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

       DynaLoader - Dynamically	load C libraries into Perl code

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

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

       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,
       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 common
       system types.  And since	the year 2000, there's also Inline::C, a
       module that allows you to write Perl subroutines	in C.  Also available
       from your local CPAN site.

       DynaLoader Interface Summary

							 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

	   The standard/default	list of	directories in which dl_findfile()
	   will	search for libraries etc.  Directories 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	manipulated 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()	function and @dl_library_path can be
	   used	to search for and return the absolute pathname for the
	   library/object that you wish	to load.

	   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/ 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/  A few platforms
	   need	to be told the location	of the dependent library explicitly.
	   Use @dl_resolve_using for this.

	   Example usage:

	       @dl_resolve_using = dl_findfile('-lsocket');

	   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

	   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_symbol() to look for a symbol in
	   any of the loaded files.

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

	   An array of file names for the shared objects that were loaded.


	       $message	= dl_error();

	   Error message text from the last failed DynaLoader function.	 Note
	   that, similar to errno in unix, a successful	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 corresponding message for later
	   retrieval.  This will avoid problems	on some	platforms (such	as
	   SunOS) where	the error message is very temporary (e.g., dlerror()).

	   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
	   information or higher for more.

	   When	specified (localised) in a module's .pm	file, indicates	the
	   extension which the module's	loadable object	will have. For

	       local $DynaLoader::dl_dlext = 'unusual_ext';

	   would indicate that the module's loadable object has	an extension
	   of "unusual_ext" instead of the more	usual $Config{dlext}.  NOTE:
	   This	also requires that the module's	Makefile.PL specify (in

	       DLEXT =>	'unusual_ext',


	       @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 independent forms.
	   Any names in	the form -lname	are converted into libname.*, where .*
	   is an appropriate suffix 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 suffix 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.


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


	       $filepath = dl_expandspec($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
	   dl_expandspec() function in  See DynaLoader_pm.PL
	   for more information.


	       $libref = dl_load_file($filename, $flags)

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

	   The $flags argument to alters dl_load_file behaviour.  Assigned

	    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 $filename.  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

	       SunOS: dlopen($filename)
	       HP-UX: shl_load($filename)
	       Linux: dld_create_reference(@dl_require_symbols); dld_link($filename)
	       VMS:   lib$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.)


	       $status = dl_unload_file($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 and	perl is	compiled with the C macro
	   "DL_UNLOAD_ALL_AT_EXIT" defined, then 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 -Duseshrplib) 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 interpreter.  As a result, any shared objects
	   opened by DynaLoader	may point to a now invalid 'ghost' of the
	   libperl shared object, causing apparently random memory 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: ???
	       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.)


	       $flags =	dl_load_flags $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().


	       $symref = dl_find_symbol($libref, $symbol)

	   Return the address of the symbol $symbol or "undef" if not found.
	   If the target system	has separate functions 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($symbol)
	       VMS:   lib$find_image_symbol($libref,$symbol)


	       $symref = dl_find_symbol_anywhere($symbol)

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


	       @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 provide it, they	just return an empty list.


	       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 newXS()/newXS_flags().  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($module [...])

	   This	is the normal entry point for automatic	dynamic	loading	in

	   It performs the following actions:

	   o	   locates an auto/$module directory by	searching @INC

	   o	   uses	dl_findfile() to determine the filename	to load

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

	   o	   executes an auto/$module/$ 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)

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

	   o	   calls dl_load_file()	to load	the file

	   o	   calls dl_undef_symbols() and	warns if any symbols are

	   o	   calls dl_find_symbol() for "boot_$module"

	   o	   calls dl_install_xsub() to install it as

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

	   All arguments to bootstrap()	are passed to the module's bootstrap
	   function.  The default code generated by xsubpp expects $module [,
	   $version] If	the optional $version argument is not given, it
	   defaults to "$XS_VERSION // $VERSION" in the	module's symbol	table.
	   The default code compares the Perl-space version with the version
	   of the compiled XS code, and	croaks with an error if	they do	not

       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

       Larry Wall designed the elegant inherited bootstrap mechanism 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.

perl v5.28.3			  2021-03-01			 DynaLoader(3)


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