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 exe-
cuted. 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.
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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 refer-
ences 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 exter-
nal 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 reloca-
tion error.
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ldd can also check dependency use. With each of the follow-
ing 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 depen-
dency 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 LDLIBRARYPATH. 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 unneces-
sary 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
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removal of unreferenced dependencies guards against a depen-
dency 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. Configura-
tion 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, LDPRELOAD.
This option is also useful for extracting addi-
tional information solely from the object under
inspection, for example, LDEBUG. 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 initializa-
tion sections. The order that is discovered can
be affected by use of the -d or -r options. See
Initialization Order under USAGE.
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-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 depen-
dencies, 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. How-
ever, under this default any auxiliary filtees
that cannot be found are silently ignored. Under
the -l option, missing auxiliary filtees gen-
erate 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 trustworthyness 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 subcom-
mand of mdb, or truss(1) to examine an untrustworthy execut-
able without too much risk of compromise. To minimize risk
when using ldd, adb :r, or truss on an untrustworthy execut-
able, use the UID "nobody".
Lazy Loading
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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 depen-
dencies might observe variations in the initialization sec-
tion order displayed by ldd due to the options used. For
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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 recom-
mended.
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 elf-
dump(1).
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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.
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Availability SUNWtoo
SEE ALSO
crle(1), dump(1), elfdump(1), lari(1), ld(1), ld.so.1(1),
mdb(1), pldd(1), pvs(1), truss(1), dlopen(3C), attributes(5)
Linker and Libraries Guide
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 preload-
ing mechanism can be employed to add dependencies to the
object being inspected. See LDPRELOAD.
ldd uses the same algorithm as the runtime linker to locate
shared objects.
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