LD.AOUT_SO(1)						    BSD General Commands Manual 					     LD.AOUT_SO(1)

NAME

     ld.aout_so -- run-time link-editor

DESCRIPTION

     ld.aout_so is a self-contained, position independent program image providing run-time support for loading and link-editing shared objects
     into a process' address space.  It uses the data structures (see link(5)) contained within dynamically linked programs to determine which
     shared libraries are needed and loads them at a convenient virtual address using the mmap(2) system call.

     After all shared libraries have been successfully loaded, ld.aout_so 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.  ld.aout_so looks for a symbol named .init in each
     object's symbol table.  If present, this symbol is assumed to represent a C-function declared as void .init(void), which is then called.
     Similarly, a void .fini(void) function is called just before an object is unloaded from the process address space as a result of calling
     dlclose(3).  Note that while an object's .init is always called, whether the object is loaded automatically at program startup or programmat-
     ically by using dlopen(3), the .fini function is called only on 'last dlclose(3)'.

     This mechanism is exploited by the system-supplied C++ constructor initialization code located in /usr/lib/c++rt.o.  This file should be
     included in the list of object-code files passed to ld(1) when building a shared C++ library.

     ld.aout_so is itself a shared object that is initially loaded by the startup module crt0.	Since a.out(5) formats do not provide easy access
     to the file header from within a running process, crt0 uses the special symbol _DYNAMIC to determine whether a program is in fact dynamically
     linked or not.  Whenever the linker ld(1) has relocated this symbol to a location other than 0, crt0 assumes the services of ld.aout_so are
     needed (see link(5) for details).	crt0 passes control to rtld's entry point before the program's main() routine is called.  Thus, ld.aout_so
     can complete the link-editing process before the dynamic program calls upon services of any dynamic library.

     To quickly locate the required shared objects in the filesystem, ld.aout_so may use a ``hints'' file, prepared by the ldconfig(8) utility, in
     which the full path specification of the shared objects can be looked up by hashing on the 3-tuple <library-name, major-version-number,
     minor-version-number>.

     ld.aout_so recognizes a number of environment variables that can be used to modify its behavior as follows:

     LD_LIBRARY_PATH	      A colon separated list of directories, overriding the default search path for shared libraries.

     LD_PRELOAD 	      A colon separated list of shared object filenames to be loaded after the main program but before its shared object
			      dependencies.

     LD_WARN_NON_PURE_CODE    When set, issue a warning whenever a link-editing operation requires modification of the text segment of some loaded
			      object.  This is usually indicative of an incorrectly built library.

     LD_SUPPRESS_WARNINGS     When set, no warning messages of any kind are issued.  Normally, a warning is given if satisfactorily versioned
			      library could not be found.

     LD_TRACE_LOADED_OBJECTS  When set, causes ld.aout_so to exit after loading the shared objects and printing a summary which includes the abso-
			      lute pathnames of all objects, to standard output.

     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.  The following conver-
			      sions 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

			      %o    The library name.

			      %m    The library's major version number.

			      %n    The library's minor version number.

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

			      %x    The library's load address.

			      Additionally, 
 and 	 are recognized and have their usual meaning.

     LD_NO_INTERN_SEARCH      When set, ld.aout_so does not process any internal search paths that were recorded in the executable.

     LD_NOSTD_PATH	      When set, do not include a set of built-in standard directory paths for searching.  This might be useful when run-
			      ning on a system with a completely non-standard filesystem layout.

FILES

     /var/run/ld.so.hints     library location hints built by ldconfig(8)

SEE ALSO

     ld(1), ld.elf_so(1), ld.so(1), link(5), ldconfig(8)

HISTORY

     The shared library model employed first appeared in SunOS 4.0.

SECURITY CONSIDERATIONS

     The environment variables LD_LIBRARY_PATH and LD_PRELOAD are not honored when executing in a set-user-ID or set-group-ID environment.  This
     action is taken to prevent malicious substitution of shared object dependencies or interposition of symbols.

BSD
								  January 1, 2011							       BSD