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
package YourPackage; require DynaLoader; @ISA = qw(... DynaLoader ...); bootstrap YourPackage;
# optional method for 'global' loading sub dl_load_flags { 0x01 }
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 C::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_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.
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_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.
$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
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 $ENV{'PERL_DL_DEBUG'}
if defined.
For the DynaLoader developer/porter there is a similar debugging variable added to the C code (see dlutils.c) and enabled if Perl was built with the -DDEBUGGING flag. This can also be set via the PERL_DL_DEBUG environment variable. Set to 1 for minimal information or higher for more.
@filepaths = dl_findfile(@names)
Determine the full paths (including file suffix) of one or more loadable
files given their generic names and optionally one or more directories.
Searches directories in @dl_library_path
by default and
returns an empty list if no files were found.
Names can be specified in a variety of platform independent forms. Any names in the form -lname are converted into libname.*, where .* is an appropriate suffix for the platform.
If a name does not already have a suitable prefix and/or suffix then the corresponding file will be searched for by trying combinations of prefix and suffix appropriate to the platform: ``$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));
$filepath = dl_expandspec($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.
$libref = dl_load_file($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.)
$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().
$symref = dl_find_symbol($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)
$symref = dl_find_symbol_anywhere($symbol)
Applies
dl_find_symbol()
to
the members of @dl_librefs
and returns the first match found.
@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($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 $filename
is not defined then ``DynaLoader'' will be used.
bootstrap($module)
This is the normal entry point for automatic dynamic loading in Perl.
It performs the following actions:
locates an auto/$module directory by searching @INC
dl_findfile()
to
determine the filename to load
@dl_require_symbols
to ("boot_$module")
@dl_resolve_using
any files which are required to load the module on the current platform)
dl_load_flags()
to
determine how to load the file.
dl_load_file()
to load
the file
dl_undef_symbols()
and warns if any symbols are undefined
dl_find_symbol()
for
``boot_$module''
dl_install_xsub()
to
install it as ``${module}::bootstrap''
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.
If rather than formatting bugs, you encounter substantive content errors in these documents, such as mistakes in the explanations or code, please use the perlbug utility included with the Perl distribution.