=begin

= Ruby/DL

Ruby/DL provides an interface to the dynamic linker such as dlopen() on UNIX
and LoadLibrary() on Windows.

= Building and Installing

  $ ruby extconf.rb    # to create the Makefile
  $ make               # to build the library 'dl.so'
  $ make libtest.so    # to build the C library 'libtest.so' for the test script
  $ make test          # to run the test script
  $ make install       # to install the library
  $ make clean         # to remove the created files without Makefile
  $ make distclean     # to remove the all created files

= Using Ruby/DL

We should usually use DL::Importable module provided by "dl/import.rb".
It has high-level functions to access library functions. We use
DL::Importable module to extend a module as follows:

  require "dl/import"
  module LIBC
    extend DL::Importable
  end

Now we can use methods dlload and extern in this module. We load the
libraries using dlload, and define wrapper methods to library functions
using extern respectively as follows:

  module LIBC
    extend DL::Importable
    dlload "libc.so.6","libm.so.6"
    extern "int strlen(char*)"
  end
  # Note that we should not include the module LIBC from some reason.

We can call the library function strlen() using LIBC.strlen. If the first
character of given function name is an uppercase, the first character of the
defined method name becomes lowercase.
We can also construct memory images of structures and unions using functions
struct and union which are defined in "dl/struct.rb" as follows:

  require "dl/import"
  require "dl/struct"
  module LIBC
    extend DL::Importable
    Timeval = struct [       # define timeval structure.
      "long tv_sec",
      "long tv_uses",
    ]
  end
  val = LIBC::Timeval.malloc # allocate memory.

Notice that the above example takes LIBC::Timeval.malloc to allocate memory,
rather than LIBC::Timeval.new. It is because DL::Timeval.new is for wrapping
an object, PtrData, which has already been created.

We can define a callback using the module function "callback" as follows:

  module Foo
    extend DL::Importable
    def my_comp(str1,str2)
      str1 <=> str2
    end
    COMPARE = callback "int my_comp(char*,char*)"
  end

where Foo::COMPARE is a Symbol object which invokes the method "my_comp".

DL::Importable module is very useful. However, we sometimes encounter a case
that we must directly use low-level functions such as dlsym(). In such case,
we would use DL module functions. They are described in next section.

= DL module

Module DL consists of three classes, a few module functions and constants.
The class Symbol represents the symbol we can call. The class PtrData
indicates a memory block such as a pointer in C. An object instantiated from
the class Handle keeps a handle to opened library.

== Constants

* VERSION
* MAJOR_VERSION
* MINOR_VERSION
* PATCH_VERSION
* RTLD_GLOBAL
* RTLD_LAZY
* RTLD_NOW
* MAX_ARG
* MAX_CBARG
* MAX_CBENT

== Functions

* handle = dlopen(lib){|handle| ... }
  * is quite equal to `Handle.new(lib)'

* sym = set_callback(cbtype, entry){|args| ... }
* sym = set_callback(cbtype, entry, proc)
  * makes entry-th pre-defined function to call the proc or given block. the 
    entry-th pre-defined function is specified by cbtype and entry. cbtype is a
    prototype of the callback. see also the section `Type specifiers' about 
    cbtype.

* sym = get_callback(cbtype, entry)
  * returns the Proc object which is given by the above function
   `set_callback'.

* ptr = malloc(size, [free = nil])
  * allocates the size bytes, and returns the pointer as a PtrData object ptr.

* ptr = strdup(str)
  * returns a PtrData object ptr which represents the pointer to a new string
    which is a duplicate of the string str.

* size = sizeof(type)
  * returns the size of type. `sizeof("C") + sizeof("L")' is not equal to
    `sizeof("CL")'. the latter is assumed to returns the enough size of the
    structure `struct foo { char c; long l; }', but the size may not equal to
    `sizeof(foo)' of C.

== Handle class

* handle = Handle.new(lib){|handle| ... }
  * opens a library lib and returns a Handle object handle. if a block is
    given, the handle is automatically closed as the block ends.

* Handle#close
  * closes the handle opened by the above Handle.new(lib).

* sym = Handle#sym(func, prototype = "0"),
  sym = Handle#[func, prototype = nil]

  * obtains the pointer to a function called func and returns a Symbol object
    or a DataPtr object. prototype is a string which consists of type
    specifiers, it indicates the function's prototype. see also the section
    `Type specifiers'.

== Symbol class

* sym = Symbol.new(addr, type = nil, name = nil)
  * creates the Symbol object sym with the type type if type is not nil. addr
    is the address where the function is allocated. If type is nil, it returns
    a DataPtr object.

* Symbol::char2type(char)
  * takes a character char that represents a type and returns the type
    specifier of the C language.

* str = Symbol#proto()
  * returns the function prototype.

* str = Symbol#name()
  * Returns the function name.

* str = Symbol#cproto(),
  str = Symbol#to_s()
  * returns the prototype of the C language.

* str = Symbol#inspect()
  * returns the inspectable string.

* r,rs = Symbol#call(arg1,arg2,...,argN),
  r,rs = Symbol#[](arg1,arg2,...,argN)
  * calls the function with parameters arg1, arg2, ..., argN. and the result
    consists of the return value r and parameters rs. rs is an array.

* ptr = Symbol#to_ptr
  * returns the corresponding PtrData object ptr.

== PtrData class

* ptr = PtrData.new(addr, [size = 0, free = nil])
  * returns the PtrData object representing the pointer which indicates the
    address addr. GC frees the memory using the free function.

* PtrData#free=(sym)
  * If you specify a symbol object sym, GC frees the memory using the function
    represented by sym.

* sym = PtrData#free
  * returns a symbol object sym which is used when GC frees the memory. it
    usually configured by `PtrData#free=' or `PtrData.new'.

* size = PtrData#size, PtrData#size=(size)
  * gets and sets allocated size of the memory.

* ary = PtrData#to_a(type, [size])
  * returns an array of the type which specified with type. type must be one of
    'S','P','I','L','D' and 'F'.

* str = PtrData#to_s([len])
  * returns a string which length is len. if len is omitted, the end of the
    string is '\0'.

* ptr = PtrData#ptr,+@
  * returns the pointed value as a PtrData object ptr.

* ptr = PtrData#ref,-@
  * returns the reference as a PtrData object ptr.

* ptr = PtrData#+
  * returns the PtrData object

* ptr = PtrData#-
  * returns the PtrData object

* PtrData#struct!(type, *members)
  * defines the data type to get access to a structure member with a symbol.
    (see also PtrData#[])

* PtrData#union!(type, *members)
  * defines the data type to get access to a union member with a symbol. (see
    also PtrData#[])

* val = PtrData#[key], PtrData#[key, num = 0]
  * if the key is a string or symbol, this method returns the value of the
    structure/union member which has the type defined by PtrData#
    {struct!,union!}. if the key is a integer value and this object represents
    the pointer ptr, it returns the value of `(ptr + key).to_s(num)'

* PtrData#[key,num]=val, PtrData#[key]=val
  * if the key is a string or symbol, this method substitute the value of the
    structure/union member with val. if the key is a integer value and val is a
    string, this method copies num bytes of val to the memory area ptr using
    memcpy(3).

== Type specifiers

the prototype consists of the following type specifiers, first element of 
prototype represents the type of return value, and remaining elements represent
the type of each argument.

    C : char
    c : char *
    H : short
    h : short *
    I : int
    i : int *
    L : long
    l : long *
    F : float
    f : float *
    D : double
    d : double *
    S : const char *
    s : char *
    A : const type[]
    a : type[] (allocates new memory space)
    P : void * (same as 'p')
    p : void * (same as 'P')
    0 : void function (this must be a first character of the prototype)

the cbtype consists of type specifiers 0, C, I, H, L, F, D, S and P.
for example:

    DL.callback('IPP'){|ptr1,ptr2|
      str1 = ptr1.ptr.to_s
      str2 = ptr2.ptr.to_s
      str1 <=> str2
    }
=end