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

NAME
       DynaLoader - Dynamically load C libraries into Perl code

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 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

	 @dl_library_path
	 @dl_resolve_using
	 @dl_require_symbols
	 $dl_debug
	 @dl_librefs
	 @dl_modules
	 @dl_shared_objects
							 Implemented in:
	 bootstrap($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.  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.

       @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/libsocket.so).  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');

       @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_symbol() 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_shared_objects
	   An array of file names for the shared objects that were loaded.

       dl_error()
	   Syntax:

	       $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()).

       $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
	   information 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 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.

	   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() function 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 reference' 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 $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
	   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 -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: ???
	       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_load_flags()
	   Syntax:

	       $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().

       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 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($symbol)
	       NeXT:  rld_lookup("_$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 provide 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 symbols 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 reference 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 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 v5.10.0			  2009-03-02			 DynaLoader(3)
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