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NAME
DynaLoader - Dynamically load C libraries into Perl code
SYNOPSIS
package YourPackage;
require DynaLoader;
@ISA = qw(... DynaLoader ...);
__PACKAGE__->bootstrap;
# 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,
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
$dl_dlext
@dl_librefs
@dl_modules
@dl_shared_objects
Implemented in:
bootstrap($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_dlext
When specified (localised) in a module's .pm file, indicates the
extension which the module's loadable object will have. For example:
local $DynaLoader::dl_dlext = 'unusual_ext';
would indicate that the module's loadable object has an extension of
"unusual_ext" instead of the more usual $Config{dlext}. NOTE: This
also requires that the module's Makefile.PL specify (in
"WriteMakefile()"):
DLEXT => 'unusual_ext',
dl_findfile()
Syntax:
@filepaths = dl_findfile(@names)
Determine the full paths (including file suffix) of one or more
loadable files given their generic names and optionally one or more
directories. Searches directories in @dl_library_path by default and
returns an empty list if no files were found.
Names can be specified in a variety of platform independent forms.
Any names in the form -lname are converted into libname.*, where .*
is an appropriate suffix for the platform.
If a name does not already have a suitable prefix and/or suffix then
the corresponding file will be searched for by trying combinations
of prefix and suffix appropriate to the platform: "$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
dl_expandspec() function in DynaLoader.pm. See DynaLoader_pm.PL 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)
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 $libref, which must be an opaque 'library
reference' as returned from dl_load_file. Returns one on success and
zero on failure. This function is optional and may not necessarily
be provided on all platforms.
If it is defined and perl is compiled with the C macro
"DL_UNLOAD_ALL_AT_EXIT" defined, then it is called automatically
when the interpreter exits for every shared object or library loaded
by DynaLoader::bootstrap. All such library references are stored in
@dl_librefs by DynaLoader::Bootstrap as it loads the libraries. The
files are unloaded in last-in, first-out order.
This unloading is usually necessary when embedding a shared-object
perl (e.g. one configured with -Duseshrplib) within a larger
application, and the perl interpreter is created and destroyed
several times within the lifetime of the application. In this case
it is possible that the system dynamic linker will unload and then
subsequently reload the shared libperl without relocating any
references to it from any files DynaLoaded by the previous
incarnation of the interpreter. As a result, any shared objects
opened by DynaLoader may point to a now invalid 'ghost' of the
libperl shared object, causing apparently random memory corruption
and crashes. This behaviour is most commonly seen when using Apache
and mod_perl built with the APXS mechanism.
SunOS: dlclose($libref)
HP-UX: ???
Linux: ???
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($symbol)
VMS: lib$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 newXS()/newXS_flags(). 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)
All arguments to bootstrap() are passed to the module's bootstrap
function. The default code generated by xsubpp expects $module [,
$version] If the optional $version argument is not given, it
defaults to "$XS_VERSION // $VERSION" in the module's symbol table.
The default code compares the Perl-space version with the version of
the compiled XS code, and croaks with an error if they do not match.
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.
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