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



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 pseudodynamic
       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 v5.12.5                                     2013-08-25                                  DynaLoader(3pm)