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diff --git a/pod/modpods/Dynaloader.pod b/pod/modpods/Dynaloader.pod new file mode 100644 index 0000000000..9810dad205 --- /dev/null +++ b/pod/modpods/Dynaloader.pod @@ -0,0 +1,316 @@ +=head1 NAME + +DynaLoader - Dynamically load C libraries into Perl code + +dl_error(), dl_findfile(), dl_expandspec(), dl_load_file(), dl_find_symbol(), dl_undef_symbols(), dl_install_xsub(), boostrap() - routines used by DynaLoader modules + +=head1 SYNOPSIS + + require DynaLoader; + push (@ISA, 'DynaLoader'); + + +=head1 DESCRIPTION + +This specification 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. + +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 allow pseudo-dynamic linking (using C<ld -A> at runtime). + +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. + +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. It is anticipated that any +glue that may be developed in the future will be implemented in a +separate dynamically loaded module. + +DynaLoader Interface Summary + + @dl_library_path + @dl_resolve_using + @dl_require_symbols + $dl_debug + Implemented in: + bootstrap($modulename) Perl + @filepaths = dl_findfile(@names) Perl + + $libref = dl_load_file($filename) C + $symref = dl_find_symbol($libref, $symbol) C + @symbols = dl_undef_symbols() C + dl_install_xsub($name, $symref [, $filename]) C + $message = dl_error C + +=over 4 + +=item @dl_library_path + +The standard/default list of directories in which dl_findfile() will +search for libraries etc. Directories are searched in order: +$dl_library_path[0], [1], ... etc + +@dl_library_path is initialised to hold the list of 'normal' directories +(F</usr/lib>, etc) determined by B<Configure> (C<$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. + +=item @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 +(F<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., +F</usr/lib/libsocket.so>). A few platforms need to to be told the location +of the dependent library explicitly. Use @dl_resolve_using for this. + +Example usage: + + @dl_resolve_using = dl_findfile('-lsocket'); + +=item @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. + +=item 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()). + +=item $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 C<$ENV{'PERL_DL_DEBUG'}> if defined. + +For the DynaLoader developer/porter there is a similar debugging +variable added to the C code (see dlutils.c) and enabled if Perl was +built with the B<-DDEBUGGING> flag. This can also be set via the +PERL_DL_DEBUG environment variable. Set to 1 for minimal information or +higher for more. + +=item dl_findfile() + +Syntax: + + @filepaths = dl_findfile(@names) + +Determine the full paths (including file suffix) of one or more +loadable files given their generic names and optionally one or more +directories. Searches directories in @dl_library_path by default and +returns an empty list if no files were found. + +Names can be specified in a variety of platform independent forms. Any +names in the form B<-lname> are converted into F<libname.*>, where F<.*> is +an appropriate suffix for the platform. + +If a name does not already have a suitable prefix and/or suffix then +the corresponding file will be searched for by trying combinations of +prefix and suffix appropriate to the platform: "$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 B<-Ldir>. Any other names +are treated as filenames to be searched for. + +Using arguments of the form C<-Ldir> and C<-lname> is recommended. + +Example: + + @dl_resolve_using = dl_findfile(qw(-L/usr/5lib -lposix)); + + +=item 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 F<dl_*.xs> file or code can be added to the autoloadable +dl_expandspec(0 function in F<DynaLoader.pm). See F<DynaLoader.pm> for more +information. + +=item dl_load_file() + +Syntax: + + $libref = dl_load_file($filename) + +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. + +(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 $libref in any way.) + +This is function that does the real work. It should use the current +values of @dl_require_symbols and @dl_resolve_using if required. + + SunOS: dlopen($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]) + + +=item dl_find_symbol() + +Syntax: + + $symref = dl_find_symbol($libref, $symbol) + +Return the address of the symbol $symbol or C<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,$symbol) + + +=item dl_undef_symbols() + +Example + + @symbols = dl_undef_symbols() + +Return a list of symbol names which remain undefined after load_file(). +Returns C<()> if not known. Don't worry if your platform does not provide +a mechanism for this. Most do not need it and hence do not provide it. + + +=item dl_install_xsub() + +Syntax: + + dl_install_xsub($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. + + +=item boostrap() + +Syntax: + +bootstrap($module) + +This is the normal entry point for automatic dynamic loading in Perl. + +It performs the following actions: + +=over 8 + +=item * + +locates an auto/$module directory by searching @INC + +=item * + +uses dl_findfile() to determine the filename to load + +=item * + +sets @dl_require_symbols to C<("boot_$module")> + +=item * + +executes an F<auto/$module/$module.bs> file if it exists +(typically used to add to @dl_resolve_using any files which +are required to load the module on the current platform) + +=item * + +calls dl_load_file() to load the file + +=item * + +calls dl_undef_symbols() and warns if any symbols are undefined + +=item * + +calls dl_find_symbol() for "boot_$module" + +=item * + +calls dl_install_xsub() to install it as "${module}::bootstrap" + +=item * + +calls &{"${module}::bootstrap"} to bootstrap the module + +=back + +=back + + +=head1 AUTHOR + +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, and others. + +Larry Wall designed the elegant inherited bootstrap mechanism and +implemented the first Perl 5 dynamic loader using it. + +Tim Bunce, 11 August 1994. |