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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ENVIRONMENT | FILES | NOTES | SEE ALSO | COLOPHON |
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ld.so(8) System Manager's Manual ld.so(8)
ld.so, ld-linux.so - dynamic linker/loader
The dynamic linker can be run either indirectly by running some
dynamically linked program or shared object (in which case no
command-line options to the dynamic linker can be passed and, in
the ELF case, the dynamic linker which is stored in the .interp
section of the program is executed) or directly by running:
/lib/ld-linux.so.* [OPTIONS] [PROGRAM [ARGUMENTS]]
The programs ld.so and ld-linux.so* find and load the shared
objects (shared libraries) needed by a program, prepare the
program to run, and then run it.
Linux binaries require dynamic linking (linking at run time)
unless the -static option was given to ld(1) during compilation.
The program ld.so handles a.out binaries, a binary format used
long ago. The program ld-linux.so* (/lib/ld-linux.so.1 for
libc5, /lib/ld-linux.so.2 for glibc2) handles binaries that are
in the more modern ELF format. Both programs have the same
behavior, and use the same support files and programs (ldd(1),
ldconfig(8), and /etc/ld.so.conf).
When resolving shared object dependencies, the dynamic linker
first inspects each dependency string to see if it contains a
slash (this can occur if a shared object pathname containing
slashes was specified at link time). If a slash is found, then
the dependency string is interpreted as a (relative or absolute)
pathname, and the shared object is loaded using that pathname.
If a shared object dependency does not contain a slash, then it
is searched for in the following order:
(1) Using the directories specified in the DT_RPATH dynamic
section attribute of the binary if present and DT_RUNPATH
attribute does not exist.
(2) Using the environment variable LD_LIBRARY_PATH, unless the
executable is being run in secure-execution mode (see
below), in which case this variable is ignored.
(3) Using the directories specified in the DT_RUNPATH dynamic
section attribute of the binary if present. Such
directories are searched only to find those objects required
by DT_NEEDED (direct dependencies) entries and do not apply
to those objects' children, which must themselves have their
own DT_RUNPATH entries. This is unlike DT_RPATH, which is
applied to searches for all children in the dependency tree.
(4) From the cache file /etc/ld.so.cache, which contains a
compiled list of candidate shared objects previously found
in the augmented library path. If, however, the binary was
linked with the -z nodefaultlib linker option, shared
objects in the default paths are skipped. Shared objects
installed in hardware capability directories (see below) are
preferred to other shared objects.
(5) In the default path /lib, and then /usr/lib. (On some
64-bit architectures, the default paths for 64-bit shared
objects are /lib64, and then /usr/lib64.) If the binary was
linked with the -z nodefaultlib linker option, this step is
skipped.
Dynamic string tokens
In several places, the dynamic linker expands dynamic string
tokens:
• In the environment variables LD_LIBRARY_PATH, LD_PRELOAD, and
LD_AUDIT,
• inside the values of the dynamic section tags DT_NEEDED,
DT_RPATH, DT_RUNPATH, DT_AUDIT, and DT_DEPAUDIT of ELF
binaries,
• in the arguments to the ld.so command line options --audit,
--library-path, and --preload (see below), and
• in the filename arguments to the dlopen(3) and dlmopen(3)
functions.
The substituted tokens are as follows:
$ORIGIN (or equivalently ${ORIGIN})
This expands to the directory containing the program or
shared object. Thus, an application located in
somedir/app could be compiled with
gcc -Wl,-rpath,'$ORIGIN/../lib'
so that it finds an associated shared object in
somedir/lib no matter where somedir is located in the
directory hierarchy. This facilitates the creation of
"turn-key" applications that do not need to be installed
into special directories, but can instead be unpacked into
any directory and still find their own shared objects.
$LIB (or equivalently ${LIB})
This expands to lib or lib64 depending on the architecture
(e.g., on x86-64, it expands to lib64 and on x86-32, it
expands to lib).
$PLATFORM (or equivalently ${PLATFORM})
This expands to a string corresponding to the processor
type of the host system (e.g., "x86_64"). On some
architectures, the Linux kernel doesn't provide a platform
string to the dynamic linker. The value of this string is
taken from the AT_PLATFORM value in the auxiliary vector
(see getauxval(3)).
Note that the dynamic string tokens have to be quoted properly
when set from a shell, to prevent their expansion as shell or
environment variables.
--argv0 string (since glibc 2.33)
Set argv[0] to the value string before running the
program.
--audit list
Use objects named in list as auditors. The objects in
list are delimited by colons.
--glibc-hwcaps-mask list
only search built-in subdirectories if in list.
--glibc-hwcaps-prepend list
Search glibc-hwcaps subdirectories in list.
--inhibit-cache
Do not use /etc/ld.so.cache.
--library-path path
Use path instead of LD_LIBRARY_PATH environment variable
setting (see below). The names ORIGIN, LIB, and PLATFORM
are interpreted as for the LD_LIBRARY_PATH environment
variable.
--inhibit-rpath list
Ignore RPATH and RUNPATH information in object names in
list. This option is ignored when running in secure-
execution mode (see below). The objects in list are
delimited by colons or spaces.
--list List all dependencies and how they are resolved.
--list-diagnostics (since glibc 2.33)
Print system diagnostic information in a machine-readable
format, such as some internal loader variables, the
auxiliary vector (see getauxval(3)), and the environment
variables. On some architectures, the command might print
additional information (like the cpu features used in GNU
indirect function selection on x86). --list-tunables
(since glibc 2.33) Print the names and values of all
tunables, along with the minimum and maximum allowed
values.
--preload list (since glibc 2.30)
Preload the objects specified in list. The objects in
list are delimited by colons or spaces. The objects are
preloaded as explained in the description of the
LD_PRELOAD environment variable below.
By contrast with LD_PRELOAD, the --preload option provides
a way to perform preloading for a single executable
without affecting preloading performed in any child
process that executes a new program.
--verify
Verify that program is dynamically linked and this dynamic
linker can handle it.
Various environment variables influence the operation of the
dynamic linker.
Secure-execution mode
For security reasons, if the dynamic linker determines that a
binary should be run in secure-execution mode, the effects of
some environment variables are voided or modified, and
furthermore those environment variables are stripped from the
environment, so that the program does not even see the
definitions. Some of these environment variables affect the
operation of the dynamic linker itself, and are described below.
Other environment variables treated in this way include:
GCONV_PATH, GETCONF_DIR, HOSTALIASES, LOCALDOMAIN, LD_AUDIT,
LD_DEBUG, LD_DEBUG_OUTPUT, LD_DYNAMIC_WEAK, LD_HWCAP_MASK,
LD_LIBRARY_PATH, LD_ORIGIN_PATH, LD_PRELOAD, LD_PROFILE,
LD_SHOW_AUXV, LOCALDOMAIN, LOCPATH, MALLOC_TRACE, NIS_PATH,
NLSPATH, RESOLV_HOST_CONF, RES_OPTIONS, TMPDIR, and TZDIR.
A binary is executed in secure-execution mode if the AT_SECURE
entry in the auxiliary vector (see getauxval(3)) has a nonzero
value. This entry may have a nonzero value for various reasons,
including:
• The process's real and effective user IDs differ, or the real
and effective group IDs differ. This typically occurs as a
result of executing a set-user-ID or set-group-ID program.
• A process with a non-root user ID executed a binary that
conferred capabilities to the process.
• A nonzero value may have been set by a Linux Security Module.
Environment variables
Among the more important environment variables are the following:
LD_ASSUME_KERNEL (from glibc 2.2.3 to glibc 2.36)
Each shared object can inform the dynamic linker of the
minimum kernel ABI version that it requires. (This
requirement is encoded in an ELF note section that is
viewable via readelf -n as a section labeled
NT_GNU_ABI_TAG.) At run time, the dynamic linker
determines the ABI version of the running kernel and will
reject loading shared objects that specify minimum ABI
versions that exceed that ABI version.
LD_ASSUME_KERNEL can be used to cause the dynamic linker
to assume that it is running on a system with a different
kernel ABI version. For example, the following command
line causes the dynamic linker to assume it is running on
Linux 2.2.5 when loading the shared objects required by
myprog:
$ LD_ASSUME_KERNEL=2.2.5 ./myprog
On systems that provide multiple versions of a shared
object (in different directories in the search path) that
have different minimum kernel ABI version requirements,
LD_ASSUME_KERNEL can be used to select the version of the
object that is used (dependent on the directory search
order).
Historically, the most common use of the LD_ASSUME_KERNEL
feature was to manually select the older LinuxThreads
POSIX threads implementation on systems that provided both
LinuxThreads and NPTL (which latter was typically the
default on such systems); see pthreads(7).
LD_BIND_NOW (since glibc 2.1.1)
If set to a nonempty string, causes the dynamic linker to
resolve all symbols at program startup instead of
deferring function call resolution to the point when they
are first referenced. This is useful when using a
debugger.
LD_LIBRARY_PATH
A list of directories in which to search for ELF libraries
at execution time. The items in the list are separated by
either colons or semicolons, and there is no support for
escaping either separator. A zero-length directory name
indicates the current working directory.
This variable is ignored in secure-execution mode.
Within the pathnames specified in LD_LIBRARY_PATH, the
dynamic linker expands the tokens $ORIGIN, $LIB, and
$PLATFORM (or the versions using curly braces around the
names) as described above in Dynamic string tokens. Thus,
for example, the following would cause a library to be
searched for in either the lib or lib64 subdirectory below
the directory containing the program to be executed:
$ LD_LIBRARY_PATH='$ORIGIN/$LIB' prog
(Note the use of single quotes, which prevent expansion of
$ORIGIN and $LIB as shell variables!)
LD_PRELOAD
A list of additional, user-specified, ELF shared objects
to be loaded before all others. This feature can be used
to selectively override functions in other shared objects.
The items of the list can be separated by spaces or
colons, and there is no support for escaping either
separator. The objects are searched for using the rules
given under DESCRIPTION. Objects are searched for and
added to the link map in the left-to-right order specified
in the list.
In secure-execution mode, preload pathnames containing
slashes are ignored. Furthermore, shared objects are
preloaded only from the standard search directories and
only if they have set-user-ID mode bit enabled (which is
not typical).
Within the names specified in the LD_PRELOAD list, the
dynamic linker understands the tokens $ORIGIN, $LIB, and
$PLATFORM (or the versions using curly braces around the
names) as described above in Dynamic string tokens. (See
also the discussion of quoting under the description of
LD_LIBRARY_PATH.)
There are various methods of specifying libraries to be
preloaded, and these are handled in the following order:
(1) The LD_PRELOAD environment variable.
(2) The --preload command-line option when invoking the
dynamic linker directly.
(3) The /etc/ld.so.preload file (described below).
LD_TRACE_LOADED_OBJECTS
If set (to any value), causes the program to list its
dynamic dependencies, as if run by ldd(1), instead of
running normally.
Then there are lots of more or less obscure variables, many
obsolete or only for internal use.
LD_AUDIT (since glibc 2.4)
A list of user-specified, ELF shared objects to be loaded
before all others in a separate linker namespace (i.e.,
one that does not intrude upon the normal symbol bindings
that would occur in the process) These objects can be used
to audit the operation of the dynamic linker. The items
in the list are colon-separated, and there is no support
for escaping the separator.
LD_AUDIT is ignored in secure-execution mode.
The dynamic linker will notify the audit shared objects at
so-called auditing checkpoints—for example, loading a new
shared object, resolving a symbol, or calling a symbol
from another shared object—by calling an appropriate
function within the audit shared object. For details, see
rtld-audit(7). The auditing interface is largely
compatible with that provided on Solaris, as described in
its Linker and Libraries Guide, in the chapter Runtime
Linker Auditing Interface.
Within the names specified in the LD_AUDIT list, the
dynamic linker understands the tokens $ORIGIN, $LIB, and
$PLATFORM (or the versions using curly braces around the
names) as described above in Dynamic string tokens. (See
also the discussion of quoting under the description of
LD_LIBRARY_PATH.)
Since glibc 2.13, in secure-execution mode, names in the
audit list that contain slashes are ignored, and only
shared objects in the standard search directories that
have the set-user-ID mode bit enabled are loaded.
LD_BIND_NOT (since glibc 2.1.95)
If this environment variable is set to a nonempty string,
do not update the GOT (global offset table) and PLT
(procedure linkage table) after resolving a function
symbol. By combining the use of this variable with
LD_DEBUG (with the categories bindings and symbols), one
can observe all run-time function bindings.
LD_DEBUG (since glibc 2.1)
Output verbose debugging information about operation of
the dynamic linker. The content of this variable is one
of more of the following categories, separated by colons,
commas, or (if the value is quoted) spaces:
help Specifying help in the value of this variable does
not run the specified program, and displays a help
message about which categories can be specified in
this environment variable.
all Print all debugging information (except statistics
and unused; see below).
bindings
Display information about which definition each
symbol is bound to.
files Display progress for input file.
libs Display library search paths.
reloc Display relocation processing.
scopes Display scope information.
statistics
Display relocation statistics.
symbols
Display search paths for each symbol look-up.
unused Determine unused DSOs.
versions
Display version dependencies.
Since glibc 2.3.4, LD_DEBUG is ignored in secure-execution
mode, unless the file /etc/suid-debug exists (the content
of the file is irrelevant).
LD_DEBUG_OUTPUT (since glibc 2.1)
By default, LD_DEBUG output is written to standard error.
If LD_DEBUG_OUTPUT is defined, then output is written to
the pathname specified by its value, with the suffix "."
(dot) followed by the process ID appended to the pathname.
LD_DEBUG_OUTPUT is ignored in secure-execution mode.
LD_DYNAMIC_WEAK (since glibc 2.1.91)
By default, when searching shared libraries to resolve a
symbol reference, the dynamic linker will resolve to the
first definition it finds.
Old glibc versions (before glibc 2.2), provided a
different behavior: if the linker found a symbol that was
weak, it would remember that symbol and keep searching in
the remaining shared libraries. If it subsequently found
a strong definition of the same symbol, then it would
instead use that definition. (If no further symbol was
found, then the dynamic linker would use the weak symbol
that it initially found.)
The old glibc behavior was nonstandard. (Standard
practice is that the distinction between weak and strong
symbols should have effect only at static link time.) In
glibc 2.2, the dynamic linker was modified to provide the
current behavior (which was the behavior that was provided
by most other implementations at that time).
Defining the LD_DYNAMIC_WEAK environment variable (with
any value) provides the old (nonstandard) glibc behavior,
whereby a weak symbol in one shared library may be
overridden by a strong symbol subsequently discovered in
another shared library. (Note that even when this
variable is set, a strong symbol in a shared library will
not override a weak definition of the same symbol in the
main program.)
Since glibc 2.3.4, LD_DYNAMIC_WEAK is ignored in secure-
execution mode.
LD_HWCAP_MASK (from glibc 2.1 to glibc 2.38)
Mask for hardware capabilities. Since glibc 2.26, the
option might be ignored if glibc does not support
tunables.
LD_ORIGIN_PATH (since glibc 2.1)
Path where the binary is found.
Since glibc 2.4, LD_ORIGIN_PATH is ignored in secure-
execution mode.
LD_POINTER_GUARD (from glibc 2.4 to glibc 2.22)
Set to 0 to disable pointer guarding. Any other value
enables pointer guarding, which is also the default.
Pointer guarding is a security mechanism whereby some
pointers to code stored in writable program memory (return
addresses saved by setjmp(3) or function pointers used by
various glibc internals) are mangled semi-randomly to make
it more difficult for an attacker to hijack the pointers
for use in the event of a buffer overrun or stack-smashing
attack. Since glibc 2.23, LD_POINTER_GUARD can no longer
be used to disable pointer guarding, which is now always
enabled.
LD_PROFILE (since glibc 2.1)
The name of a (single) shared object to be profiled,
specified either as a pathname or a soname. Profiling
output is appended to the file whose name is:
$LD_PROFILE_OUTPUT/$LD_PROFILE.profile.
Since glibc 2.2.5, LD_PROFILE uses a different default
path in secure-execution mode.
LD_PROFILE_OUTPUT (since glibc 2.1)
Directory where LD_PROFILE output should be written. If
this variable is not defined, or is defined as an empty
string, then the default is /var/tmp.
LD_PROFILE_OUTPUT is ignored in secure-execution mode;
instead /var/profile is always used.
LD_SHOW_AUXV (since glibc 2.1)
If this environment variable is defined (with any value),
show the auxiliary array passed up from the kernel (see
also getauxval(3)).
Since glibc 2.3.4, LD_SHOW_AUXV is ignored in secure-
execution mode.
LD_TRACE_PRELINKING (from glibc 2.4 to glibc 2.35)
If this environment variable is defined, trace prelinking
of the object whose name is assigned to this environment
variable. (Use ldd(1) to get a list of the objects that
might be traced.) If the object name is not recognized,
then all prelinking activity is traced.
LD_USE_LOAD_BIAS (from glibc 2.3.3 to glibc 2.35)
By default (i.e., if this variable is not defined),
executables and prelinked shared objects will honor base
addresses of their dependent shared objects and
(nonprelinked) position-independent executables (PIEs) and
other shared objects will not honor them. If
LD_USE_LOAD_BIAS is defined with the value 1, both
executables and PIEs will honor the base addresses. If
LD_USE_LOAD_BIAS is defined with the value 0, neither
executables nor PIEs will honor the base addresses.
Since glibc 2.3.3, this variable is ignored in secure-
execution mode.
LD_VERBOSE (since glibc 2.1)
If set to a nonempty string, output symbol versioning
information about the program if the
LD_TRACE_LOADED_OBJECTS environment variable has been set.
LD_WARN (since glibc 2.1.3)
If set to a nonempty string, warn about unresolved
symbols.
LD_PREFER_MAP_32BIT_EXEC (x86-64 only; since glibc 2.23)
According to the Intel Silvermont software optimization
guide, for 64-bit applications, branch prediction
performance can be negatively impacted when the target of
a branch is more than 4 GB away from the branch. If this
environment variable is set (to any value), the dynamic
linker will first try to map executable pages using the
mmap(2) MAP_32BIT flag, and fall back to mapping without
that flag if that attempt fails. NB: MAP_32BIT will map
to the low 2 GB (not 4 GB) of the address space.
Because MAP_32BIT reduces the address range available for
address space layout randomization (ASLR),
LD_PREFER_MAP_32BIT_EXEC is always disabled in secure-
execution mode.
/lib/ld.so
a.out dynamic linker/loader
/lib/ld-linux.so.{1,2}
ELF dynamic linker/loader
/etc/ld.so.cache
File containing a compiled list of directories in which to
search for shared objects and an ordered list of candidate
shared objects. See ldconfig(8).
/etc/ld.so.preload
File containing a whitespace-separated list of ELF shared
objects to be loaded before the program. See the
discussion of LD_PRELOAD above. If both LD_PRELOAD and
/etc/ld.so.preload are employed, the libraries specified
by LD_PRELOAD are preloaded first. /etc/ld.so.preload has
a system-wide effect, causing the specified libraries to
be preloaded for all programs that are executed on the
system. (This is usually undesirable, and is typically
employed only as an emergency remedy, for example, as a
temporary workaround to a library misconfiguration issue.)
lib*.so*
shared objects
Legacy Hardware capabilities (from glibc 2.5 to glibc 2.37)
Some shared objects are compiled using hardware-specific
instructions which do not exist on every CPU. Such objects
should be installed in directories whose names define the
required hardware capabilities, such as /usr/lib/sse2/. The
dynamic linker checks these directories against the hardware of
the machine and selects the most suitable version of a given
shared object. Hardware capability directories can be cascaded
to combine CPU features. The list of supported hardware
capability names depends on the CPU. The following names are
currently recognized:
Alpha ev4, ev5, ev56, ev6, ev67
MIPS loongson2e, loongson2f, octeon, octeon2
PowerPC
4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp,
efpdouble, efpsingle, fpu, ic_snoop, mmu, notb, pa6t,
power4, power5, power5+, power6x, ppc32, ppc601, ppc64,
smt, spe, ucache, vsx
SPARC flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
s390 dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp,
msa, stfle, z900, z990, z9-109, z10, zarch
x86 (32-bit only)
acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486,
i586, i686, mca, mmx, mtrr, pat, pbe, pge, pn, pse36, sep,
ss, sse, sse2, tm
The legacy hardware capabilities support has the drawback that
each new feature added grows the search path exponentially,
because it has to be added to every combination of the other
existing features.
For instance, on x86 32-bit, if the hardware supports i686 and
sse2, the resulting search path will be i686/sse2:i686:sse2:.. A
new capability newcap will set the search path to
newcap/i686/sse2:newcap/i686:newcap/sse2:newcap:i686/sse2:i686:sse2:.
glibc Hardware capabilities (from glibc 2.33)
glibc 2.33 added a new hardware capability scheme,
where under each CPU architecture, certain levels can be
defined, grouping support for certain features or special
instructions. Each architecture level has a fixed set of
paths that it adds to the dynamic linker search list,
depending on the hardware of the machine. Since each new
architecture level is not combined with previously
existing ones, the new scheme does not have the drawback
of growing the dynamic linker search list uncontrollably.
For instance, on x86 64-bit, if the hardware supports x86_64-v3
(for instance Intel Haswell or AMD Excavator), the resulting
search path will be glibc-hwcaps/x86-64-v3:glibc-
hwcaps/x86-64-v2:. The following paths are currently supported,
in priority order.
PowerPC (64-bit little-endian only)
power10, power9
s390 (64-bit only)
z16, z15, z14, z13
x86 (64-bit only)
x86-64-v4, x86-64-v3, x86-64-v2
glibc 2.37 removed support for the legacy hardware capabilities.
ld(1), ldd(1), pldd(1), sprof(1), dlopen(3), getauxval(3),
elf(5), capabilities(7), rtld-audit(7), ldconfig(8), sln(8)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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Linux man-pages 6.9.1 2024-05-08 ld.so(8)
Pages that refer to this page: ldd(1), memusage(1), pldd(1), sprof(1), execve(2), PR_SET_FP_MODE(2const), PR_SET_MM(2const), uselib(2), dladdr(3), dlinfo(3), dl_iterate_phdr(3), dlopen(3), dlsym(3), getauxval(3), lttng-ust(3), lttng-ust-cyg-profile(3), elf(5), capabilities(7), environ(7), rtld-audit(7), execstack(8), ldconfig(8), prelink(8), sln(8)
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