DynaLoader - Dynamically load C libraries into Perl code
dl_error(), dl_findfile(), dl_expandspec(),
dl_load_file(), dl_unload_file(), dl_find_symbol(),
dl_find_symbol_anywhere(), dl_undef_symbols(),
dl_install_xsub(), dl_load_flags(), bootstrap() - routines
used by DynaLoader modules
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 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 com
mon system types.
DynaLoader Interface Summary
@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
$status = dl_unload_file($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. Directo
ries 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 manipu
lated 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() func
tion 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().
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/lib
socket.so). A few platforms need to be told the loca
tion 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_sym
bol() 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 suc
cessful 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 cor
responding 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
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 informa
tion 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 inde
pendent forms. Any names in the form -lname are con
verted into libname.*, where .* is an appropriate suf
fix 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 suf
fix 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 added to the autoloadable dl_expandspec() func
tion 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 refer
ence' 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 $file
name. 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 $libref, which must be an opaque
'library reference' as returned from dl_load_file.
Returns one on success and zero on failure.
This function is optional and may not necessarily be
called automatically when the interpreter exits for
every shared object or library loaded by
DynaLoader::bootstrap. All such library references
are stored in @dl_librefs by DynaLoader::Bootstrap as
it loads the libraries. The files are unloaded in
last-in, first-out order.
This unloading is usually necessary when embedding a
shared-object perl (e.g. one configured with -Duse
shrplib) within a larger application, and the perl
interpreter is created and destroyed several times
within the lifetime of the application. In this case
it is possible that the system dynamic linker will
unload and then subsequently reload the shared libperl
without relocating any references to it from any files
DynaLoaded by the previous incarnation of the inter
preter. As a result, any shared objects opened by
DynaLoader may point to a now invalid 'ghost' of the
libperl shared object, causing apparently random mem
ory corruption and crashes. This behaviour is most
commonly seen when using Apache and mod_perl built
with the APXS mechanism.
SunOS: dlclose($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_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 func
tions to search for symbols of different types then
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 pro
vide 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)
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 sym
bols 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 ref
erence 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 mecha
nism 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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