DynaLoader - Dynamically load C libraries into Perl code

- NAME
- SYNOPSIS
- DESCRIPTION
- AUTHOR

# NAME

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

# 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. 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
      $dl_debug
      @dl_librefs
      @dl_modules
                                                      Implemented in:
      bootstrap($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 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_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 -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 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:

  - 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 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, January 1996.