


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


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









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         @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().



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           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



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           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



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           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



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           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



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           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)



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

           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/$module/$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)

           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 sym-
                   bols are undefined

           o       calls dl_find_symbol() for "boot_$module"

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

           o       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.








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