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RTLD(1) 		     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  deter-
     mine  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  pro-
     ceeds to resolve external references from both the main program and all ob-
     jects  loaded.   A  mechanism is provided for initialization routines to be
     called on a per-object basis, giving a shared object an opportunity to per-
     form any extra set-up before execution of the program proper begins.   This
     is useful for C++ libraries that contain static constructors.

     When  resolving dependencies for the loaded objects, ld-elf.so.1 translates
     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 per-
     formed at the object load time, see ld(1).  The following strings are  rec-
     ognized 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 load-
     ing,  relocating,	and initializing the program and its required shared ob-
     jects, it transfers control to the entry point of the program.  The follow-
     ing search order is used to locate required shared objects:

	   1.	DT_RPATH of the referencing object unless that object also  con-
		tains 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 object
		was linked using the "-z nodefaultlib" option

     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  over-
			ride  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 variable
			is unset for set-user-ID and set-group-ID programs.

     LD_LIBRARY_PATH_RPATH
			If  the  variable  is specified and has a value starting
			with any of 'y', 'Y' or '1' symbols, the path  specified
			by  LD_LIBRARY_PATH  variable is allowed to override 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 li-
			braries.  If the directory is not specified then the di-
			rectories specified by LD_LIBRARY_PATH will be	searched
			first  followed by the set of built-in standard directo-
			ries.  This variable is unset for set-user-ID  and  set-
			group-ID  programs.   LD_LIBRARY_PATH_FDS  A colon sepa-
			rated list of file descriptor numbers for library direc-
			tories.  This is intended  for	use  within  capsicum(4)
			sandboxes, when global namespaces such as the filesystem
			are  unavailable.   It	is  consulted  just after LD_LI-
			BRARY_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 combina-
			tion 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 ld-elf.so.1 to re-
			locate all external function calls before starting  exe-
			cution	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  unexpectedly
			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 ob-
			jects, to standard output.

     LD_TRACE_LOADED_OBJECTS_ALL
			When set to a nonempty string, causes ld-elf.so.1 to ex-
			pand  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  depen-
			dency	   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 li-
			      braries 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 loading
			and unloading of shared objects via utrace(2).

     LD_LOADFLTR	If set, ld-elf.so.1 will process the filtee dependencies
			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
     ld-elf.so.1 is typically used implicitly, loaded by the kernel as requested
     by the PT_INTERP program header of the executed binary.  FreeBSD also  sup-
     ports  a  direct  execution mode for the dynamic linker.  In this mode, the
     user explicitly executes ld-elf.so.1 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  bi-
     nary  or  without changing the installed dynamic linker.  Execution options
     may be specified.

     The syntax of the direct invocation is

	   /libexec/ld-elf.so.1 [-f fd] [-p] [--] image_path [image arguments]

     The options are as follows:

     -f fd   File descriptor  fd  references  the  binary  to  be  activated  by
	     ld-elf.so.1.  It must already be opened in the process when execut-
	     ing  ld-elf.so.1.	 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  contain
	     a slash "/" character, ld-elf.so.1 uses the search path provided by
	     the environment variable PATH to find the binary to execute.

     --      Ends  the ld-elf.so.1 options.  The argument following -- is inter-
	     preted as the path of binary to execute.

     To conform to user expectation to not break some naively restricted  execu-
     tion  environments, in the direct execution mode ld-elf.so.1 emulates veri-
     fication of the binary execute permission for current user.  The  verifica-
     tion  only uses Unix DACs, ignores ACLs and is racy by its nature.  The en-
     vironments which rely on such restrictions are weak and  breakable  on  its
     own.

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  bina-
				 ries on 64-bit system.

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

FreeBSD 11.1			  May 20, 2017				 RTLD(1)
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