1DynaLoader(3pm)        Perl Programmers Reference Guide        DynaLoader(3pm)
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4

NAME

6       DynaLoader - Dynamically load C libraries into Perl code
7

SYNOPSIS

9           package YourPackage;
10           require DynaLoader;
11           @ISA = qw(... DynaLoader ...);
12           __PACKAGE__->bootstrap;
13
14           # optional method for 'global' loading
15           sub dl_load_flags { 0x01 }
16

DESCRIPTION

18       This document defines a standard generic interface to the dynamic
19       linking mechanisms available on many platforms.  Its primary purpose is
20       to implement automatic dynamic loading of Perl modules.
21
22       This document serves as both a specification for anyone wishing to
23       implement the DynaLoader for a new platform and as a guide for anyone
24       wishing to use the DynaLoader directly in an application.
25
26       The DynaLoader is designed to be a very simple high-level interface
27       that is sufficiently general to cover the requirements of SunOS, HP-UX,
28       Linux, VMS and other platforms.
29
30       It is also hoped that the interface will cover the needs of OS/2, NT
31       etc and also allow pseudo-dynamic linking (using "ld -A" at runtime).
32
33       It must be stressed that the DynaLoader, by itself, is practically
34       useless for accessing non-Perl libraries because it provides almost no
35       Perl-to-C 'glue'.  There is, for example, no mechanism for calling a C
36       library function or supplying arguments.  A C::DynaLib module is
37       available from CPAN sites which performs that function for some common
38       system types.  And since the year 2000, there's also Inline::C, a
39       module that allows you to write Perl subroutines in C.  Also available
40       from your local CPAN site.
41
42       DynaLoader Interface Summary
43
44         @dl_library_path
45         @dl_resolve_using
46         @dl_require_symbols
47         $dl_debug
48         $dl_dlext
49         @dl_librefs
50         @dl_modules
51         @dl_shared_objects
52                                                         Implemented in:
53         bootstrap($modulename)                               Perl
54         @filepaths = dl_findfile(@names)                     Perl
55         $flags = $modulename->dl_load_flags                  Perl
56         $symref  = dl_find_symbol_anywhere($symbol)          Perl
57
58         $libref  = dl_load_file($filename, $flags)           C
59         $status  = dl_unload_file($libref)                   C
60         $symref  = dl_find_symbol($libref, $symbol)          C
61         @symbols = dl_undef_symbols()                        C
62         dl_install_xsub($name, $symref [, $filename])        C
63         $message = dl_error                                  C
64
65       @dl_library_path
66           The standard/default list of directories in which dl_findfile()
67           will search for libraries etc.  Directories are searched in order:
68           $dl_library_path[0], [1], ... etc
69
70           @dl_library_path is initialised to hold the list of 'normal'
71           directories (/usr/lib, etc) determined by Configure
72           ($Config{'libpth'}).  This should ensure portability across a wide
73           range of platforms.
74
75           @dl_library_path should also be initialised with any other
76           directories that can be determined from the environment at runtime
77           (such as LD_LIBRARY_PATH for SunOS).
78
79           After initialisation @dl_library_path can be manipulated by an
80           application using push and unshift before calling dl_findfile().
81           Unshift can be used to add directories to the front of the search
82           order either to save search time or to override libraries with the
83           same name in the 'normal' directories.
84
85           The load function that dl_load_file() calls may require an absolute
86           pathname.  The dl_findfile() function and @dl_library_path can be
87           used to search for and return the absolute pathname for the
88           library/object that you wish to load.
89
90       @dl_resolve_using
91           A list of additional libraries or other shared objects which can be
92           used to resolve any undefined symbols that might be generated by a
93           later call to load_file().
94
95           This is only required on some platforms which do not handle
96           dependent libraries automatically.  For example the Socket Perl
97           extension library (auto/Socket/Socket.so) contains references to
98           many socket functions which need to be resolved when it's loaded.
99           Most platforms will automatically know where to find the
100           'dependent' library (e.g., /usr/lib/libsocket.so).  A few platforms
101           need to be told the location of the dependent library explicitly.
102           Use @dl_resolve_using for this.
103
104           Example usage:
105
106               @dl_resolve_using = dl_findfile('-lsocket');
107
108       @dl_require_symbols
109           A list of one or more symbol names that are in the library/object
110           file to be dynamically loaded.  This is only required on some
111           platforms.
112
113       @dl_librefs
114           An array of the handles returned by successful calls to
115           dl_load_file(), made by bootstrap, in the order in which they were
116           loaded.  Can be used with dl_find_symbol() to look for a symbol in
117           any of the loaded files.
118
119       @dl_modules
120           An array of module (package) names that have been bootstrap'ed.
121
122       @dl_shared_objects
123           An array of file names for the shared objects that were loaded.
124
125       dl_error()
126           Syntax:
127
128               $message = dl_error();
129
130           Error message text from the last failed DynaLoader function.  Note
131           that, similar to errno in unix, a successful function call does not
132           reset this message.
133
134           Implementations should detect the error as soon as it occurs in any
135           of the other functions and save the corresponding message for later
136           retrieval.  This will avoid problems on some platforms (such as
137           SunOS) where the error message is very temporary (e.g., dlerror()).
138
139       $dl_debug
140           Internal debugging messages are enabled when $dl_debug is set true.
141           Currently setting $dl_debug only affects the Perl side of the
142           DynaLoader.  These messages should help an application developer to
143           resolve any DynaLoader usage problems.
144
145           $dl_debug is set to $ENV{'PERL_DL_DEBUG'} if defined.
146
147           For the DynaLoader developer/porter there is a similar debugging
148           variable added to the C code (see dlutils.c) and enabled if Perl
149           was built with the -DDEBUGGING flag.  This can also be set via the
150           PERL_DL_DEBUG environment variable.  Set to 1 for minimal
151           information or higher for more.
152
153       $dl_dlext
154           When specified (localised) in a module's .pm file, indicates the
155           extension which the module's loadable object will have. For
156           example:
157
158               local $DynaLoader::dl_dlext = 'unusual_ext';
159
160           would indicate that the module's loadable object has an extension
161           of "unusual_ext" instead of the more usual $Config{dlext}.  NOTE:
162           This also requires that the module's Makefile.PL specify (in
163           "WriteMakefile()"):
164
165               DLEXT => 'unusual_ext',
166
167       dl_findfile()
168           Syntax:
169
170               @filepaths = dl_findfile(@names)
171
172           Determine the full paths (including file suffix) of one or more
173           loadable files given their generic names and optionally one or more
174           directories.  Searches directories in @dl_library_path by default
175           and returns an empty list if no files were found.
176
177           Names can be specified in a variety of platform independent forms.
178           Any names in the form -lname are converted into libname.*, where .*
179           is an appropriate suffix for the platform.
180
181           If a name does not already have a suitable prefix and/or suffix
182           then the corresponding file will be searched for by trying
183           combinations of prefix and suffix appropriate to the platform:
184           "$name.o", "lib$name.*"  and "$name".
185
186           If any directories are included in @names they are searched before
187           @dl_library_path.  Directories may be specified as -Ldir.  Any
188           other names are treated as filenames to be searched for.
189
190           Using arguments of the form "-Ldir" and "-lname" is recommended.
191
192           Example:
193
194               @dl_resolve_using = dl_findfile(qw(-L/usr/5lib -lposix));
195
196       dl_expandspec()
197           Syntax:
198
199               $filepath = dl_expandspec($spec)
200
201           Some unusual systems, such as VMS, require special filename
202           handling in order to deal with symbolic names for files (i.e.,
203           VMS's Logical Names).
204
205           To support these systems a dl_expandspec() function can be
206           implemented either in the dl_*.xs file or code can be added to the
207           dl_expandspec() function in DynaLoader.pm.  See DynaLoader_pm.PL
208           for more information.
209
210       dl_load_file()
211           Syntax:
212
213               $libref = dl_load_file($filename, $flags)
214
215           Dynamically load $filename, which must be the path to a shared
216           object or library.  An opaque 'library reference' is returned as a
217           handle for the loaded object.  Returns undef on error.
218
219           The $flags argument to alters dl_load_file behaviour.  Assigned
220           bits:
221
222            0x01  make symbols available for linking later dl_load_file's.
223                  (only known to work on Solaris 2 using dlopen(RTLD_GLOBAL))
224                  (ignored under VMS; this is a normal part of image linking)
225
226           (On systems that provide a handle for the loaded object such as
227           SunOS and HPUX, $libref will be that handle.  On other systems
228           $libref will typically be $filename or a pointer to a buffer
229           containing $filename.  The application should not examine or alter
230           $libref in any way.)
231
232           This is the function that does the real work.  It should use the
233           current values of @dl_require_symbols and @dl_resolve_using if
234           required.
235
236               SunOS: dlopen($filename)
237               HP-UX: shl_load($filename)
238               Linux: dld_create_reference(@dl_require_symbols); dld_link($filename)
239               VMS:   lib$find_image_symbol($filename,$dl_require_symbols[0])
240
241           (The dlopen() function is also used by Solaris and some versions of
242           Linux, and is a common choice when providing a "wrapper" on other
243           mechanisms as is done in the OS/2 port.)
244
245       dl_unload_file()
246           Syntax:
247
248               $status = dl_unload_file($libref)
249
250           Dynamically unload $libref, which must be an opaque 'library
251           reference' as returned from dl_load_file.  Returns one on success
252           and zero on failure.  This function is optional and may not
253           necessarily be provided on all platforms.
254
255           If it is defined and perl is compiled with the C macro
256           "DL_UNLOAD_ALL_AT_EXIT" defined, then it is called automatically
257           when the interpreter exits for every shared object or library
258           loaded by DynaLoader::bootstrap.  All such library references are
259           stored in @dl_librefs by DynaLoader::Bootstrap as it loads the
260           libraries.  The files are unloaded in last-in, first-out order.
261
262           This unloading is usually necessary when embedding a shared-object
263           perl (e.g.  one configured with -Duseshrplib) within a larger
264           application, and the perl interpreter is created and destroyed
265           several times within the lifetime of the application.  In this case
266           it is possible that the system dynamic linker will unload and then
267           subsequently reload the shared libperl without relocating any
268           references to it from any files DynaLoaded by the previous
269           incarnation of the interpreter.  As a result, any shared objects
270           opened by DynaLoader may point to a now invalid 'ghost' of the
271           libperl shared object, causing apparently random memory corruption
272           and crashes.  This behaviour is most commonly seen when using
273           Apache and mod_perl built with the APXS mechanism.
274
275               SunOS: dlclose($libref)
276               HP-UX: ???
277               Linux: ???
278               VMS:   ???
279
280           (The dlclose() function is also used by Solaris and some versions
281           of Linux, and is a common choice when providing a "wrapper" on
282           other mechanisms as is done in the OS/2 port.)
283
284       dl_load_flags()
285           Syntax:
286
287               $flags = dl_load_flags $modulename;
288
289           Designed to be a method call, and to be overridden by a derived
290           class (i.e. a class which has DynaLoader in its @ISA).  The
291           definition in DynaLoader itself returns 0, which produces standard
292           behavior from dl_load_file().
293
294       dl_find_symbol()
295           Syntax:
296
297               $symref = dl_find_symbol($libref, $symbol)
298
299           Return the address of the symbol $symbol or "undef" if not found.
300           If the target system has separate functions to search for symbols
301           of different types then dl_find_symbol() should search for function
302           symbols first and then other types.
303
304           The exact manner in which the address is returned in $symref is not
305           currently defined.  The only initial requirement is that $symref
306           can be passed to, and understood by, dl_install_xsub().
307
308               SunOS: dlsym($libref, $symbol)
309               HP-UX: shl_findsym($libref, $symbol)
310               Linux: dld_get_func($symbol) and/or dld_get_symbol($symbol)
311               VMS:   lib$find_image_symbol($libref,$symbol)
312
313       dl_find_symbol_anywhere()
314           Syntax:
315
316               $symref = dl_find_symbol_anywhere($symbol)
317
318           Applies dl_find_symbol() to the members of @dl_librefs and returns
319           the first match found.
320
321       dl_undef_symbols()
322           Example
323
324               @symbols = dl_undef_symbols()
325
326           Return a list of symbol names which remain undefined after
327           load_file().  Returns "()" if not known.  Don't worry if your
328           platform does not provide a mechanism for this.  Most do not need
329           it and hence do not provide it, they just return an empty list.
330
331       dl_install_xsub()
332           Syntax:
333
334               dl_install_xsub($perl_name, $symref [, $filename])
335
336           Create a new Perl external subroutine named $perl_name using
337           $symref as a pointer to the function which implements the routine.
338           This is simply a direct call to newXS()/newXS_flags().  Returns a
339           reference to the installed function.
340
341           The $filename parameter is used by Perl to identify the source file
342           for the function if required by die(), caller() or the debugger.
343           If $filename is not defined then "DynaLoader" will be used.
344
345       bootstrap()
346           Syntax:
347
348           bootstrap($module [...])
349
350           This is the normal entry point for automatic dynamic loading in
351           Perl.
352
353           It performs the following actions:
354
355           •       locates an auto/$module directory by searching @INC
356
357           •       uses dl_findfile() to determine the filename to load
358
359           •       sets @dl_require_symbols to "("boot_$module")"
360
361           •       executes an auto/$module/$module.bs file if it exists
362                   (typically used to add to @dl_resolve_using any files which
363                   are required to load the module on the current platform)
364
365           •       calls dl_load_flags() to determine how to load the file.
366
367           •       calls dl_load_file() to load the file
368
369           •       calls dl_undef_symbols() and warns if any symbols are
370                   undefined
371
372           •       calls dl_find_symbol() for "boot_$module"
373
374           •       calls dl_install_xsub() to install it as
375                   "${module}::bootstrap"
376
377           •       calls &{"${module}::bootstrap"} to bootstrap the module
378                   (actually it uses the function reference returned by
379                   dl_install_xsub for speed)
380
381           All arguments to bootstrap() are passed to the module's bootstrap
382           function.  The default code generated by xsubpp expects $module [,
383           $version] If the optional $version argument is not given, it
384           defaults to "$XS_VERSION // $VERSION" in the module's symbol table.
385           The default code compares the Perl-space version with the version
386           of the compiled XS code, and croaks with an error if they do not
387           match.
388

AUTHOR

390       Tim Bunce, 11 August 1994.
391
392       This interface is based on the work and comments of (in no particular
393       order): Larry Wall, Robert Sanders, Dean Roehrich, Jeff Okamoto, Anno
394       Siegel, Thomas Neumann, Paul Marquess, Charles Bailey, myself and
395       others.
396
397       Larry Wall designed the elegant inherited bootstrap mechanism and
398       implemented the first Perl 5 dynamic loader using it.
399
400       Solaris global loading added by Nick Ing-Simmons with design/coding
401       assistance from Tim Bunce, January 1996.
402
403
404
405perl v5.32.1                      2021-05-31                   DynaLoader(3pm)
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