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LD.AOUT_SO(1)               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 programmatically
     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 absolute 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 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

                              %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, \n and \t 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 running 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.

NetBSD 7.1.2                    January 1, 2011                   NetBSD 7.1.2