LD.SO(8) Linux Programmer'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
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 as 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 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:
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 exe-
cutable 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. 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
dependency tree.
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 node-
flib 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 archi-
tectures, 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
The dynamic linker understands certain token strings in an rpath speci-
fication (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 inter-
preted 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. (For details, see the discus-
sion of individual environment variables below.) 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 (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 load-
ing 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 fea-
ture 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 refer-
enced. 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 tokens $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 $ORI-
GIN 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 sepa-
rated 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. 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 Rpath token expansion. (See also the discussion of
quoting under the description of LD_LIBRARY_PATH.)
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.
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 Rpath token expansion. (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 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 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-execu-
tion 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.pro-
file".
Since glibc 2.2.5, LD_PROFILE is ignored 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. (This detail is relevant only
before glibc 2.2.5, since in later glibc versions, LD_PROFILE is
also ignored in secure-execution mode.)
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 (since glibc 2.4)
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 activ-
ity 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 executa-
bles and PIEs will honor the base addresses. If
LD_USE_LOAD_BIAS is defined with the value 0, neither executa-
bles 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 informa-
tion about the program if the LD_TRACE_LOADED_OBJECTS environ-
ment 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 exe-
cutable 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_PRE-
FER_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. 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
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 recog-
nized:
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
SEE ALSO
ld(1), ldd(1), pldd(1), sprof(1), dlopen(3), getauxval(3), elf(5),
capabilities(7), rtld-audit(7), ldconfig(8), sln(8)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2017-09-15 LD.SO(8)
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