ld.so(8) System Manager's Manual ld.so(8)
NAME
ld.so, ld-linux.so - dynamic linker/loader
SYNOPSIS
The dynamic linker can be run either indirectly by running some dynami-
cally linked program or shared object (in which case no command-line op-
tions to the dynamic linker can be passed and, in the ELF case, the dy-
namic linker which is stored in the .interp section of the program is
executed) or directly by running:
/lib/ld-linux.so.* [OPTIONS] [PROGRAM [ARGUMENTS]]
DESCRIPTION
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 in-
spects 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 inter-
preted 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 at-
tribute of the binary if present and DT_RUNPATH attribute does not
exist.
(2) Using the environment variable LD_LIBRARY_PATH, unless the exe-
cutable 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 dependen-
cies) 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 de-
pendency 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 node-
faultlib linker option, shared objects in the default paths are
skipped. Shared objects installed in hardware capability directo-
ries (see below) are preferred to other shared objects.
(5) In the default path /lib, and then /usr/lib. (On some 64-bit ar-
chitectures, 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_AU-
DIT,
• 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, --li-
brary-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 ob-
jects.
$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 vari-
ables.
OPTIONS
--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 architec-
tures, 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 ex-
plained 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.
ENVIRONMENT
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 af-
fect the operation of the dynamic linker itself, and are described be-
low. 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, LOC-
PATH, 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 ef-
fective group IDs differ. This typically occurs as a result of exe-
cuting 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 en-
coded 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 ex-
ceed that ABI version.
LD_ASSUME_KERNEL can be used to cause the dynamic linker to as-
sume that it is running on a system with a different kernel ABI
version. For example, the following command line causes the dy-
namic 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 im-
plementation 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 ex-
ecution 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 ex-
ecuted:
$ LD_LIBRARY_PATH='$ORIGIN/$LIB' prog
(Note the use of single quotes, which prevent expansion of $ORI-
GIN 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 selec-
tively 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. Ob-
jects 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 pre-
loaded, 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 de-
pendencies, 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 ob-
ject, 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 dy-
namic 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 sym-
bol 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 path-
name 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 defini-
tion it finds.
Old glibc versions (before glibc 2.2), provided a different be-
havior: if the linker found a symbol that was weak, it would re-
member that symbol and keep searching in the remaining shared li-
braries. 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 getaux-
val(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.
FILES
/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 ob-
jects. 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_PRE-
LOAD above. If both LD_PRELOAD and /etc/ld.so.preload are em-
ployed, 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 ex-
ecuted on the system. (This is usually undesirable, and is typi-
cally employed only as an emergency remedy, for example, as a
temporary workaround to a library misconfiguration issue.)
lib*.so*
shared objects
NOTES
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, efp-
double, 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:new-
cap/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 draw-
back of growing the dynamic linker search list uncontrollably.
For instance, on x86 64-bit, if the hardware supports x86_64-v3 (for in-
stance 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.
SEE ALSO
ld(1), ldd(1), pldd(1), sprof(1), dlopen(3), getauxval(3), elf(5), capa-
bilities(7), rtld-audit(7), ldconfig(8), sln(8)
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