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RTLD(1)			  BSD General Commands Manual		       RTLD(1)

NAME
     ld-elf.so.1, ld.so, rtld — run-time link-editor

DESCRIPTION
     The ld-elf.so.1 utility is a self-contained shared object providing run-
     time support for loading and link-editing shared objects into a process'
     address space.  It is also commonly known as the dynamic linker.  It uses
     the data structures contained within dynamically linked programs to
     determine which shared libraries are needed and loads them using the
     mmap(2) system call.

     After all shared libraries have been successfully loaded, ld-elf.so.1
     proceeds to resolve external references from both the main program and
     all objects loaded.  A mechanism is provided for initialization routines
     to be called on a per-object basis, giving a shared object an opportunity
     to perform any extra set-up before execution of the program proper
     begins.  This is useful for C++ libraries that contain static construc‐
     tors.

     When resolving dependencies for the loaded objects, ld-elf.so.1 may be
     allowed to translate dynamic token strings in rpath and soname by setting
     -z origin option of the static linker ld(1).  The following strings are
     recognized now:

     $ORIGIN	Translated to the full path of the loaded object.

     $OSNAME	Translated to the name of the operating system implementation.

     $OSREL	Translated to the release level of the operating system.

     $PLATFORM	Translated to the machine hardware platform.

     The ld-elf.so.1 utility itself is loaded by the kernel together with any
     dynamically-linked program that is to be executed.	 The kernel transfers
     control to the dynamic linker.  After the dynamic linker has finished
     loading, relocating, and initializing the program and its required shared
     objects, it transfers control to the entry point of the program.

     To locate the required shared objects in the file system, ld-elf.so.1 may
     use a “hints” file prepared by the ldconfig(8) utility.

     The ld-elf.so.1 utility recognizes a number of environment variables that
     can be used to modify its behaviour.  On 64-bit architectures, the linker
     for 32-bit objects recognizes all the environment variables listed below,
     but is being prefixed with LD_32_, for example:
     LD_32_TRACE_LOADED_OBJECTS.

     LD_DUMP_REL_POST	If set, ld-elf.so.1 will print a table containing all
			relocations after symbol binding and relocation.

     LD_DUMP_REL_PRE	If set, ld-elf.so.1 will print a table containing all
			relocations before symbol binding and relocation.

     LD_LIBMAP		A library replacement list in the same format as
			libmap.conf(5).	 For convenience, the characters ‘=’
			and ‘,’ can be used instead of a space and a newline.
			This variable is parsed after libmap.conf(5), and will
			override its entries.  This variable is unset for set-
			user-ID and set-group-ID programs.

     LD_LIBMAP_DISABLE	If set, disables the use of libmap.conf(5) and
			LD_LIBMAP.  This variable is unset for set-user-ID and
			set-group-ID programs.

     LD_ELF_HINTS_PATH	This variable will override the default location of
			“hints” file.  This variable is unset for set-user-ID
			and set-group-ID programs.

     LD_LIBRARY_PATH	A colon separated list of directories, overriding the
			default search path for shared libraries.  This vari‐
			able is unset for set-user-ID and set-group-ID pro‐
			grams.

     LD_PRELOAD		A list of shared libraries, separated by colons and/or
			white space, to be linked in before any other shared
			libraries.  If the directory is not specified then the
			directories specified by LD_LIBRARY_PATH will be
			searched first followed by the set of built-in stan‐
			dard directories.  This variable is unset for set-
			user-ID and set-group-ID programs.

     LD_BIND_NOW	When set to a nonempty string, causes ld-elf.so.1 to
			relocate all external function calls before starting
			execution of the program.  Normally, function calls
			are bound lazily, at the first call of each function.
			LD_BIND_NOW increases the start-up time of a program,
			but it avoids run-time surprises caused by unexpect‐
			edly undefined functions.

     LD_TRACE_LOADED_OBJECTS
			When set to a nonempty string, causes ld-elf.so.1 to
			exit after loading the shared objects and printing a
			summary which includes the absolute pathnames of all
			objects, to standard output.

     LD_TRACE_LOADED_OBJECTS_ALL
			When set to a nonempty string, causes ld-elf.so.1 to
			expand the summary to indicate which objects caused
			each object to be loaded.

     LD_TRACE_LOADED_OBJECTS_FMT1

     LD_TRACE_LOADED_OBJECTS_FMT2
			When set, these variables are interpreted as format
			strings a la printf(3) to customize the trace output
			and are used by ldd(1)'s -f option and allows ldd(1)
			to be operated as a filter more conveniently.  If the
			dependency name starts with string lib,
			LD_TRACE_LOADED_OBJECTS_FMT1 is used, otherwise
			LD_TRACE_LOADED_OBJECTS_FMT2 is used.  The following
			conversions can be used:

			%a    The main program's name (also known as
			      “__progname”).

			%A    The value of the environment variable
			      LD_TRACE_LOADED_OBJECTS_PROGNAME.	 Typically
			      used to print both the names of programs and
			      shared libraries being inspected using ldd(1).

			%o    The library name.

			%p    The full pathname as determined by rtld's
			      library search rules.

			%x    The library's load address.

			Additionally, ‘\n’ and ‘\t’ are recognized and have
			their usual meaning.

     LD_UTRACE		If set, ld-elf.so.1 will log events such as the load‐
			ing and unloading of shared objects via utrace(2).

FILES
     /var/run/ld-elf.so.hints	 Hints file.
     /var/run/ld-elf32.so.hints	 Hints file for 32-bit binaries on 64-bit sys‐
				 tem.
     /etc/libmap.conf		 The libmap configuration file.
     /etc/libmap32.conf		 The libmap configuration file for 32-bit
				 binaries on 64-bit system.

SEE ALSO
     ld(1), ldd(1), elf(5), libmap.conf(5), ldconfig(8)

BSD				 April 1, 2009				   BSD
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