illumos: 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)