Tribblix: manual page: ldd.1

LDD(1) User Commands LDD(1)

NAME

ldd - list dynamic dependencies of executable files or shared objects

SYNOPSIS

ldd [-d | -r] [-c] [-e envar] [-f] [-i] [-L] [-l] [-p] [-s]
[-U | -u] [-v] [-w] filename...

DESCRIPTION

The ldd utility lists the dynamic dependencies of executable files or
shared objects. ldd uses the runtime linker, ld.so.1, to generate the
diagnostics. The runtime linker takes the object being inspected and
prepares the object as would occur in a running process. By default,
ldd triggers the loading of any lazy dependencies.

ldd lists the path names of all shared objects that would be loaded
when filename is loaded. ldd expects the shared objects that are
being inspected to have execute permission. If a shared object does
not have execute permission, ldd issues a warning before attempting
to process the file.

ldd processes its input one file at a time. For each file, ldd
performs one of the following:

o Lists the object dependencies if the dependencies exist.

o Succeeds quietly if dependencies do not exist.

o Prints an error message if processing fails.

The dynamic objects that are inspected by ldd are not executed.
Therefore, ldd does not list any shared objects explicitly attached
using dlopen(3C). To display all the objects in use by a process, or
a core file, use pldd(1).

OPTIONS

ldd can also check the compatibility of filename with the shared
objects filename uses. With the following options, ldd prints
warnings for any unresolved symbol references that would occur when
filename is loaded.

-d
Check immediate references.

-r
Check both immediate references and lazy references.

Only one of the options -d or -r can be specified during any single
invocation of ldd.

immediate references are typically to data items used by the
executable or shared object code. immediate references are also
pointers to functions, and even calls to functions made from a
position dependent shared object. lazy references are typically calls
to global functions made from a position independent shared object,
or calls to external functions made from an executable. For more
information on these types of reference, see When Relocations Are
Performed in the Linker and Libraries Guide. Object loading can also
be affected by relocation processing. See Lazy Loading under USAGE
for more details.

Some unresolved symbol references are not reported by default. These
unresolved references can be reported with the following options.
These options are only useful when combined with either the -d or the
-r options.

-p
Expose any unresolved symbol errors to explicit parent and
external references.

-w
Expose any unresolved weak symbol references.

A shared object can make reference to symbols that should be supplied
by the caller of the shared object. These references can be
explicitly classified when the shared object is created, as being
available from a parent, or simply as being external. See the -M
mapfile option of ld(1), and the PARENT and EXTERN symbol definition
keywords. When examining a dynamic executable, a parent or external
reference that can not be resolved is flagged as an error. However by
default, when examining a shared object, a parent or external
reference that can not be resolved is not flagged as an error. The -p
option, when used with either the -d or -r options, causes any
unresolved parent or external reference to be flagged as a relocation
error.

Symbols that are used by relocations may be defined as weak
references. By default, if a weak symbol reference can not be
resolved, the relocation is ignored and a zero written to the
relocation offset. The -w option, when used with either the -d or the
-r options, causes any unresolved relocation against a weak symbol
reference to be flagged as a relocation error.

ldd can also check dependency use. With each of the following
options, ldd prints warnings for any unreferenced, or unused
dependencies that are loaded when filename is loaded. Only when a
symbol reference is bound to a dependency, is that dependency deemed
used. These options are therefore only useful when symbol references
are being checked. If the -r option is not in effect, the -d option
is enabled.

A dependency that is defined by an object but is not bound to from
that object is an unreferenced dependency. A dependency that is not
bound to by any other object when filename is loaded is an unused
object.

Dependencies can be located in default system locations, or in
locations that must be specified by search paths. Search paths may be
specified globally, such as the environment variable LD_LIBRARY_PATH.
Search paths can also be defined in dynamic objects as runpaths. See
the -R option to ld(1). Search paths that are not used to satisfy
any dependencies cause unnecessary file system processing.

-U
Displays any unreferenced, or unused dependencies. If an
unreferenced dependency is not bound to by other objects loaded
with filename, the dependency is also flagged as unused. Cyclic
dependencies that are not bound to from objects outside of the
cycle are also deemed unreferenced.

This option also displays any unused search paths.

-u
Displays any unused objects.

Only one of the options -U or -u can be specified during any single
invocation of ldd, although -U is a superset of -u. Objects that are
found to be unreferenced, or unused when using the -r option, should
be removed as dependencies. These objects provide no references, but
result in unnecessary overhead when filename is loaded. When using
the -d option, any objects that are found to be unreferenced, or
unused are not immediately required when filename is loaded. These
objects are candidates for lazy loading. See Lazy Loading under USAGE
for more details.

The removal of unused dependencies reduces runtime-linking overhead.
The removal of unreferenced dependencies reduces runtime-linking
overhead to a lesser degree. However, the removal of unreferenced
dependencies guards against a dependency being unused when combined
with different objects, or as the other object dependencies evolve.

The removal of unused search paths can reduce the work required to
locate dependencies. This can be significant when accessing files
from a file server over a network. Note, a search path can be encoded
within an object to satisfy the requirements of dlopen(3C). This
search path might not be required to obtain the dependencies of this
object, and hence will look unused to ldd.

The following additional options are supported:

-c
Disables any configuration file use. Configuration files
can be employed to alter default search paths, and
provide alternative object dependencies. See crle(1).

-e envar
Sets the environment variable envar.

This option is useful for experimenting with environment
variables that are recognized by the runtime linker that
can adversely affect ldd, for example, LD_PRELOAD.

This option is also useful for extracting additional
information solely from the object under inspection, for
example, LD_DEBUG. See ld.so.1(1) and lari(1).

-f
Forces ldd to check for an executable file that is not
secure. When ldd is invoked by a superuser, by default
ldd does not process any executable that is not secure.
An executable is not considered secure if the interpreter
that the executable specifies does not reside under /lib,
/usr/lib or /etc/lib. An executable is also not
considered secure if the interpreter cannot be
determined. See Security under USAGE.

-i
Displays the order of execution of initialization
sections. The order that is discovered can be affected by
use of the -d or -r options. See Initialization Order
under USAGE.

-L
Enables lazy loading. Lazy loading is the default mode of
operation when the object under inspection is loaded as
part of a process. In this case, any lazy dependencies,
or filters, are only loaded into the process when
reference is made to a symbol that is defined within the
lazy object. The -d or -r options, together with the -L
option, can be used to inspect the dependencies, and
their order of loading as would occur in a running
process.

-l
Forces the immediate processing of any filters so that
all filtees, and their dependencies, are listed. The
immediate processing of filters is now the default mode
of operation for ldd. However, under this default any
auxiliary filtees that cannot be found are silently
ignored. Under the -l option, missing auxiliary filtees
generate an error message.

-s
Displays the search path used to locate shared object
dependencies.

-v
Displays all dependency relationships incurred when
processing filename. This option also displays any
dependency version requirements. See pvs(1).

USAGE

Security

A superuser should use the -f option only if the executable to be
examined is known to be trustworthy. The use of -f on an
untrustworthy executable while superuser can compromise system
security. If an executables trustworthiness is unknown, a superuser
should temporarily become a regular user. Then invoke ldd as this
regular user.

Untrustworthy objects can be safely examined with dump(1) and with
mdb(1), as long as the :r subcommand is not used. In addition, a non-
superuser can use either the :r subcommand of mdb, or truss(1) to
examine an untrustworthy executable without too much risk of
compromise. To minimize risk when using ldd, adb :r, or truss on an
untrustworthy executable, use the UID "nobody".

Lazy Loading

Lazy loading can be applied directly by specified lazy dependencies.
See the -z lazyload option of ld(1). Lazy loading can also be applied
indirectly through filters. See the -f option and -F option of ld(1).
Objects that employ lazy loading techniques can experience variations
in ldd output due to the options used. If an object expresses all its
dependencies as lazy, the default operation of ldd lists all
dependencies in the order in which the dependencies are recorded in
that object:

example% ldd main
libelf.so.1 => /lib/libelf.so.1
libnsl.so.1 => /lib/libnsl.so.1
libc.so.1 => /lib/libc.so.1

The lazy loading behavior that occurs when this object is used at
runtime can be enabled using the -L option. In this mode, lazy
dependencies are loaded when reference is made to a symbol that is
defined within the lazy object. Therefore, combining the -L option
with use of the -d and -r options reveals the dependencies that are
needed to satisfy the immediate, and lazy references respectively:

example% ldd -L main
example% ldd -d main
libc.so.1 => /lib/libc.so.1
example% ldd -r main
libc.so.1 => /lib/libc.so.1
libelf.so.1 => /lib/libelf.so.1

Notice that in this example, the order of the dependencies that are
listed is not the same as displayed from ldd with no options. Even
with the -r option, the lazy reference to dependencies might not
occur in the same order as would occur in a running program.

Observing lazy loading can also reveal objects that are not required
to satisfy any references. These objects, in this example,
libnsl.so.1, are candidates for removal from the link-line used to
build the object being inspected.

Initialization Order

Objects that do not explicitly define their required dependencies
might observe variations in the initialization section order
displayed by ldd due to the options used. For example, a simple
application might reveal:

example% ldd -i main
libA.so.1 => ./libA.so.1
libc.so.1 => /lib/libc.so.1
libB.so.1 => ./libB.so.1

init object=./libB.so.1
init object=./libA.so.1
init object=/lib/libc.so.1

whereas, when relocations are applied, the initialization section
order is:

example% ldd -ir main
.........

init object=/lib/libc.so.1
init object=./libB.so.1
init object=./libA.so.1

In this case, libB.so.1 makes reference to a function in
/usr/lib/libc.so.1. However, libB.so.1 has no explicit dependency on
this library. Only after a relocation is discovered is a dependency
then established. This implicit dependency affects the initialization
section order.

Typically, the initialization section order established when an
application is executed, is equivalent to ldd with the -d option. The
optimum order can be obtained if all objects fully define their
dependencies. Use of the ld(1) options -zdefs and -zignore when
building dynamic objects is recommended.

Cyclic dependencies can result when one or more dynamic objects
reference each other. Cyclic dependencies should be avoided, as a
unique initialization sort order for these dependencies can not be
established.

Users that prefer a more static analysis of object files can inspect
dependencies using tools such as dump(1) and elfdump(1).

FILES

/usr/lib/lddstub
Fake 32-bit executable loaded to check the
dependencies of shared objects.

/usr/lib/64/lddstub
Fake 64-bit executable loaded to check the
dependencies of shared objects.

DIAGNOSTICS

ldd prints the record of shared object path names to stdout. The
optional list of symbol resolution problems is printed to stderr. If
filename is not an executable file or a shared object, or if filename
cannot be opened for reading, a non-zero exit status is returned.

NOTES

Use of the -d or -r option with shared objects can give misleading
results. ldd does a worst case analysis of the shared objects.
However, in practice, the symbols reported as unresolved might be
resolved by the executable file referencing the shared object. The
runtime linkers preloading mechanism can be employed to add
dependencies to the object being inspected. See LD_PRELOAD.

ldd uses the same algorithm as the runtime linker to locate shared
objects.

April 9, 2016 LDD(1)