DynaLoader(3)
NNAAMMEE
DynaLoader - Dynamically load C libraries into Perl code
dl_error(), dl_findfile(), dl_expandspec(),
dl_load_file(), dl_find_symbol(),
dl_find_symbol_anywhere(), dl_undef_symbols(),
dl_install_xsub(), dl_load_flags(), bootstrap() - routines
used by DynaLoader modules
SSYYNNOOPPSSIISS
package YourPackage;
require DynaLoader;
@ISA = qw(... DynaLoader ...);
bootstrap YourPackage;
# optional method for 'global' loading
sub dl_load_flags { 0x01 }
DDEESSCCRRIIPPTTIIOONN
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 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 ExtUtils::DynaLib module is
available from CPAN sites which performs that function for
some common system types.
DynaLoader Interface Summary
@dl_library_path
@dl_resolve_using
@dl_require_symbols
$dl_debug
@dl_librefs
@dl_modules
Implemented in:
bootstrap($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
$symref = dl_find_symbol($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:
$dl_library_path[0], [1], ... etc
@dl_library_path is initialised to hold the list of
'normal' directories (/usr/lib, etc) determined by
CCoonnffiigguurree ($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_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 $ENV{'PERL_DL_DEBUG'} if defined.
For the DynaLoader developer/porter there is a similar
debugging variable added to the C code (see dlutils.c)
and enabled if Perl was built with the --DDDDEEBBUUGGGGIINNGG
flag. This can also be set via the PERL_DL_DEBUG
environment variable. Set to 1 for minimal
information or higher for more.
dl_findfile()
Syntax:
@filepaths = dl_findfile(@names)
Determine the full paths (including file suffix) of
one or more loadable files given their generic names
and optionally one or more directories. Searches
directories in @dl_library_path by default and returns
an empty list if no files were found.
Names can be specified in a variety of platform
independent forms. Any names in the form --llnnaammee are
converted into libname.*, where .* is an appropriate
suffix for the platform.
If a name does not already have a suitable prefix
and/or suffix then the corresponding file will be
searched for by trying combinations of prefix and
suffix appropriate to the platform: "$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 --LLddiirr. 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_loadflags()
Syntax:
$flags = dl_loadflags $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:
+o locates an auto/$module directory by searching
@INC
+o uses dl_findfile() to determine the filename
to load
+o sets @dl_require_symbols to ("boot_$module")
+o 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)
+o calls dl_load_flags() to determine how to load
the file.
+o calls dl_load_file() to load the file
+o calls dl_undef_symbols() and warns if any
symbols are undefined
+o calls dl_find_symbol() for "boot_$module"
+o calls dl_install_xsub() to install it as
"${module}::bootstrap"
+o calls &{"${module}::bootstrap"} to bootstrap
the module (actually it uses the function
reference returned by dl_install_xsub for
speed)
AAUUTTHHOORR
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.