lld/docs/ld.lld.1

.\" This file is distributed under the University of Illinois Open Source
.\" License. See LICENSE.TXT for details.
.\"
.\" This man page documents only lld's ELF linking support, obtained originally
.\" from FreeBSD.
.Dd September 26, 2018
.Dt LD.LLD 1
.Os
.Sh NAME
.Nm ld.lld
.Nd ELF linker from the LLVM project
.Sh SYNOPSIS
.Nm ld.lld
.Op Ar options
.Ar objfile ...

.Sh DESCRIPTION
A linker takes one or more object, archive, and library files, and combines
them into an output file (an executable, a shared library, or another object
file).
It relocates code and data from the input files and resolves symbol
references between them.
.Pp
.Nm
is a drop-in replacement for the GNU BFD and gold linkers.
It accepts most of the same command line arguments and linker scripts
as GNU linkers.
.Pp
.Nm
currently supports i386, x86-64, ARM, AArch64, PowerPC32, PowerPC64,
MIPS32, MIPS64, RISC-V, AMDGPU, Hexagon and SPARC V9 targets.
.Nm
acts as a Microsoft link.exe-compatible linker if invoked as
.Nm lld-link
and as macOS's ld if invoked as
.Nm ld.ld64.
All these targets are always supported however
.Nm
was built, so you can always use
.Nm
as a native linker as well as a cross linker.

.Sh OPTIONS
Many options have both a single-letter and long form.
When using the long form options other than those beginning with the
letter
.Cm o
may be specified using either one or two dashes preceding the option name.
Long options beginning with
.Cm o
require two dashes to avoid confusion with the
.Fl o Ar path
option.
.Pp
.Bl -tag -width indent
.It Fl -allow-multiple-definition
Do not error if a symbol is defined multiple times.
The first definition will be used.
.It Fl -allow-shlib-undefined
Allow unresolved references in shared libraries.
This option is enabled by default when linking a shared library.
.It Fl -apply-dynamic-relocs
Apply link-time values for dynamic relocations.
.It Fl -as-needed
Only set
.Dv DT_NEEDED
for shared libraries if used.
.It Fl -auxiliary Ns = Ns Ar value
Set the
.Dv DT_AUXILIARY
field to the specified name.
.It Fl -Bdynamic , Fl -dy
Link against shared libraries.
.It Fl -Bstatic , Fl -static , Fl -dn
Do not link against shared libraries.
.It Fl -Bsymbolic
Bind defined symbols locally.
.It Fl -Bsymbolic-functions
Bind defined function symbols locally.
.It Fl -build-id Ns = Ns Ar value
Generate a build ID note.
.Ar value
may be one of
.Cm fast ,
.Cm md5 ,
.Cm sha1 ,
.Cm tree ,
.Cm uuid ,
.Cm 0x Ns Ar hex-string ,
and
.Cm none .
.Cm tree
is an alias for
.Cm sha1 .
Build-IDs of type
.Cm fast ,
.Cm md5 ,
.Cm sha1 ,
and
.Cm tree
are calculated from the object contents.
.Cm fast
is not intended to be cryptographically secure.
.It Fl -build-id
Synonym for
.Fl -build-id Ns = Ns Cm fast .
.It Fl -color-diagnostics Ns = Ns Ar value
Use colors in diagnostics.
.Ar value
may be one of
.Cm always ,
.Cm auto ,
and
.Cm never .
.Cm auto
enables color if and only if output is to a terminal.
.It Fl -color-diagnostics
Alias for
.Fl -color-diagnostics Ns = Ns Cm auto .
.It Fl -compress-debug-sections Ns = Ns Ar value
Compress DWARF debug sections.
.Ar value
may be
.Cm none
or
.Cm zlib .
.It Fl -cref
Output cross reference table.
.It Fl -define-common , Fl d
Assign space to common symbols.
.It Fl -defsym Ns = Ns Ar symbol Ns = Ns Ar expression
Define a symbol alias.
.Ar expression
may be another symbol or a linker script expression.
For example,
.Ql --defsym=foo=bar
or
.Ql --defsym=foo=bar+0x100 .
.It Fl -demangle
Demangle symbol names.
.It Fl -disable-new-dtags
Disable new dynamic tags.
.It Fl -discard-all , Fl x
Delete all local symbols.
.It Fl -discard-locals , Fl X
Delete temporary local symbols.
.It Fl -discard-none
Keep all symbols in the symbol table.
.It Fl -dynamic-linker Ns = Ns Ar value
Specify the dynamic linker to be used for a dynamically linked executable.
This is recorded in an ELF segment of type
.Dv PT_INTERP .
.It Fl -dynamic-list Ns = Ns Ar file
Read a list of dynamic symbols from
.Ar file .
.It Fl -eh-frame-hdr
Request creation of
.Li .eh_frame_hdr
section and
.Dv PT_GNU_EH_FRAME
segment header.
.It Fl -emit-relocs , Fl q
Generate relocations in the output.
.It Fl -enable-new-dtags
Enable new dynamic tags.
.It Fl -end-lib
End a grouping of objects that should be treated as if they were together
in an archive.
.It Fl -entry Ns = Ns Ar entry
Name of entry point symbol.
.It Fl -error-limit Ns = Ns Ar value
Maximum number of errors to emit before stopping.
A value of zero indicates that there is no limit.
.It Fl -error-unresolved-symbols
Report unresolved symbols as errors.
.It Fl -execute-only
Mark executable sections unreadable. This option is currently only
supported on AArch64.
.It Fl -exclude-libs Ns = Ns Ar value
Exclude static libraries from automatic export.
.It Fl -export-dynamic , Fl E
Put symbols in the dynamic symbol table.
.It Fl -export-dynamic-symbol Ns = Ns Ar symbol
Include
.Ar symbol
in the dynamic symbol table.
.It Fl -fatal-warnings
Treat warnings as errors.
.It Fl -filter Ns = Ns Ar value , Fl F Ar value
Set the
.Dv DT_FILTER
field to the specified value.
.It Fl -fini Ns = Ns Ar symbol
Specify a finalizer function.
.It Fl -format Ns = Ns Ar input-format , Fl b Ar input-format
Specify the format of the inputs following this option.
.Ar input-format
may be one of
.Cm binary ,
.Cm elf ,
and
.Cm default .
.Cm default
is a synonym for
.Cm elf .
.It Fl -gc-sections
Enable garbage collection of unused sections.
.It Fl -gdb-index
Generate
.Li .gdb_index
section.
.It Fl -hash-style Ns = Ns Ar value
Specify hash style.
.Ar value
may be
.Cm sysv ,
.Cm gnu ,
or
.Cm both .
.Cm both
is the default.
.It Fl -help
Print a help message.
.It Fl -icf Ns = Ns Cm all
Enable identical code folding.
.It Fl -icf Ns = Ns Cm safe
Enable safe identical code folding.
.It Fl -icf Ns = Ns Cm none
Disable identical code folding.
.It Fl -image-base Ns = Ns Ar value
Set the base address to
.Ar value .
.It Fl -init Ns = Ns Ar symbol
Specify an initializer function.
.It Fl -keep-unique Ns = Ns Ar symbol
Do not fold
.Ar symbol
during ICF.
.It Fl l Ar libName, Fl -library Ns = Ns Ar libName
Root name of library to use.
.It Fl L Ar dir , Fl -library-path Ns = Ns Ar dir
Add a directory to the library search path.
.It Fl -lto-aa-pipeline Ns = Ns Ar value
AA pipeline to run during LTO.
Used in conjunction with
.Fl -lto-newpm-passes .
.It Fl -lto-newpm-passes Ns = Ns Ar value
Passes to run during LTO.
.It Fl -lto-O Ns Ar opt-level
Optimization level for LTO.
.It Fl -lto-partitions Ns = Ns Ar value
Number of LTO codegen partitions.
.It Fl m Ar value
Set target emulation.
.It Fl -Map Ns = Ns Ar file , Fl M Ar file
Print a link map to
.Ar file .
.It Fl -no-allow-shlib-undefined
Do not allow unresolved references in shared libraries.
This option is enabled by default when linking an executable.
.It Fl -no-as-needed
Always set
.Dv DT_NEEDED
for shared libraries.
.It Fl -no-color-diagnostics
Do not use colors in diagnostics.
.It Fl -no-define-common
Do not assign space to common symbols.
.It Fl -no-demangle
Do not demangle symbol names.
.It Fl -no-dynamic-linker
Inhibit output of an
.Li .interp
section.
.It Fl -no-gc-sections
Disable garbage collection of unused sections.
.It Fl -no-gnu-unique
Disable STB_GNU_UNIQUE symbol binding.
.It Fl -no-rosegment
Do not put read-only non-executable sections in their own segment.
.It Fl -no-threads
Do not run the linker multi-threaded.
.It Fl -no-undefined-version
Report version scripts that refer undefined symbols.
.It Fl -no-undefined
Report unresolved symbols even if the linker is creating a shared library.
.It Fl -no-whole-archive
Restores the default behavior of loading archive members.
.It Fl -no-pie
Do not create a position independent executable.
.It Fl -noinhibit-exec
Retain the executable output file whenever it is still usable.
.It Fl -nostdlib
Only search directories specified on the command line.
.It Fl o Ar path
Write the output executable, library, or object to
.Ar path .
If not specified,
.Dv a.out
is used as a default.
.It Fl O Ns Ar value
Optimize output file size.
.Ar value
may be:
.Pp
.Bl -tag -width 2n -compact
.It Cm 0
Disable string merging.
.It Cm 1
Enable string merging.
.It Cm 2
Enable string tail merging.
.El
.Pp
.Fl O Ns Cm 1
is the default.
.It Fl -oformat Ns = Ns Ar format
Specify the format for the output object file.
The only supported
.Ar format
is
.Cm binary ,
which produces output with no ELF header.
.It Fl -omagic , Fl N
Set the text and data sections to be readable and writable.
.It Fl -opt-remarks-filename Ar file
Write optimization remarks in YAML format to
.Ar file .
.It Fl -opt-remarks-with-hotness
Include hotness information in the optimization remarks file.
.It Fl -pic-veneer
Always generate position independent thunks.
.It Fl -pie
Create a position independent executable.
.It Fl -print-gc-sections
List removed unused sections.
.It Fl -print-icf-sections
List identical folded sections.
.It Fl -print-map
Print a link map to the standard output.
.It Fl -push-state
Save the current state of
.Fl -as-needed ,
.Fl -static ,
and
.Fl -whole-archive.
.It Fl -pop-state
Undo the effect of
.Fl -push-state.
.It Fl -relocatable , Fl r
Create relocatable object file.
.It Fl -reproduce Ns = Ns Ar value
Dump linker invocation and input files for debugging.
.It Fl -retain-symbols-file Ns = Ns Ar file
Retain only the symbols listed in the file.
.It Fl -rpath Ns = Ns Ar value , Fl R Ar value
Add a
.Dv DT_RUNPATH
to the output.
.It Fl -rsp-quoting Ns = Ns Ar value
Quoting style for response files.
The supported values are
.Cm windows
and
.Cm posix .
.It Fl -script Ns = Ns Ar file , Fl T Ar file
Read linker script from
.Ar file .
.It Fl -section-start Ns = Ns Ar section Ns = Ns Ar address
Set address of section.
.It Fl -shared , Fl -Bsharable
Build a shared object.
.It Fl -soname Ns = Ns Ar value , Fl h Ar value
Set
.Dv DT_SONAME
to
.Ar value .
.It Fl -sort-section Ns = Ns Ar value
Specifies sections sorting rule when linkerscript is used.
.It Fl -start-lib
Start a grouping of objects that should be treated as if they were together
in an archive.
.It Fl -strip-all , Fl s
Strip all symbols.
.It Fl -strip-debug , Fl S
Strip debugging information.
.It Fl -symbol-ordering-file Ns = Ns Ar file
Lay out sections in the order specified by
.Ar file .
.It Fl -sysroot Ns = Ns Ar value
Set the system root.
.It Fl -target1-abs
Interpret
.Dv R_ARM_TARGET1
as
.Dv R_ARM_ABS32 .
.It Fl -target1-rel
Interpret
.Dv R_ARM_TARGET1
as
.Dv R_ARM_REL32 .
.It Fl -target2 Ns = Ns Ar type
Interpret
.Dv R_ARM_TARGET2
as
.Ar type ,
where
.Ar type
is one of
.Cm rel ,
.Cm abs ,
or
.Cm got-rel .
.It Fl -Tbss Ns = Ns Ar value
Same as
.Fl -section-start
with
.Li .bss
as the sectionname.
.It Fl -Tdata Ns = Ns Ar value
Same as
.Fl -section-start
with
.Li .data
as the sectionname.
.It Fl -Ttext Ns = Ns Ar value
Same as
.Fl -section-start
with
.Li .text
as the sectionname.
.It Fl -thinlto-cache-dir Ns = Ns Ar value
Path to ThinLTO cached object file directory.
.It Fl -thinlto-cache-policy Ns = Ns Ar value
Pruning policy for the ThinLTO cache.
.It Fl -thinlto-jobs Ns = Ns Ar value
Number of ThinLTO jobs.
.It Fl -threads
Run the linker multi-threaded.
This option is enabled by default.
.It Fl -trace
Print the names of the input files.
.It Fl -trace-symbol Ns = Ns Ar symbol , Fl y Ar symbol
Trace references to
.Ar symbol .
.It Fl -undefined Ns = Ns Ar symbol , Fl u Ar symbol
Force
.Ar symbol
to be an undefined symbol during linking.
.It Fl -unresolved-symbols Ns = Ns Ar value
Determine how to handle unresolved symbols.
.It Fl v
Display the version number and proceed with linking if object files are
specified.
.It Fl V , Fl -version
Display the version number and exit.
.It Fl -verbose
Verbose mode.
.It Fl -version-script Ns = Ns Ar file
Read version script from
.Ar file .
.It Fl -warn-backrefs
Warn about reverse or cyclic dependencies to or between static archives.
This can be used to ensure linker invocation remains compatible with
traditional Unix-like linkers.
.It Fl -warn-common
Warn about duplicate common symbols.
.It Fl -warn-ifunc-textrel
Warn about using ifunc symbols in conjunction with text relocations.
Older versions of glibc library (2.28 and earlier) has a bug that causes
the segment that includes ifunc symbols to be marked as not executable when
they are relocated. As a result, although the program compiles and links
successfully, it gives segmentation fault when the instruction pointer reaches
an ifunc symbol. Use -warn-ifunc-textrel to let lld give a warning, if the
code may include ifunc symbols, may do text relocations and be linked with
an older glibc version. Otherwise, there is no need to use it, as the default
value does not give a warning. This flag has been introduced in late 2018,
has no counter part in ld and gold linkers, and may be removed in the future.
.It Fl -warn-unresolved-symbols
Report unresolved symbols as warnings.
.It Fl -whole-archive
Force load of all members in a static library.
.It Fl -wrap Ns = Ns Ar symbol
Use wrapper functions for symbol.
.It Fl z Ar option
Linker option extensions.
.Bl -tag -width indent
.It Cm execstack
Make the main stack executable.
Stack permissions are recorded in the
.Dv PT_GNU_STACK
segment.
.It Cm global
Sets the
.Dv DF_1_GLOBAL flag in the
.Dv DYNAMIC
section.
Different loaders can decide how to handle this flag on their own.
.It Cm ifunc-noplt
Do not emit PLT entries for GNU ifuncs.
Instead, preserve relocations for ifunc call sites so that they may
be applied by a run-time loader.
Note that this feature requires special loader support and will
generally result in application crashes when used outside of freestanding
environments.
.It Cm initfirst
Sets the
.Dv DF_1_INITFIRST
flag to indicate the module should be initialized first.
.It Cm interpose
Set the
.Dv DF_1_INTERPOSE
flag to indicate to the runtime linker that the object is an interposer.
During symbol resolution interposers are searched after the application
but before other dependencies.
.It Cm muldefs
Do not error if a symbol is defined multiple times.
The first definition will be used.
This is a synonym for
.Fl -allow-multiple-definition.
.It Cm nocombreloc
Disable combining and sorting multiple relocation sections.
.It Cm nocopyreloc
Disable the creation of copy relocations.
.It Cm nodefaultlib
Set the
.Dv DF_1_NODEFLIB
flag to indicate that default library search paths should be ignored.
.It Cm nodelete
Set the
.Dv DF_1_NODELETE
flag to indicate that the object cannot be unloaded from a process.
.It Cm nodlopen
Set the
.Dv DF_1_NOOPEN
flag to indicate that the object may not be opened by
.Xr dlopen 3 .
.It Cm norelro
Do not indicate that portions of the object shold be mapped read-only
after initial relocation processing.
The object will omit the
.Dv PT_GNU_RELRO
segment.
.It Cm notext
Allow relocations against read-only segments.
Sets the
.Dv DT_TEXTREL flag in the
.Dv DYNAMIC
section.
.It Cm now
Set the
.Dv DF_BIND_NOW
flag to indicate that the run-time loader should perform all relocation
processing as part of object initialization.
By default relocations may be performed on demand.
.It Cm origin
Set the
.Dv DF_ORIGIN
flag to indicate that the object requires
$ORIGIN
processing.
.It Cm retpolineplt
Emit retpoline format PLT entries as a mitigation for CVE-2017-5715.
.It Cm rodynamic
Make the
.Li .dynamic
section read-only.
The
.Dv DT_DEBUG
tag will not be emitted.
.It Cm stack-size Ns = Ns Ar size
Set the main thread's stack size to
.Ar size .
The stack size is recorded as the size of the
.Ar size .
.Dv PT_GNU_STACK
program segment.
.It Cm text
Do not allow relocations against read-only segments.
This is the default.
.It Cm wxneeded
Create a
.Dv PT_OPENBSD_WXNEEDED
segment.
.El
.El
.Sh IMPLEMENTATION NOTES
.Nm Ap s
handing of archive files (those with a
.Pa .a
file extension) is different from traditional linkers used on Unix-like
systems.
.Pp
Traditional linkers maintain a set of undefined symbols during linking.
The linker processes each file in the order in which it appears on the
command line, until the set of undefined symbols becomes empty.
An object file is linked into the output object when it is encountered,
with its undefined symbols added to the set.
Upon encountering an archive file a traditional linker searches the objects
contained therein, and processes those that satisfy symbols in the unresolved
set.
.Pp
Handling mutually dependent archives may be awkward when using a traditional
linker.
Archive files may have to be specified multiple times, or the special command
line options
.Fl -start-group
and
.Fl -end-group
may be used to have the linker loop over the files in the group until no new
symbols are added to the set.
.Pp
.Nm
records all symbols found in objects and archives as it iterates over
command line arguments.
When
.Nm
encounters an undefined symbol that can be resolved by an object file
contained in a previously processed archive file, it immediately extracts
and links it into the output object.
.Pp
With certain archive inputs
.Nm
may produce different results compared to traditional linkers.
In practice, large bodies of third party software have been linked with
.Nm
without material issues.
.Pp
The
.Fl -warn-backrefs
option may be used to identify a linker invocation that may be incompatible
with traditional Unix-like linker behavior.