LD.AOUT_SO(1)           NetBSD 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 pro-
     viding run-time support for loading and link-editing shared objects into
     a process' address space.  It uses the data structures (see link(5)) con-
     tained 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 pro-
     ceeds 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 sym-
     bol 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 automat-
     ically 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 ini-
     tialization 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 com-
     plete the link-editing process before the dynamic program calls upon ser-
     vices 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, overrid-
                              ing the default search path for shared
                              libraries.

     LD_PRELOAD               A colon separated list of shared object file-
                              names 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-edit-
                              ing 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 satis-
                              factorily versioned library could not be found.

     LD_TRACE_LOADED_OBJECTS  When set, causes ld.aout_so to exit after load-
                              ing 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 inter-
                              nal search paths that were recorded in the exe-
                              cutable.

     LD_NOSTD_PATH            When set, do not include a set of built-in stan-
                              dard directory paths for searching.  This might
                              be useful when running on a system with a com-
                              pletely 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 7.0                      January 1, 2011                     NetBSD 7.0

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©1996-2014 Modified for NetBSD by Kimmo Suominen