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

NAME,, rtld -- run-time link-editor

     The 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 de-
     termine which shared libraries are	needed and loads them using the
     mmap(2) system call.

     After all shared libraries	have been successfully loaded,
     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	be-
     gins.  This is useful for C++ libraries that contain static constructors.

     When resolving dependencies for the loaded	objects, trans-
     lates dynamic token strings in rpath and soname.  If the -z origin	option
     of	the static linker was set when linking the binary, the token expansion
     is	performed at the object	load time, see 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 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.  The
     following search order is used to locate required shared objects:

	   1.	DT_RPATH of the	referencing object unless that object also
		contains a DT_RUNPATH tag
	   2.	DT_RPATH of the	program	unless the referencing object contains
		a DT_RUNPATH tag
	   3.	Path indicated by LD_LIBRARY_PATH environment variable
	   4.	DT_RUNPATH of the referencing object
	   5.	Hints file produced by the ldconfig(8) utility
	   6.	The /lib and /usr/lib directories, unless the referencing ob-
		ject was linked	using the "-z nodefaultlib" option

     The 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_DUMP_REL_POST	If set, will print a table containing all
			relocations after symbol binding and relocation.

     LD_DUMP_REL_PRE	If set, 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-

			If the variable	is specified and has a value starting
			with any of 'y', 'Y' or	'1' symbols, the path speci-
			fied by	LD_LIBRARY_PATH	variable is allowed to over-
			ride the path from DT_RPATH for	binaries which does
			not contain DT_RUNPATH tag.  For such binaries,	when
			the variable LD_LIBRARY_PATH_RPATH is set, "-z
			nodefaultlib" link-time	option is ignored as well.

     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_LIBRARY_PATH_FDS A colon separated list of file de-
			scriptor numbers for library directories.  This	is in-
			tended for use within capsicum(4) sandboxes, when
			global namespaces such as the filesystem are unavail-
			able.  It is consulted just after LD_LIBRARY_PATH.
			This variable is unset for set-user-ID and set-group-
			ID programs.

     LD_BIND_NOT	When set to a nonempty string, prevents	modifications
			of the PLT slots when doing bindings.  As result, each
			call of	the PLT-resolved function is resolved.	In
			combination with debug output, this provides complete
			account	of all bind actions at runtime.	 This variable
			is unset for set-user-ID and set-group-ID programs.

     LD_BIND_NOW	When set to a nonempty string, causes 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.

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

			When set to a nonempty string, causes to
			expand the summary to indicate which objects caused
			each object to be loaded.


			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

			%A    The value	of the environment variable
			      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	li-
			      brary search rules.

			%x    The library's load address.

			Additionally, `\n' and `\t' are	recognized and have
			their usual meaning.

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

     LD_LOADFLTR	If set, will process the filtee dependen-
			cies of	the loaded objects immediately,	instead	of
			postponing it until required.  Normally, the filtees
			are opened at the time of the first symbol resolution
			from the filter	object.

DIRECT EXECUTION MODE is typically used implicitly, loaded by the kernel as re-
     quested by	the PT_INTERP program header of	the executed binary.  FreeBSD
     also supports a direct execution mode for the dynamic linker.  In this
     mode, the user explicitly executes and	provides the path of
     the program to be linked and executed as an argument.  This mode allows
     use of a non-standard dynamic linker for a	program	activation without
     changing the binary or without changing the installed dynamic linker.
     Execution options may be specified.

     The syntax	of the direct invocation is

	   /libexec/	[-f fd]	[-p] [--] image_path [image arguments]

     The options are as	follows:

     -f	fd   File descriptor fd	references the binary to be activated by  It must already be opened in	the process when exe-
	     cuting  If this option is specified, image_path is
	     only used to provide the argv[0] value to the program.

     -p	     If	the image_path argument	specifies a name which does not	con-
	     tain a slash "/" character, uses the search path pro-
	     vided by the environment variable PATH to find the	binary to exe-

     --	     Ends the options.  The	argument following -- is in-
	     terpreted as the path of binary to	execute.

     To	conform	to user	expectation to not break some naively restricted exe-
     cution environments, in the direct	execution mode emulates
     verification of the binary	execute	permission for current user.  The ver-
     ification only uses Unix DACs, ignores ACLs and is	racy by	its nature.
     The environments which rely on such restrictions are weak and breakable
     on	its own.

     /var/run/	 Hints file.
     /var/run/	 Hints file for	32-bit binaries	on 64-bit sys-
     /etc/libmap.conf		 The libmap configuration file.
     /etc/libmap32.conf		 The libmap configuration file for 32-bit bi-
				 naries	on 64-bit system.

     ld(1), ldd(1), capsicum(4), elf(5), libmap.conf(5), ldconfig(8)

BSD				 May 20, 2017				   BSD


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