ld.so(8)
NAME
ld.so, ld-linux.so* - dynamic linker/loader
SYNOPSIS
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]]
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 format used long ago; ld-
linux.so* (/lib/ld-linux.so.1 for libc5, /lib/ld-linux.so.2 for glibc2)
handles ELF, which everybody has been using for years now. Otherwise,
both 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:
o Using the directories specified in the DT_RPATH dynamic section
attribute of the binary if present and DT_RUNPATH attribute does not
exist. Use of DT_RPATH is deprecated.
o Using the environment variable LD_LIBRARY_PATH (unless the
executable is being run in secure-execution mode; see below). in
which case it is ignored.
o Using the directories specified in the DT_RUNPATH dynamic section
attribute of the binary if present.
o 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
nodeflib 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.
o 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
nodeflib linker option, this step is skipped.
Rpath token expansion
ld.so understands certain strings in an rpath specification (DT_RPATH
or DT_RUNPATH); those strings are substituted 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)).
OPTIONS
--list List all dependencies and how they are resolved.
--verify
Verify that program is dynamically linked and this dynamic
linker can handle it.
--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).
--audit list
Use objects named in list as auditors.
ENVIRONMENT
Various environment variables influence the operation of the dynamic
linker.
Secure-execution mode
For security reasons, the effects of some environment variables are
voided or modified if the dynamic linker determines that the binary
should be run in secure-execution mode. This determination is made by
checking whether 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
permitted or effective capabilities.
* 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 (since glibc 2.2.3)
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. Similar to the PATH environment variable.
This variable is ignored in secure-execution mode.
Within the pathnames specified in LD_LIBRARY_PATH, the dynamic
linker expands the strings $ORIGIN, $LIB, and $PLATFORM (or the
versions using curly braces around the names) as described above
in Rpath token expansion. 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. The items of the list can be
separated by spaces or colons. This can be used to selectively
override functions in other shared objects. The objects are
searched for using the rules given under DESCRIPTION. In
secure-execution mode, preload pathnames containing slashes are
ignored, and only shared objects in the standard search
directories that have the set-user-ID mode bit enabled are
loaded.
Within the pathnames specified in LD_PRELOAD, the dynamic linker
understands the strings $ORIGIN, $LIB, and $PLATFORM (or the
versions using curly braces around the names) as described above
in Rpath token expansion.
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 colon-separated 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. 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.
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 symbol.
LD_DEBUG (since glibc 2.1)
Output verbose debugging information about the dynamic linker.
If set to all, print all debugging information, Setting this
variable to help does not run the specified program, and
displays a help message about which categories can be specified
in this environment variable. The categories are:
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.
The value in LD_DEBUG can specify multiple categories, separated
by colons, commas, or (if the value is quoted) spaces.
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)
File in which LD_DEBUG output should be written. The default is
standard error. LD_DEBUG_OUTPUT is ignored in secure-execution
mode.
LD_DYNAMIC_WEAK (since glibc 2.1.91)
If this environment variable is defined (with any value), allow
weak symbols to be overridden (reverting to old glibc behavior).
Since glibc 2.3.4, LD_DYNAMIC_WEAK is ignored in secure-
execution mode.
LD_HWCAP_MASK (since glibc 2.1)
Mask for hardware capabilities.
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 (glibc from 2.4 to 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".
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 2.3.5, LD_SHOW_AUXV is ignored in
secure-execution mode.
LD_TRACE_PRELINKING (since glibc 2.4)
If this environment variable is defined (with any value), 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 (since glibc 2.3.3)
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. 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 4GB
away from the branch. If this environment variable is set (to
any value), ld.so 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 2GB (not 4GB) 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 objects.
/etc/ld.so.preload
File containing a whitespace-separated list of ELF shared
objects to be loaded before the program.
lib*.so*
shared objects
NOTES
Hardware capabilities
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
SEE ALSO
ld(1), ldd(1), pldd(1), sprof(1), dlopen(3), getauxval(3),
capabilities(7), rtld-audit(7), ldconfig(8), sln(8)
COLOPHON
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