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: $dl_library_path[0], [1], ... etc
@dl_library_path is initialised to hold the list of 'normal' directories (/usr/lib, etc) deter-mined determined
mined by Configure ($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 devel-oper developer
oper 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 sym-bolic symbolic
bolic 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 con-taining containing
taining $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 $libref, which must be an opaque 'library reference' as returned from
dl_load_file. Returns one on success and zero on failure.
This function is optional and may not necessarily be provided on all platforms. If it is
defined, it is called automatically when the interpreter exits for every shared object or library
loaded by DynaLoader::bootstrap. All such library references are stored in @dl_librefs by
DynaLoader::Bootstrap as it loads the libraries. The files are unloaded in last-in, first-out
order.
This unloading is usually necessary when embedding a shared-object perl (e.g. one configured
with -Duseshrplib) within a larger application, and the perl interpreter is created and destroyed
several times within the lifetime of the application. In this case it is possible that the sys-tem system
tem dynamic linker will unload and then subsequently reload the shared libperl without relocating
any references to it from any files DynaLoaded by the previous incarnation of the interpreter.
As a result, any shared objects opened by DynaLoader may point to a now invalid 'ghost' of the
libperl shared object, causing apparently random memory corruption and crashes. This behaviour
is most commonly seen when using Apache and mod_perl built with the APXS mechanism.
SunOS: dlclose($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 sepa-rate separate
rate 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 plat-form) platform)
form)
* 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, Jan-uary January
uary 1996.
perl v5.8.8 2001-09-21 DynaLoader(3pm)
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