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DynaLoader(3)	 Perl Programmers Reference Guide   DynaLoader(3)

NAME
       DynaLoader - Dynamically load C libraries into Perl code

       dl_error(), dl_findfile(), dl_expandspec(),
       dl_load_file(), dl_unload_file(), dl_find_symbol(),
       dl_find_symbol_anywhere(), dl_undef_symbols(),
       dl_install_xsub(), dl_load_flags(), bootstrap() - routines
       used by DynaLoader modules

SYNOPSIS
	   package YourPackage;
	   require DynaLoader;
	   @ISA = qw(... DynaLoader ...);
	   bootstrap YourPackage;

	   # optional method for 'global' loading
	   sub dl_load_flags { 0x01 }

DESCRIPTION
       This document defines a standard generic interface to the
       dynamic linking mechanisms available on many platforms.
       Its primary purpose is to implement automatic dynamic
       loading of Perl modules.

       This document serves as both a specification for anyone
       wishing to implement the DynaLoader for a new platform and
       as a guide for anyone wishing to use the DynaLoader
       directly in an application.

       The DynaLoader is designed to be a very simple high-level
       interface that is sufficiently general to cover the
       requirements of SunOS, HP-UX, NeXT, Linux, VMS and other
       platforms.

       It is also hoped that the interface will cover the needs
       of OS/2, NT etc and also allow pseudo-dynamic linking
       (using "ld -A" at runtime).

       It must be stressed that the DynaLoader, by itself, is
       practically useless for accessing non-Perl libraries
       because it provides almost no Perl-to-C 'glue'.	There is,
       for example, no mechanism for calling a C library function
       or supplying arguments.	A C::DynaLib module is available
       from CPAN sites which performs that function for some com
       mon system types.

       DynaLoader Interface Summary

	 @dl_library_path
	 @dl_resolve_using
	 @dl_require_symbols
	 $dl_debug
	 @dl_librefs
	 @dl_modules
							 Implemented in:
	 bootstrap($modulename)				      Perl
	 @filepaths = dl_findfile(@names)		      Perl
	 $flags = $modulename->dl_load_flags		      Perl
	 $symref  = dl_find_symbol_anywhere($symbol)	      Perl

	 $libref  = dl_load_file($filename, $flags)	      C
	 $status  = dl_unload_file($libref)		      C
	 $symref  = dl_find_symbol($libref, $symbol)	      C
	 @symbols = dl_undef_symbols()			      C
	 dl_install_xsub($name, $symref [, $filename])	      C
	 $message = dl_error				      C

       @dl_library_path
	   The standard/default list of directories in which
	   dl_findfile() will search for libraries etc.	 Directo
	   ries are searched in order: $dl_library_path[0], [1],
	   ... etc

	   @dl_library_path is initialised to hold the list of
	   'normal' directories (/usr/lib, etc) determined by
	   Configure ("$Config{'libpth'}").  This should ensure
	   portability across a wide range of platforms.

	   @dl_library_path should also be initialised with any
	   other directories that can be determined from the
	   environment at runtime (such as LD_LIBRARY_PATH for
	   SunOS).

	   After initialisation @dl_library_path can be manipu
	   lated by an application using push and unshift before
	   calling dl_findfile().  Unshift can be used to add
	   directories to the front of the search order either to
	   save search time or to override libraries with the
	   same name in the 'normal' directories.

	   The load function that dl_load_file() calls may
	   require an absolute pathname.  The dl_findfile() func
	   tion and @dl_library_path can be used to search for
	   and return the absolute pathname for the
	   library/object that you wish to load.

       @dl_resolve_using
	   A list of additional libraries or other shared objects
	   which can be used to resolve any undefined symbols
	   that might be generated by a later call to
	   load_file().

	   This is only required on some platforms which do not
	   handle dependent libraries automatically.  For example
	   the Socket Perl extension library
	   (auto/Socket/Socket.so) contains references to many
	   socket functions which need to be resolved when it's
	   loaded.  Most platforms will automatically know where
	   to find the 'dependent' library (e.g., /usr/lib/lib_
	   socket.so).	A few platforms need to be told the loca
	   tion of the dependent library explicitly.  Use
	   @dl_resolve_using for this.

	   Example usage:

	       @dl_resolve_using = dl_findfile('-lsocket');

       @dl_require_symbols
	   A list of one or more symbol names that are in the
	   library/object file to be dynamically loaded.  This is
	   only required on some platforms.

       @dl_librefs
	   An array of the handles returned by successful calls
	   to dl_load_file(), made by bootstrap, in the order in
	   which they were loaded.  Can be used with dl_find_sym_
	   bol() to look for a symbol in any of the loaded files.

       @dl_modules
	   An array of module (package) names that have been
	   bootstrap'ed.

       dl_error()
	   Syntax:

	       $message = dl_error();

	   Error message text from the last failed DynaLoader
	   function.  Note that, similar to errno in unix, a suc
	   cessful function call does not reset this message.

	   Implementations should detect the error as soon as it
	   occurs in any of the other functions and save the cor
	   responding message for later retrieval.  This will
	   avoid problems on some platforms (such as SunOS) where
	   the error message is very temporary (e.g., dlerror()).

       $dl_debug
	   Internal debugging messages are enabled when $dl_debug
	   is set true.	 Currently setting $dl_debug only affects
	   the Perl side of the DynaLoader.  These messages
	   should help an application developer to resolve any
	   DynaLoader usage problems.

	   $dl_debug is set to "$ENV{'PERL_DL_DEBUG'}" if
	   defined.

	   For the DynaLoader developer/porter there is a similar
	   debugging variable added to the C code (see dlutils.c)
	   and enabled if Perl was built with the -DDEBUGGING
	   flag.  This can also be set via the PERL_DL_DEBUG
	   environment variable.  Set to 1 for minimal informa
	   tion or higher for more.

       dl_findfile()
	   Syntax:

	       @filepaths = dl_findfile(@names)

	   Determine the full paths (including file suffix) of
	   one or more loadable files given their generic names
	   and optionally one or more directories.  Searches
	   directories in @dl_library_path by default and returns
	   an empty list if no files were found.

	   Names can be specified in a variety of platform inde
	   pendent forms.  Any names in the form -lname are con
	   verted into libname.*, where .* is an appropriate suf
	   fix for the platform.

	   If a name does not already have a suitable prefix
	   and/or suffix then the corresponding file will be
	   searched for by trying combinations of prefix and suf
	   fix appropriate to the platform: "$name.o",
	   "lib$name.*"	 and "$name".

	   If any directories are included in @names they are
	   searched before @dl_library_path.  Directories may be
	   specified as -Ldir.	Any other names are treated as
	   filenames to be searched for.

	   Using arguments of the form "-Ldir" and "-lname" is
	   recommended.

	   Example:

	       @dl_resolve_using = dl_findfile(qw(-L/usr/5lib -lposix));

       dl_expandspec()
	   Syntax:

	       $filepath = dl_expandspec($spec)

	   Some unusual systems, such as VMS, require special
	   filename handling in order to deal with symbolic names
	   for files (i.e., VMS's Logical Names).

	   To support these systems a dl_expandspec() function
	   can be implemented either in the dl_*.xs file or code
	   can be added to the autoloadable dl_expandspec() func
	   tion in DynaLoader.pm.  See DynaLoader.pm for more
	   information.

       dl_load_file()
	   Syntax:

	       $libref = dl_load_file($filename, $flags)

	   Dynamically load $filename, which must be the path to
	   a shared object or library.	An opaque 'library refer
	   ence' is returned as a handle for the loaded object.
	   Returns undef on error.

	   The $flags argument to alters dl_load_file behaviour.
	   Assigned bits:

	    0x01  make symbols available for linking later dl_load_file's.
		  (only known to work on Solaris 2 using dlopen(RTLD_GLOBAL))
		  (ignored under VMS; this is a normal part of image linking)

	   (On systems that provide a handle for the loaded
	   object such as SunOS and HPUX, $libref will be that
	   handle.  On other systems $libref will typically be
	   $filename or a pointer to a buffer containing $file
	   name.  The application should not examine or alter
	   $libref in any way.)

	   This is the function that does the real work.  It
	   should use the current values of @dl_require_symbols
	   and @dl_resolve_using if required.

	       SunOS: dlopen($filename)
	       HP-UX: shl_load($filename)
	       Linux: dld_create_reference(@dl_require_symbols); dld_link($filename)
	       NeXT:  rld_load($filename, @dl_resolve_using)
	       VMS:   lib$find_image_symbol($filename,$dl_require_symbols[0])

	   (The dlopen() function is also used by Solaris and
	   some versions of Linux, and is a common choice when
	   providing a "wrapper" on other mechanisms as is done
	   in the OS/2 port.)

       dl_unload_file()
	   Syntax:

	       $status = dl_unload_file($libref)

	   Dynamically unload $libref, which must be an opaque
	   'library reference' as returned from dl_load_file.
	   Returns one on success and zero on failure.

	   This function is optional and may not necessarily be
	   provided on all platforms.  If it is defined, it is
	   called automatically when the interpreter exits for
	   every shared object or library loaded by
	   DynaLoader::bootstrap.  All such library references
	   are stored in @dl_librefs by DynaLoader::Bootstrap as
	   it loads the libraries.  The files are unloaded in
	   last-in, first-out order.

	   This unloading is usually necessary when embedding a
	   shared-object perl (e.g.  one configured with -Duse
	   shrplib) within a larger application, and the perl
	   interpreter is created and destroyed several times
	   within the lifetime of the application.  In this case
	   it is possible that the system dynamic linker will
	   unload and then subsequently reload the shared libperl
	   without relocating any references to it from any files
	   DynaLoaded by the previous incarnation of the inter
	   preter.  As a result, any shared objects opened by
	   DynaLoader may point to a now invalid 'ghost' of the
	   libperl shared object, causing apparently random mem
	   ory corruption and crashes.	This behaviour is most
	   commonly seen when using Apache and mod_perl built
	   with the APXS mechanism.

	       SunOS: dlclose($libref)
	       HP-UX: ???
	       Linux: ???
	       NeXT:  ???
	       VMS:   ???

	   (The dlclose() function is also used by Solaris and
	   some versions of Linux, and is a common choice when
	   providing a "wrapper" on other mechanisms as is done
	   in the OS/2 port.)

       dl_loadflags()
	   Syntax:

	       $flags = dl_loadflags $modulename;

	   Designed to be a method call, and to be overridden by
	   a derived class (i.e. a class which has DynaLoader in
	   its @ISA).  The definition in DynaLoader itself
	   returns 0, which produces standard behavior from
	   dl_load_file().

       dl_find_symbol()
	   Syntax:

	       $symref = dl_find_symbol($libref, $symbol)

	   Return the address of the symbol $symbol or "undef" if
	   not found.  If the target system has separate func
	   tions to search for symbols of different types then
	   dl_find_symbol() should search for function symbols
	   first and then other types.

	   The exact manner in which the address is returned in
	   $symref is not currently defined.  The only initial
	   requirement is that $symref can be passed to, and
	   understood by, dl_install_xsub().

	       SunOS: dlsym($libref, $symbol)
	       HP-UX: shl_findsym($libref, $symbol)
	       Linux: dld_get_func($symbol) and/or dld_get_symbol($symbol)
	       NeXT:  rld_lookup("_$symbol")
	       VMS:   lib$find_image_symbol($libref,$symbol)

       dl_find_symbol_anywhere()
	   Syntax:

	       $symref = dl_find_symbol_anywhere($symbol)

	   Applies dl_find_symbol() to the members of @dl_librefs
	   and returns the first match found.

       dl_undef_symbols()
	   Example

	       @symbols = dl_undef_symbols()

	   Return a list of symbol names which remain undefined
	   after load_file().  Returns "()" if not known.  Don't
	   worry if your platform does not provide a mechanism
	   for this.  Most do not need it and hence do not pro
	   vide it, they just return an empty list.

       dl_install_xsub()
	   Syntax:

	       dl_install_xsub($perl_name, $symref [, $filename])

	   Create a new Perl external subroutine named $perl_name
	   using $symref as a pointer to the function which
	   implements the routine.  This is simply a direct call
	   to newXSUB().  Returns a reference to the installed
	   function.

	   The $filename parameter is used by Perl to identify
	   the source file for the function if required by die(),
	   caller() or the debugger.  If $filename is not defined
	   then "DynaLoader" will be used.

       bootstrap()
	   Syntax:

	   bootstrap($module)

	   This is the normal entry point for automatic dynamic
	   loading in Perl.

	   It performs the following actions:

		  locates an auto/$module directory by searching
		   @INC

		  uses dl_findfile() to determine the filename
		   to load

		  sets @dl_require_symbols to "("boot_$module")"

		  executes an auto/$module/$module.bs file if it
		   exists (typically used to add to
		   @dl_resolve_using any files which are required
		   to load the module on the current platform)

		  calls dl_load_flags() to determine how to load
		   the file.

		  calls dl_load_file() to load the file

		  calls dl_undef_symbols() and warns if any sym
		   bols are undefined

		  calls dl_find_symbol() for "boot_$module"

		  calls dl_install_xsub() to install it as
		   "${module}::bootstrap"

		  calls &{"${module}::bootstrap"} to bootstrap
		   the module (actually it uses the function ref
		   erence returned by dl_install_xsub for speed)

AUTHOR
       Tim Bunce, 11 August 1994.

       This interface is based on the work and comments of (in no
       particular order): Larry Wall, Robert Sanders, Dean
       Roehrich, Jeff Okamoto, Anno Siegel, Thomas Neumann, Paul
       Marquess, Charles Bailey, myself and others.

       Larry Wall designed the elegant inherited bootstrap mecha
       nism and implemented the first Perl 5 dynamic loader using
       it.

       Solaris global loading added by Nick Ing-Simmons with
       design/coding assistance from Tim Bunce, January 1996.

2001-03-18		   perl v5.6.1		    DynaLoader(3)
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