlib.
- -h libname, --soname=libname
DT_SONAMEdynamic 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.
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.
By default, mold doesn't merge input sections by name when merging input object files into a single output object file for -r. For example, .text.foo and .text.bar aren't merged for -r even though they are merged into .text according to the default section merging rules.
This option changes the behavior so that mold merges input sections by name by the default section merging rules.
- -s, --strip-all
.symtabsection 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.
Link against shared libraries.
Do not link against shared libraries.
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.
Have the same effect as --Bsymbolic but works only for function symbols. Data symbols remains being both imported and exported.
Cancel --Bsymbolic and --Bsymbolic-functions.
Write map file to file.
Alias for --section-start=.bss=address.
Alias for --section-start=.data=address.
Alias for --section-start=.text=address.
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.
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=[none | md5 | sha1 | sha256 | uuid | 0xhexstring]
.note.gnu.build-idsection 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.
- --compress-debug-sections=[none | zlib | zlib-gabi | zstd]
Compress DWARF debug info (.debug_* sections) using the zlib or zstd compression algorithm.
-zlib-gabiis an alias for
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.
Use soname as a symbol version and append that version to all symbols.
Demangle C++ symbols in log messages.
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 files 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
Read a list of dynamic symbols from file. Same as --export-dynamic-symbol-list, except that it implies --Bsymbolic.
If file does not exist in the current directory, it is searched from library search paths for the sake of compatibility with GNU ld.
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.
By default, mold emits DT_RUNPATH for --rpath. If you pass --disable-new-dtags, mold emits DT_RPATH for --rpath instead.
Traditionally, most processors require both executable and readable bits to 1 to make the page executable, which allows machine code to read itself as data at runtime. This is actually what an attacker often does after gaining a limited control of a process to find pieces of machine code they can use to gain the full control of the process.
As a mitigation, some recent processors allows "execute-only" pages. If a page is execute-only, you can call a function there as long as you know its address but can't read it as data.
This option marks text segments execute-only. This option currently works only on some ARM64 processors.
Mark all symbols in the given libraries hidden.
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.
Read a list of dynamic symbols from file.
Treat warnings as errors.
Call symbol at unload-time.
Remove unreferenced sections.
.gdb_indexsection to speed up GNU debugger. To use this, you need to compile source files with the
- --hash-style=[sysv | gnu | both]
Set hash style.
- --icf=[none | safe | all]
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 pointers 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.
Make ICF to merge not only functions but also data. This option should be used in combination with --icf=all.
Set the base address to addr.
Call symbol at load-time.
Report undefined symbols (even with --shared).
Create an output file even if errors occur.
- --pack-dyn-relocs=[none | relr]
If relr is specified, all
R_*_RELATIVErelocations are put into
.relr.dynsection instead of
.relr.dynsection 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.dynto run executables or shared libraries linked with --pack-dyn-relocs=relr, and only ChromeOS, Android and Fuchsia support it as of now in 2022.
Embed string to a .note.package section. This option in intended to be used by a package management command such as
rpmto embed metadata regarding a package to each executable file.
- --pie, --pic-executable
- --no-pie, --no-pic-executable
Create a position-independent executable.
Print removed unreferenced sections.
Print folded identical sections.
--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.
Rewrite machine instructions with more efficient ones for some relocations. The feature is enabled by default.
Like --undefined, except the new symbol must be defined by the end of the link.
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 symbol that are not in file. Note that this option removes symbols only from
.symtabsection and does not affect
.dynsymsection, which is used for dynamic linking.
Add dir to runtime search path.
Set address to section. address is a hexadecimal number that may start with an optional ‘0x’.
- --shared, --Bshareable
Create a share library.
Reserve given number of tags in
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).
Do not link against shared libraries.
Set target system root directory to dir.
Print name of each input file.
By default, mold warns on a symbol specified by a version script or by --export-dynamic-symbol if it is not defined. You can silence the warning by --undefined-version.
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.
Read version script from file. If file does not exist in the current directory, it is searched from library search paths for the sake of compatibility with GNU ld.
Warn about common symbols.
Only warn once for each undefined symbol instead of warn for each relocation referring an undefined symbol.
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.
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.
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
cet-reportflag is used to make sure that all object files were compiled with a correct
-fcf-protectionflag. 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
.note.gnu.propertysection to determine whether or not an object file was compiled with
- -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
nowmarks an executable or a shared library file so that all dynamic symbols are loaded when a file is loaded to memory. -z
lazyrestores the default behavior.
- -z origin
Mark object requiring immediate
$ORIGINprocessing at runtime.
- -z ibt
.note.gnu.propertysection to indicate that the output uses IBT-enabled PLT. This option implies -z
- -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
execstackmakes it executable. -z
noexecstackrestores the default behavior.
- -z keep-text-section-prefix
- -z nokeep-text-section-prefix
.text.exitas separate sections in the final binary.
- -z relro
- -z norelro
Some sections such as
.dynamichave 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.
relroputs 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
norelrodisables 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 satisfied for all segments. This usually happens at a boundary of two segments with two different attributes.
separate-loadable-segmentsadds paddings between segments with different attributes so that they do not share the same page. This is the default.
separate-codeadds paddings only between executable and non-executable segments.
noseparate-codedoes 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.propertyoutput 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
textoffare given, mold creates such dynamic relocations without reporting an error. -z
textrestores 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.
gold(1), ld(1), elf(5), ld.so(8)
Rui Ueyama <email@example.com>
Report bugs to https://github.com/rui314/mold/issues.
The man page ld.mold(1) is an alias of mold(1).