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

6       DynaLoader - Dynamically load C libraries into Perl code
7

SYNOPSIS

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

AUTHOR

369       Tim Bunce, 11 August 1994.
370
371       This interface is based on the work and comments of (in no particular
372       order): Larry Wall, Robert Sanders, Dean Roehrich, Jeff Okamoto, Anno
373       Siegel, Thomas Neumann, Paul Marquess, Charles Bailey, myself and
374       others.
375
376       Larry Wall designed the elegant inherited bootstrap mechanism and
377       implemented the first Perl 5 dynamic loader using it.
378
379       Solaris global loading added by Nick Ing-Simmons with design/coding
380       assistance from Tim Bunce, January 1996.
381
382
383
384perl v5.12.4                      2011-11-04                   DynaLoader(3pm)
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