An ONC RPC Framework for Guile

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An ONC RPC Framework for Guile

This document describes GNU Guile-RPC version 0.3. It was last updated in August 2008.


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1 Introduction

GNU Guile-RPC is a framework for distributed programming under GNU Guile. It is a pure Scheme implementation of the ONC RPC standard, i.e., the “Open Network Computing” Remote Procedure Call standard. ONC RPC is standardized by the Internet Engineering Task Force (IETF) as RFC 1831. It is based on the External Data Representation standard (XDR), known as RFC 4506 (see References).

Remote procedure calls allow programs on different, potentially remote machines to interact together. A remote procedure call is the invocation of the procedure of a program located on a remote host (the RPC server), as the name implies. Doing so requires the procedure arguments on the client-side to be encoded, or marshalled, i.e., converted to a representation suitable for transfer over the network. On the server-side, upon reception of the RPC, those arguments must be decoded or unmarshalled, i.e., converted back to a form that is directly understandable by the server program—for instance, data using Scheme data types, should the server program be written in Scheme. The value returned by the RPC server must be encoded and decoded similarly.

When using the ONC RPC protocol, the way data items are encoded is dictated by the XDR standard. This encoding has the advantage of being particularly compact, allowing for relatively low bandwidth requirements and fast implementations, especially compared to more verbose RPC protocols such as XML-RPC and SOAP.

The XDR encoding is not self-describing, i.e., it is impossible given an arbitrary XDR encoded sequence to determine the XDR type of the encoded data. This is different from D-Bus, for example, which uses a compact and self-describing encoding. In practice, this is sufficient for a wide range of applications.

GNU Guile-RPC provides an easy access to the ONC RPC protocol for the Guile Scheme programmer. In particular, it allows standard Scheme types to be mapped to XDR data types, so that Scheme objects are easily encoded to or decoded from XDR.

Please send bug reports, comments and patches to the Guile-RPC mailing list.


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2 Obtaining and Installing GNU Guile-RPC

The GNU Guile-RPC web page is located at http://www.gnu.org/software/guile-rpc/. You can obtain copy of GNU Guile-RPC from ftp://alpha.gnu.org/gnu/guile-rpc/.

In order to use GNU Guile-RPC, all that is needed is GNU Guile 1.8 along with the R6RS library package for Guile 1.8, known as Guile-R6RS-Libs1.

Before version 1.8.3, Guile 1.8 did not contain an implementation of SRFI-35. Thus, if you are using one of these versions of Guile, you need to install Guile-Lib, which contains the relevant module.

GNU Guile-RPC comes with a compiler for the XDR/RPC interface description language (see Invoking grpc-compile). To use that, Guile-Lib version 0.1.7 or later is needed2.


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3 Quick Start

This section illustrates how ONC RPC clients and servers can be implemented using Guile-RPC. ONC RPC defines a language to describe RPCs and the data types they depend on (see RFC 4506, Section 6 and RFC 1831, Section 11). This language, which we refer to as the XDR/RPC language or simply RPC language, is essentially an interface description language (IDL). It resembles the C programming language and borrows C's type definition constructs and adds the program and version constructs that introduce RPC definitions.

Consider the following RPC definition, written using the XDR/RPC language:

     struct result_t
     {
       int          integer_part;
       unsigned int decimal_part;
     };
     
     program ARITHMETIC_PROGRAM
     {
       version ARITHMETIC_VERSION
       {
         /* Return the integer part and the 1000th of the
            given double-precision floating point number.  */
         result_t split_number (double) = 1;
       } = 0;
     } = 80000;

It defines a simple RPC interface named ARITHMETIC which contains only one procedure called split_number (). The interface itself has a program number that identifies it (here, 80000). Normally, program numbers below 20000000 (hexadecimal) are assigned by Sun Microsystems, Inc. and thus should not be used unless the number has been properly registered (see RFC 1831, for details). It also has a version number (here, 0) that is user-defined and should be increased when the interface changes (e.g., when procedures are added, modified, etc.). Finally, the procedure split_number () has a procedure number (here, 1) that allows it to be distinguished from other procedures.

People vaguely familiar with the C programming language should have guessed by now that this simple interface defines a procedure that takes a double-precision floating-point number and returns a structure that contains two fields.

Client and server creation are two-step. Since the first step—data type definition—is the same for both, that leaves us with a total of three steps, described below. Nicely, each of these steps can be automated using the XDR/RPC language compiler (see grpc-compile).

More details about the XDR type definition as well as client and server creation are available in the API reference (see API Reference).


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3.0.1 Defining Data Types

Before actually creating a client or server for this interface, one must define the types it uses. The simplest way to define one or several data types is to pipe their definition in XDR/RPC language through the compiler (see grpc-compile):

     $ grpc-compile --xdr --constants < input.x > types.scm

Given the description from input.x in RPC language, this command generates code that provides access to the constants and data types defined therein. The resulting Scheme code, types.scm, can then be loaded in other Scheme files (see Loading).

In addition, code in types.scm depends on Guile-RPC modules that it uses at run-time. Thus, it must first import the relevant modules:

     (use-modules (rpc xdr)
                  (rpc xdr types))

Then, the result_t type is defined (this is the code generated by the compiler but it can also be written “by hand”):

     (define result-type
       (make-xdr-struct-type (list xdr-integer             ;; `integer_part'
                                   xdr-unsigned-integer))) ;; `decimal_part'


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3.0.2 Creating the Client

Producing a client to invoke split_number () is as simple as this:

     (use-modules (rpc rpc))
     
     (define invoke-split-number
       (make-synchronous-rpc-call 80000 0    ;; program and version
                                  1          ;; procedure number
                                  xdr-double ;; argument type
                                  result-type))

Again, this definition, minus the use-modules clause, can alternatively be generated by the compiler from the RPC description in XDR/RPC language (see grpc-compile):

     $ grpc-compile --client < input.x > client.scm

Once this is done, invoking the procedure is as simple as this:

     (invoke-split-number 3.14 #x7777 socket)

The first argument to invoke-split-number is the argument of split_number (); the second argument is a transaction ID, i.e., an arbitrarily chosen number that identifies the remote procedure call; the third argument should be an output port, typically one bound to a connection to the RPC server:

     (define socket (socket PF_INET SOCK_STREAM 0))
     (connect socket AF_INET INADDR_LOOPBACK 6666)

This example creates an IPv4 connection to the local host on port 6666 (see Network Sockets and Communication).

On success, invoke-split-number returns a two-element list where the first element corresponds to the integer_part field of the result and the second element correspond to the decimal_part field of the result, both represented as Scheme exact integers.


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3.0.3 Creating the Server

Creating a TCP server for our RPC interface should be quite easy as well. We can re-use our previous type definitions (see Defining Data Types). Then, all we have to do is to create a definition for our program.

     (use-modules (rpc rpc server))
     
     (define (split-number-handler number)
       ;; Handle a `split-number' request.
       (let* ((int (floor number))
              (dec (floor (* 1000 (- number int)))))
         (list (inexact->exact int)
               (inexact->exact dec))))
     
     (define my-rpc-program
       ;; Define our RPC program.
       (let* ((proc    (make-rpc-procedure 1 xdr-double result-type
                                           split-number-handler))
              (version (make-rpc-program-version 0 (list proc))))
         (make-rpc-program 80000 (list version))))

Alternatively, using the compiler allows the above definition of my-rpc-program to be automatically generated from the XDR/RPC definition file (see grpc-compile):

     $ grpc-compile --server < input.x > server.scm

However, there is a slight difference compared to the above “hand-written” approach: server.scm does not contain the actual definition of my-rpc-program since it does not know about your split-number-handler procedure. Instead, given the RPC/XDR definition given earlier, it contains a make-ARITHMETIC-PROGRAM-server procedure; this procedure can be passed a list associating RPC names to Scheme procedures, and returns the resulting RPC program object:

     (define my-rpc-program
       (make-ARITHMETIC-PROGRAM-server
         `(("ARITHMETIC_VERSION"  ;; specify the target version
     
             ;; list all supported RPCs for this version
             ("split_number" . ,split-number-handler)))))

As can be seen, using the compiler-generated server stub, one doesn't have to deal explicitly with program, version and RPC numbers, which clarifies the code.

Finally, we can make the server listen for incoming connections and handle client requests, using Guile's networking primitives.

     ;; Creating a listening TCP socket.
     (define server-socket (socket PF_INET SOCK_STREAM 0))
     
     ;; Listen for connections on 127.0.0.1, port 6666.
     (bind server-socket AF_INET INADDR_LOOPBACK 6666)
     (listen server-socket 1024)
     
     ;; Go ahead and serve requests.
     (run-stream-rpc-server (list (cons server-socket my-rpc-program))
                            1000000 ;; a one-second timeout
                            #f      ;; we don't care about closed connections
                            (lambda () ;; our idle thunk
                              (format #t "one second passed~%")))

And now we're all set: We have a working TCP client and server for this wonderful RPC interface! This would work similarly for other stream-oriented transports such as Unix-domain sockets: only the socket and bind calls need to be adapted.


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4 API Reference

This section first details facilities available to manipulate XDR types. It then describes Scheme procedures that should be used to build ONC RPC clients and servers.


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4.1 Usage Notes

Guile-RPC makes use of the SRFI-34 exception mechanism along with SRFI-35 error conditions to deal with the various protocol errors3. Thus, users are expected to use these mechanisms to handle the error conditions described in the following sections. Hopefully, in most cases, error conditions raised by Guile-RPC code provide users with detailed information about the error.


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4.2 Implementation of XDR

This section describes how XDR types are represented in Guile-RPC, as well as how one can encode Scheme values to XDR and decode XDR binary data to Scheme values.


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4.2.1 XDR Type Representations

The XDR standard defines various basic data types and allows for the definition of compound data types (“structs”), fixed-size and variable-size arrays as well as “unions”. Fixed-size arrays and structs can actually be thought as the same type: Their size if known in advance and they are encoded as the succession of the data they contain. Thus, those types can be summarized as 4 great classes: “basic” types, variable-length arrays, structs and unions.

The (rpc xdr) module provides facilities to create Scheme objects representing XDR data types and to manipulate them. These Scheme objects, described below, are all immutable, i.e., they cannot be modified after creation.

— Scheme Procedure: make-xdr-basic-type name size type-pred encoder decoder [vector-encoder [vector-decoder]]

This returns an <xdr-basic-type> object describing a type whose encoding fits on size octets, and which is encoded/decoded using the encoder and decoder procedures. type-pred should be a predicate checking the validity of an input Scheme value for encoding into this type.

Optionally, vector-encoder and vector-decoder can be passed and should be procedures that efficiently encode/decode sequences of data items of this type (for instance, the vector decoder could use the bytevector->int-list procedure of the (r6rs bytevector) module to speed up decoding). The vector encoder is invoked as (vector-encoder type value bv index) while the vector decoder is invoked as (vector-decoder type count port).

Users should normally not need to define new basic types since all the basic types defined by XDR are already available in (rpc xdr types) (see XDR Standard Data Types). Thus, we will not describe its use further.

— Scheme Procedure: make-xdr-struct-type base-types

Return a new XDR struct type made of a sequence of XDR data items whose types are listed in base-types.

Struct types encode from/decode to Scheme lists whose length is that of base-types.

— Scheme Procedure: make-xdr-vector-type base-type max-element-count

Return an object describing an XDR variable-length array of elements of types base-type (again, an XDR type object). If max-element-count is a number, then it describes the maximum number of items of type base-type that are allow in actual arrays of this type. If base-type is #f, then arrays of this type may contain up to 2^32 - 1 items of type base-type.

Vector types are encoded from generalized vectors such as Scheme vectors, SRFI-4 vectors or strings (see Generalized Vectors). By default, vector types decode to vectors, but any other kind of generalized vector can be used: it only needs to be specified as the vector-decoder argument of make-xdr-basic-type for the corresponding base type. Of course, SRFI-4 vectors, for example, may only be used when an XDR integer vector with a matching integer range is expected.

If max-element-count is specified and a vector to be encoded contains more than max-element-count elements, then an &xdr-vector-size-exceeded-error error condition is raised. Likewise, if XDR data to be decoded describes vectors larger than max-element-count, this error condition is raised.

— Scheme Procedure: make-xdr-union-type discr-type discr/type-alist default-type

Return a new XDR discriminated union type, using discr-type as the discriminant type (which must be a 32-bit basic type) and discr/type-alist to select the “arm” type depending on the discriminant value. If no suitable value is found in discr/type-alist and default-type is not #f, then default type is used as the arm type.

Union types encode from/decode to Scheme pairs whose car is the discriminant's value and whose cdr is the actual union value.

— Scheme Procedure: xdr-union-arm-type union discriminant

Return the type that should be used for union's arm given discriminant (a Scheme value).

Sometimes, one may want to define recursive types, i.e., types that refer to themselves. This is particularly useful to implement lists. For example, in XDR language, a list of integers can be defined as follows:

     struct integer_list_t
     {
       int x;
       integer_list_t *next;
     };

This notation is a shortcut for the following structure:

     struct integer_list_t
     {
       int x;
       union switch (bool opted)
       {
         case TRUE:
           integer_list_t value;
         case FALSE:
           void;
       } next;
     };

The integer_list_t type references itself. Defining it using our API seems impossible at first: one cannot pass a self-reference to make-xdr-struct-type (since the object is not yet created!), and the self-reference cannot be added after the fact since objects returned by make-xdr-struct-type are immutable.

The API addresses this problem by allowing thunks (zero-argument procedures) to be used as types. Together with Scheme's letrec recursive binding construct or a top-level define (see letrec), it makes it possible to create such recursive types:

     (letrec ((integer-list
               (make-xdr-struct-type
                (list xdr-integer
                      (make-xdr-union-type xdr-boolean
                                           `((TRUE . ,(lambda ()
                                                        integer-list))
                                             (FALSE . ,xdr-void))
                                           #f)))))
       integer-list)

The trick here is that using the thunk effectively defers the evaluation of the self-reference4.

It is often useful to know the size in octets it takes to encode a given value according to an XDR type. However, as we just saw, the size of some XDR types (variable-length arrays and unions) cannot be known in advance: The encoding size depends on the actual value to encode. The following procedure allow the computation of the size of the XDR representation of some value.

— Scheme Procedure: xdr-type-size type value

Return the size (in octets) of type when applied to value. type must be an XDR type object returned by one of the above procedures, while value should be a Scheme value suitable for encoding with type.

The following section lists the standard XDR data types.


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4.2.2 XDR Standard Data Types

All the basic data types defined by XDR are defined in the (rpc xdr types) module.

— Scheme Variable: xdr-integer
— Scheme Variable: xdr-unsigned-integer
— Scheme Variable: xdr-hyper-integer
— Scheme Variable: xdr-unsigned-hyper-integer

XDR's 32-bit and 64-bit signed and unsigned integer types. This type decodes to/encodes from Scheme exact numbers.

— Scheme Variable: xdr-float
— Scheme Variable: xdr-double

32-bit and 64-bit IEEE-754 floating point numbers. This type decodes to/encodes from Scheme inexact numbers. Note that XDR also defines a “quadruple-precision floating point type” (i.e., 128-bit long) that is currently not available (FIXME).

— Scheme Variable: xdr-void

The “void” type that yields zero bits. Any Scheme value is suitable as an input when encoding with this type. When decoding this type, the %void value (which may be compared via eq?) is returned.

XDR provides support for “enumerations”, similar to that found in C. An enumeration type maps symbols to integers and are actually encoded as 32-bit integers.

— Scheme Procedure: make-xdr-enumeration name enum-alist

Return an enumeration type that obeys the symbol-integer mapping provided in enum-alist which should be a list of symbol-integer pairs. The returned type decodes to/encodes from Scheme symbols, as provided in enum-alist. Upon decoding/encoding of an enumeration, an &xdr-enumeration-error is raised if an incorrect value (i.e., one not listed in enum-alist) is found.

— Scheme Variable: xdr-boolean

XDR's boolean type which is an enumeration. It encodes to/decodes from Scheme symbols TRUE and FALSE.

Several fixed-size and variable-size are predefined in the standard.

— Scheme Procedure: make-xdr-fixed-length-opaque-array size

Return a fixed-length “opaque” array of size octets. An opaque array is simply a sequence of octets.

The returned XDR type object is actually an <xdr-struct-type> object. Thus, it encodes from/decodes to Scheme lists of exact integers. Conversion to a Scheme string, if needed, is left to the user.

— Scheme Procedure: make-xdr-variable-length-opaque-array limit

Return a variable-length opaque array. As for make-xdr-vector-type (see XDR Type Representations), limit can be either a number specifying the maximum number of elements that can be held by the created type, or #f in which case the variable-length array can hold up to 2^32 - 1 octets.

The returned XDR type object is actually an <xdr-vector-type> object. Thus, it encodes from/decodes to Scheme vectors of exact integers.

— Scheme Procedure: make-xdr-string limit

This is a synonym of make-xdr-variable-length-opaque-array since XDR's string type actually represents ASCII strings, i.e., sequences of octets.

— Scheme Variable: xdr-variable-length-opaque-array
— Scheme Variable: xdr-string

These convenience variables contain the unlimited variable-length opaque array.


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4.2.3 XDR Encoding and Decoding

The following primitives are exported by the (rpc xdr) module. They implement the encoding of Scheme values to XDR data types, and the decoding of binary XDR data to Scheme values. The exact mapping between XDR data types and Scheme data types has been discussed earlier.

— Scheme Procedure: xdr-encode! bv index type value

Encode value (a suitable Scheme value), using XDR type type, into bytevector bv at index. Return the index where encoding ended.

bv should be an R6RS bytevector large enough to hold the XDR representation of value according to type. To that end, users may rely on xdr-type-size (see XDR Type Representations).

Error conditions sub-classing &xdr-error may be raised during encoding. The exact exceptions that may be raised are type-dependent and have been discussed in the previous sections.

— Scheme Procedure: xdr-decode type port

Decode from port (a binary input port) a value of XDR type type. Return the decoded value.

Error conditions sub-classing &xdr-error may be raised during encoding. The exact exceptions that may be raised are type-dependent and have been discussed in the previous sections.


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4.3 Implementation of ONC RPC

This section describes facilities available for the creation of ONC RPC clients and servers, as well as lower-level details about raw RPC messages.


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4.3.1 Building an RPC Client

Basic building blocks for the creation of RPC clients are provided by the (rpc rpc) module. The easiest way to build an RPC client is through make-synchronous-rpc-call.

— Scheme Procedure: make-synchronous-rpc-call program version procedure arg-type result-type

Return a procedure that may be applied to a list of arguments, transaction ID (any unsigned number representable on 32 bits), and I/O port, to make a synchronous RPC call to the remote procedure numbered procedure in program, version version. On success, the invocation result is eventually returned. Otherwise, an error condition is raised. arg-type and result-type should be XDR type objects (see XDR Type Representations).

Error conditions that may be raised include those related to XDR encoding and decoding (see XDR Encoding and Decoding), as well as RPC-specific error conditions inheriting from &rpc-error (i.e., conditions that pass the rpc-error? predicate). These are detailed in assert-successful-reply.

For an example, see Creating the Client.

It is also possible to create “one-way” calls, i.e., RPC calls that do not expect a reply (i.e., no return value, not even void). This is useful, for instance, to implement batched calls where clients do not wait for the server to reply (see Section 7.4.1 of RFC 1831). Asynchronous calls can be implemented in terms of this, too.

— Scheme Procedure: make-one-way-rpc-call program version procedure arg-type result-type

Similar to make-synchronous-rpc-call, except that the returned procedure does not wait for a reply.


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4.3.2 Building an RPC Server

The (rpc rpc server) module provides helpful facilities for building an ONC RPC server. In particular, it provides tools to decode RPC call messages, as well as an event loop mechanisms that allows RPC calls to be automatically dispatched to the corresponding Scheme handlers.

— Scheme Procedure: procedure-call-information call-msg

Return an <rpc-call> object that denotes the procedure call requested in call-msg (the result of an (xdr-decode rpc-message port) operation). If call-msg is not an appropriate RPC call message, an error condition is raised.

The error condition raised may be either onc-rpc-version-mismatch-error? or rpc-invalid-call-message-error?.

The returned object can be queried using the rpc-call- procedures described below.

— Scheme Procedure: rpc-call-xid call

Return the transaction ID (“xid” for short) of call.

— Scheme Procedure: rpc-call-program call
— Scheme Procedure: rpc-call-version call
— Scheme Procedure: rpc-call-procedure call

Return the program, version or procedure number of call.

— Scheme Procedure: rpc-call-credentials call
— Scheme Procedure: rpc-call-verifier call

Return the credentials and verifier provided by the client for call. FIXME: As of version 0.3, this information is not usable.

The following procedures allow the description of RPC “programs”. Such descriptions can then be readily used to produced a full-blown RPC processing loop.

— Scheme Procedure: make-rpc-program number versions

Return a new object describing the RPC program identified by number and consisting of the versions listed in versions.

— Scheme Procedure: make-rpc-program-version number procedures

Return a new object describing the RPC program version identified by number and consisting of the procedures listed in procedures.

— Scheme Procedure: make-rpc-procedure number argument-xdr-type result-xdr-type handler [one-way?]

Return a new object describing a procedure whose number is number, whose argument type is argument-xdr-type and whose result type is result-xdr-type (see XDR Type Representations). handler should be a procedure that will be invoked upon reception of an RPC call for that procedure.

If synchronous RPC processing is used, i.e., through serve-one-stream-request, then handler is passed the decoded argument and should return a result type that may be encoded as result-xdr-type. If asynchronous processing is used, i.e., through serve-one-stream-request/asynchronous, then handler is passed the decoded argument along with a continuation that must be invoked to actually return the result.

If one-way? is passed and is true, then the returned procedure is marked as “one-way” (see make-one-way-rpc-call). For one-way procedures, run-stream-rpc-server and similar procedures ignores the return value of handler and don't send any reply when procedure number is invoked.

Once a program, its versions and procedures have been defined, an RPC server for that program (and possibly others) can be run using the following procedures.

— Scheme Procedure: run-stream-rpc-server sockets+rpc-programs timeout close-connection-proc idle-thunk

Run a full-blown connection-oriented (i.e., SOCK_STREAM, be it PF_UNIX or PF_INET) RPC server for the given listening sockets and RPC programs. sockets+rpc-programs should be a list of listening socket-RPC program pairs (where “RPC programs” are objects as returned by make-rpc-program). timeout should be a number of microseconds that the loop should wait for input; when no input is available, idle-thunk is invoked, thus at most every timeout microseconds. If close-connection-proc is a procedure, it is called when a connection is being closed is passed the corresponding <stream-connection> object.

While an RPC server is running over a stream-oriented transport such as TCP using run-stream-rpc-server, its procedure handlers can get information about the current connection and client:

— Scheme Procedure: current-stream-connection

This procedure returns a <stream-connection> object describing the current TCP connection (when within a run-stream-rpc-server invocation). This object can be queried with the procedures described below.

— Scheme Procedure: stream-connection? obj

Return #t if obj is a <stream-connection> object.

— Scheme Procedure: stream-connection-port connection

Return the I/O port (not the TCP port) for connection.

— Scheme Procedure: stream-connection-peer-address connection

Return the IP address of the peer/client of connection (see Network Socket Address).

— Scheme Procedure: stream-connection-rpc-program connection

Return the RPC program object corresponding to connection.

For a complete RPC server example, Creating the Server.

The run-stream-rpc-server mechanism is limited to servers managing only RPC connections, and only over stream-oriented transports. Should your server need to handle other input sources, a more geneneral event handling mechanism is available. This works by first creating a set of I/O managers and then passing run-input-event-loop a list of I/O manager-file descriptor pairs to actually handle I/O events.

— Scheme Procedure: make-i/o-manager exception-handler read-handler

Return an I/O manager. When data is available for reading, read-handler will be called and passed a port to read from; when an exception occurs on a port, exception-handler is called and passed the failing port.

— Scheme Procedure: i/o-manager? obj

Return #t if obj is an I/O manager.

— Scheme Procedure: i/o-manager-exception-handler manager
— Scheme Procedure: i/o-manager-read-handler manager

Return, respectively, the exception handler and the read handler of manager.

— Scheme Procedure: run-input-event-loop fd+manager-list timeout idle-thunk

Run an input event loop based on fd+manager-list, a list of pairs of input ports (or file descriptors) and I/O managers. I/O managers are invoked are invoked and passed the corresponding port when data becomes readable or when an exception occurs. I/O manager handlers can:

When timeout (a number of microseconds) is reached, idle-thunk is invoked. If timeout is #f, then an infinite timeout is assumed and idle-thunk is never run. The event loop returns when no watched port is left.

The event loop provided by run-input-event-loop should cover a wide range of applications. However, it will turn out to be insufficient in situations where tasks must be executed at specific times, and where the interval between consecutive executions varies over the program's lifetime.

Finally, a lower-level mechanism is available to handle a single incoming RPC:

— Scheme Procedure: serve-one-stream-request program port

Serve one RPC for program, reading the RPC from port (using the record-marking protocol) and writing the reply to port. If port is closed or the end-of-file was reached, an &rpc-connection-lost-error is raised.

— Scheme Procedure: serve-one-stream-request/asynchronous program port

Same as serve-one-stream-request except that the RPC is to be handled in an asynchronous fashion.

Concretely, the procedure handler passed to make-rpc-procedure is called with two arguments instead of one: the first one is the actual procedure argument, and the second one is a one-argument procedure that must be invoked to return the procedure's result—in other words, procedure call processing is decoupled from procedure call return using continuation-passing style.


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4.3.3 ONC RPC Message Types

The (rpc rpc types) module provides a representation of the various XDR types defined in the standard to represent RPC messages (see References). We only describe the most important ones as well as procedures from the (rpc rpc) module that help use it.

— Scheme Variable: rpc-message

This variable contains a XDR struct type representing all possible RPC messages—the rpc_msg struct type defined in RFC 1831. By “rpc message” we mean the header that is transmitted before the actual procedure argument to describe the procedure call being made.

Roughly, this header contains a transaction ID allowing clients to match call/reply pairs, plus information describing either the call or the reply being made. Calls essentially contain a program, version and procedure numbers. Replies, on the other hand, can be more complex since they can describe a large class of errors.

— Scheme Variable: rpc-message-type

This variable is bound to an XDR enumeration. Its two possible values are CALL and REPLY (both represented in Scheme using symbols), denoting a procedure call and a reply to a procedure call, respectively.

— Scheme Procedure: make-rpc-message xid type args ...

Return an rpc-message datum. type should be either CALL or REPLY (the two values of the rpc-message-type enumeration). The arguments args are message-type-specific. For example, a message denoting a procedure call to procedure number 5 of version 1 of program 77 can be created as follows:

          (define my-call-msg
            (make-rpc-message #x123 ;; the transaction ID
                              'CALL ;; the message type
                              77 1 5))

It can then be encoded in the usual way (see XDR Encoding and Decoding):

          (let* ((size (xdr-type-size rpc-message my-call-msg))
                 (bv   (make-bytevector size)))
            (xdr-encode! bv 0 rpc-message my-call-msg)
            ;;; ...
            )

Likewise, a reply message denoting a successful RPC call can be produced as follows:

          (make-rpc-message xid 'REPLY 'MSG_ACCEPTED 'SUCCESS)

It is worth noting that in practice, “messages” of type rpc-message are followed by additional data representing either the procedure call arguments (if the message is a CALL message) or the procedure return value (if the message is a REPLY message).

— Scheme Procedure: assert-successful-reply rpc-msg xid

Return true if rpc-msg (an RPC message as returned by a previous (xdr-decode rpc-message port) call) is a valid reply for the invocation labeled with transaction ID xid indicating that it was accepted. If xid is #t, any reply transaction ID is accepted and it is returned (provided the rest of the message denotes an accepted message). On failure, an appropriate error condition is raised.

The error conditions that may be raised obey rpc-error? and rpc-call-error?. More precisely, error conditions include the following:

rpc-program-unavailable-error?
If rpc-msg denotes the fact that the program requested by the corresponding RPC call is not available.
rpc-program-mismatch-error?
If the corresponding RPC call requested a program version that is not available. The procedures rpc-program-mismatch-error:low-version and rpc-program-mismatch-error:high-version return, respectively, the lowest and highest version numbers supported by the remote server.
rpc-procedure-unavailable-error?
If the corresponding RPC call requested a procedure that is not available.
rpc-garbage-arguments-error?
If the remote server failed to decode the procedure arguments.
rpc-system-error?
If the remote server failed to allocate enough memory for argument decoding, for instance.


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4.3.4 Record Marking Standard

The ONC RPC standard defines a record-marking protocol for stream-oriented transport layers such as TCP whereby (1) each RPC message is sent out as a single record and (2) where records may be split into several fragments. This allows implementations to “delimit one message from another in order to detect and possibly recover from protocol errors” (see RFC 1831, Section 10).

This protocol is implemented by the (rpc rpc transports) module. It is automatically used by the high-level client and server facilities, namely make-synchronous-rpc-call and run-stream-rpc-server. The available facilities are described below.

— Scheme Procedure: send-rpc-record port bv offset len

Send the RPC message of len octets encoded at offset offset in bv (a bytevector) to port. This procedure sends the len octets of the record without fragmenting them.

— Scheme Procedure: make-rpc-record-sender fragment-len

This procedure is a generalization of send-rpc-record.

Return a procedure that sends data according to the record marking standard, chopping its input bytevector into fragments of size fragment-len octets.

— Scheme Procedure: rpc-record-marking-input-port port

Return a binary input port that proxies port in order to implement decoding of the record marking standard (RFC 1831, Section 10).


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4.4 Standard RPC Programs

GNU Guile-RPC provides client-side and/or server-side of some commonly found ONC RPC program, which are described below. Currently, this is limited to the portmapper interface, but other interfaces (e.g., “mount”, NFSv2) may follow.


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4.4.1 The Portmapper Program

The (rpc rpc portmap) module implements the widespread portmapper RPC program defined in RFC 1833 (see RFC 1833, Section 3). As the name suggests, the portmapper interface allows servers to be queried for the association between an RPC service and the port it is listening to. It also allows clients to query the list of services registered.

In practice, most machines run a system-wide portmap daemon on port 111 (TCP or UDP), and it is this server that is queried for information about locally hosted RPC programs. The grpc-rpcinfo program is a portmapper client that can be used to query a portmapper server (see the grpc-rpcinfo tool)

Note that registering RPC programs with the portmapper is optional: it is basically a directory mechanism that allows servers to be located quite easily, but other existing mechanisms could be used for that purpose, e.g., decentralized service discovery (see service discovery with DNS-SD in Guile-Avahi).

The module exports client-side procedures, as returned by make-synchronous-rpc-call (see Building an RPC Client), for the various portmapper procedures. They are listed below.

— Scheme Procedure: portmapper-null arg xid port

Invoke the null RPC over port, ignoring arg, and return %void.

— Scheme Procedure: portmapper-set arg xid port

Invoke the set RPC over port with argument arg. The invoked server should register the RPC program specified by arg, where arg must be an XDR struct (i.e., a Scheme list) containing these four elements: the RPC program number, its version number, its protocol and its port. The protocol number should be one of IPPROTO_TCP or IPPROTO_UDP (see Network Sockets and Communication). An XDR boolean is returned, indicating whether the request successful.

— Scheme Procedure: portmapper-unset arg xid port

Invoke the unset RPC over port with argument arg. The invoked server should unregister the RPC program specified by arg, where arg must have the same form as for portmapper-set. Again, an XDR boolean is returned, indicating whether the request was successful.

— Scheme Procedure: portmapper-get-port arg xid port

Invoke the get-port RPC over port with argument arg, which must have the same form as previously mentioned (except that its port number is ignored). The invoked server returns an unsigned integer indicating the port of that RPC program.

— Scheme Procedure: portmapper-dump arg xid port

Invoke the dump RPC over port, ignoring arg. The invoked server should return a list of 4-element lists describing the registered RPC programs. Those four element list are the same as for portmapper-set and portmapper-get, namely the RPC program number and version, its protocol and its port.

— Scheme Procedure: portmapper-call-it arg xid port

Invoke the call-it procedure over port. Quoting RFC 1833, this procedure “allows a client to call another remote procedure on the same machine without knowing the remote procedure's port number”. Concretely, it makes the portmapper invoke over UDP the procedure of the program matching the description in arg, where arg is an XDR struct (i.e., a Scheme list) containing an RPC program and version number, a procedure number, and an opaque array denoting the procedure arguments (an xdr-variable-length-opaque-array).

On success, it returns a struct consisting of the port number of the matching program and an opaque array representing the RPC reply. On failure, it does not return. Therefore, this synchronous call version may be inappropriate. We recommend that you do not use it.

The portmap module also provides convenience functions to retrieve the symbolic name associated with common RPC program numbers. The association between program numbers and their name is usually stored in /etc/rpc on Unix systems and it can be parsed using the read-rpc-service-list procedure.

— Scheme Procedure: read-rpc-service-list port

Return a list of name-program pairs read from port (e.g., the /etc/rpc file), showing the connection between an RPC program human-readable name and its program number.

— Scheme Procedure: lookup-rpc-service-name service-list program

Lookup RPC program numbered program in service-list (a list as returned by read-rpc-service-list) and return its human-readable name.

— Scheme Procedure: lookup-rpc-service-number service-list program

Lookup RPC program named program in service-list (a list as returned by read-rpc-service-list) and return its RPC program number.


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4.5 XDR/RPC Language Compiler

This section describes the compiler's programming interface. Most of the time, its command-line interface is all is needed; it is described in grpc-compile. This section is intended for users who need more flexibility than is provided by grpc-compile.

The compiler consists of a lexer, a parser and two compiler back-ends. The lexer separates input data into valid XDR/RPC language tokens; the parser then validates the input syntax and produces an abstract syntax tree of the input. Finally, the back-ends are responsible for actually “compiling” the input into something usable by the programmer. The back-end used by the grpc-compile command is the code generation back-end. In addition, an experimental run-time compiler back-end is available, making it possible to compile dynamically definitions in the XDR/RPC language at run-time; this technology paves the way for a wide range of crazy distributed applications, the programmer's imagination being the only real limitation5.


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4.5.1 Parser

The parser is available under the (rpc compiler parser) module. The main procedure, rpc-language->sexp, reads XDR/RPC language descriptions and returns the abstract syntax tree (AST) in the form of an S-expression. The AST can be shown using the --intermediate option of the grpc-compile command-line tool (see Invoking grpc-compile). Below is an illustration of the mapping between the XDR/RPC language and the S-exp representation.

     const SIZE = 10;
     struct foo
     {
       int x;
       enum { NO = 0, YES = 1 } y;
       float z[SIZE];
     };

... yields:

     (define-constant "SIZE" 10)
     (define-type
       "foo"
       (struct
         ("x" "int")
         ("y" (enum ("NO" 0) ("YES" 1)))
         ("z" (fixed-length-array "float" "SIZE"))))
— Scheme Procedure: rpc-language->sexp port

Read a specification written in the XDR Language from port and return the corresponding sexp-based representation. This procedure can raise a &compiler-error exception (see below).

The behavior of the parser can be controlled using the *parser-options* parameter object:

— Scheme Variable: *parser-options*

This SRFI-39 parameter object must be a list of symbols or the empty list. Each symbol describes an option. For instance, allow-unsigned instructs the parser to recognize unsigned as if it were unsigned int (see Sun XDR/RPC language extensions).

Source location information is attached to the S-expressions returned by rpc-language->sexp. It can be queried using the procedures below. Note that not-only top-level S-expressions (such as define-type or define-constant expressions) can be queried, but also sub-expressions, e.g., the enum S-expression above.

— Scheme Procedure: sexp-location sexp

Return the source location associated with sexp or #f if no source location information is available.

— Scheme Procedure: location-line loc
— Scheme Procedure: location-column loc
— Scheme Procedure: location-file loc

Return the line number, column number or file name from location loc, an object returned by sexp-location.

In case of parse errors or other compiler errors, a &compiler-error error condition (or an instance of a sub-type thereof) may be raise.

— Scheme Variable: &compiler-error

The “compiler error” error condition type.

— Scheme Procedure: compiler-error? c

Return #t if c is a compiler error.

— Scheme Procedure: compiler-error:location c

Return the source location information associated with c, or #f if that information is not available.


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4.5.2 Code Generation Compiler Back-End

The code generation back-end is provided by the (rpc compiler) module. Given an XDR/RPC description, it returns a list of S-expressions, each of which is a top-level Scheme expression implementing an element of the input description. These expressions are meant to be dumped to a Scheme file; this is what the command-line interface of the compiler does (see grpc-compile).

Here is an example XDR/RPC description and the resulting client code, as obtained, e.g., with grpc-compile --xdr --constants --client:

     const max = 010;
     
     struct foo
     {
       int   x;
       float y<max>;
     };
     
     =>
     
     (define max 8)
     (define foo
       (make-xdr-struct-type
         (list xdr-integer
               (make-xdr-vector-type xdr-float max))))

As can be seen here, the generated code uses the run-time support routines described earlier (see Implementation of XDR); an optimization would consist in generating specialized code that does not depend on the run-time support, but it is not implemented yet.

This front-end consists of two procedures:

— Scheme Procedure: rpc-language->scheme-client input type-defs? constant-defs?
— Scheme Procedure: rpc-language->scheme-server input type-defs? constant-defs?

These procedures return a list of top-level Scheme expressions implementing input for an RPC client or, respectively, a server.

input can be either an input port, a string, or an AST as returned by rpc-language->sexp (see Parser). If type-defs? is #t, then type definition code is produced; if constant-defs? is #t, then constant definition code is produced.

Both procedures can raise error conditions having a sub-type of &compiler-error.


Previous: Code Generation Compiler Back-End, Up: Compiler

4.5.3 Run-Time Compiler Back-End

The run-time compiler back-end is also provided by the (rpc compiler) module. It reads XDR/RPC definitions and returns data structures readily usable to deal with the XDR data types or RPC programs described, at run-time. Actually, as of version 0.3, it does not have an API to deal with RPC programs, only with XDR data types.

— Scheme Procedure: rpc-language->xdr-types input

Read XDR type definitions from input and return an alist; element of the returned alist is a pair whose car is a string naming an XDR data type and whose cdr is an XDR data type object (see XDR Type Representations). input can be either an input port, a string, or an AST as returned by rpc-language->sexp (see Parser).

This procedure can raise error conditions having a sub-type of &compiler-error.

Here is an example of two procedures that, given XDR type definitions, decode (respectively encode) an object of that type:

     (use-modules (rpc compiler)
                  (rpc xdr)
                  (r6rs bytevector)
                  (r6rs io ports))
     
     (define (decode-data type-defs type-name port)
       ;; Read binary data from PORT as an object of type
       ;; TYPE-NAME whose definition is given in TYPE-DEFS.
       (let* ((types (rpc-language->xdr-types type-defs))
              (type  (cdr (assoc type-name types))))
         (xdr-decode type port)))
     
     (define (encode-data type-defs type-name object)
       ;; Encode OBJECT as XDR data type named TYPE-NAME from
       ;; the XDR type definitions in TYPE-DEFS.
       (let* ((types (rpc-language->xdr-types type-defs))
              (type  (cdr (assoc type-name types)))
              (size  (xdr-type-size type object))
              (bv    (make-bytevector size)))
         (xdr-encode! bv 0 type object)
         (open-bytevector-input-port bv)))

These procedures can then be used as follows:

     (let ((type-defs (string-append "typedef hyper chbouib<>;"
                                     "struct foo { "
                                     "  int x; float y; chbouib z;"
                                     "};"))
           (type-name "foo")
           (object    '(1 2.0 #(3 4 5))))
       (equal? (decode-data type-defs type-name
                            (encode-data type-defs type-name
                                         object))
               object))
     
     =>
     
     #t

Note that in this example type-defs contains two type definitions, which is why the type-name argument is absolutely needed.


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5 Stand-Alone Tools

GNU Guile-RPC comes with stand-alone tools that can be used from the command-line.


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5.1 Invoking grpc-compile

The grpc-compile command provides a simple command-line interface to the XDR/RPC language compiler (see Compiler). It reads a RPC definitions written in the XDR/RPC language on the standard input and, depending on the options, write Scheme code containing client, server, data type or constant definitions on the standard output.

--help
Print a summary of the command-line options and exit.
--version
Print the version number of GNU Guile-RPC and exit.
--xdr
-x
Compile XDR type definitions.
--constants
-C
Compile XDR constant definitions.
--client
-c
Compile client RPC stubs.
--server
-s
Compile server RPC stubs.
--strict
-S
Use strict XDR standard compliance per RFC 4506, Section 6. By default, the compiler recognizes extensions implemented by Sun Microsystems, Inc., and also available in the GNU C Library's rpcgen. These extensions include:

Also note that some XDR/RPC definition files (.x files) originally designed to be used in C programs with rpcgen include C preprocessor directives. Unlike rpcgen, which automatically invokes cpp, such input files need to be piped through cpp -P before being fed to grpc-compile.

--intermediate
Output the intermediate form produced by the parser (see Parser).

Code generation options can be combined. For instance, the command line below writes data type and constant definitions as well as client stubs in a single file:

     $ grpc-compile --xdr --constants --client < input.x > client-stubs.scm

The various pieces of generated code can also be stored in separate files. The following example shows how to create one file containing constant and type definitions, another one containing client stubs, and a third one containing server stubs. Since the two last files depend on the first one, care must be taken to load them beforehand.

     $ grpc-compile --xdr --constants < input.x > types+constants.scm
     $ echo '(load "types+constants.scm")' > client-stubs.scm
     $ grpc-compile --client < input.x >> client-stubs.scm
     $ echo '(load "types+constants.scm")' > server-stubs.scm
     $ grpc-compile --server < input.x >> server-stubs.scm

In the future, there may be additional --use-module and --define-module options to make it easier to use Guile's module system in generated code.


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5.2 Invoking grpc-rpcinfo

This program is equivalent to the rpcinfo program available on most Unix systems and notably provided by the GNU C Library. In is a client of the portmapper RPC program (see The Portmapper Program). Among the options supported by rpcinfo, only a few of them are supported at this moment:

--help
Print a summary of the command-line options and exit.
--version
Print the version number of GNU Guile-RPC and exit.
--dump
-p
Query the portmapper and list the registered RPC services.
--delete
-d
Unregister the RPC program with the given RPC program and version numbers from the portmapper.

Note that the host where the portmapper lives can be specified as the last argument to grpc-rpcinfo:

     # Query the portmapper at host `klimt'.
     
     $ grpc-rpcinfo -p klimt
     program vers    proto   port    name
     100000  2       tcp     111     portmapper
     100000  2       udp     111     portmapper
     $ grpc-rpcinfo -d 100000 2 klimt
     ERROR: `portmapper-unset' failed FALSE


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5.3 Invoking grpc-nfs-export

Guile-RPC comes with an example NFS (Network File System) server, provided by the grpc-nfs-export command. More precisely, it implements NFS version 2, i.e., the NFS_PROGRAM RPC program version 2 along with the MOUNTPROG program version 1, which are closely related (see RFC 1094). It is a TCP server.

Enough technical details. The important thing about grpc-nfs-export is this: although it's of little use in one's everyday life, this NFS server is nothing less than life-changing. It's different from any file system you've seen before. It's the ultimate debugging aid for any good Guile hacker.

The “file hierarchy” served by grpc-nfs-export is—guess what?—Guile's module hierarchy! In other words, when mounting the file system exported by grpc-nfs-export, the available files are bindings, while directories represent modules (see The Guile module system). The module hierarchy can also be browsed from the REPL using Guile's nested-ref procedure. Here's a sample session:

     $ ./grpc-nfs-export &
     
     $ sudo mount -t nfs -o nfsvers=2,tcp,port=2049 localhost: /nfs/
     
     $ ls /nfs/%app/modules/
     guile/  guile-rpc/  guile-user/  ice-9/  r6rs/  rpc/  srfi/
     
     $ ls /nfs/%app/modules/rpc/rpc/portmap/%module-public-interface/
     lookup-rpc-service-name    %portmapper-port
     lookup-rpc-service-number  %portmapper-program-number
     portmapper-call-it         portmapper-set
     portmapper-dump            portmapper-unset
     portmapper-get-port        %portmapper-version-number
     portmapper-null            read-rpc-service-list
     
     $ cat /nfs/%app/modules/rpc/xdr/xdr-decode
     #<procedure xdr-decode (type port)>
     
     $ cat /nfs/%app/modules/rpc/xdr/%xdr-endianness
     big

Here is the option reference:

--help
Print a summary of the command-line options and exit.
--version
Print the version number of GNU Guile-RPC and exit.
--nfs-port=port
-p port
Listen for NFS connections on port (default: 2049).
--mount-port=mount-port
-P mount-port
Listen for mount connections on mount-port (default: 6666).
--debug
Produce debugging messages.

In addition, grpc-nfs-export can be passed the name of a Scheme source file, in which case it will load that file in a separate thread while still serving NFS and mount requests. This allows the program's global variables to be monitored via the NFS mount.

As of version 0.3, this toy server exhibits poor performance, notably when running ls (which translates into a few readdir and many lookup RPCs, the latter being costly) in directories containing a lot of files. This is probably partly due to the use of TCP, and partly due to other inefficiencies that we hope to fix soon.


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6 References

RFC 1831
R. Srinivasan et al., “RPC: Remote Procedure Call Protocol Specification Version 2”, August 1995.
RFC 4506
M. Eisler et al., “XDR: External Data Representation Standard”, May 2006.
RFC 1833
R. Srinivasan et al., “Binding Protocols for ONC RPC Version 2”, August 2005.
RFC 1094
B. Nowicki, “NFS: Network File System Protocol Specification”, March 1989.


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Appendix A Portability Notes

This appendix is about Guile-RPC's portability. Of course, Guile-RPC can be ported to any OS/architecture Guile runs on. What this section deals with is portability among Scheme implementations.

Although implemented on top of GNU Guile, Guile-RPC uses mostly portable APIs such as SRFIs. Thus, it should be relatively easy to port to other Scheme implementations or to systems like Snow. Below are a few notes on portability, listing APIs and tools Guile-RPC depends on.

Portability patches can be posted to the Guile-RPC mailing list where they will be warmly welcomed!


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Appendix B GNU Free Documentation License

Version 1.2, November 2002
     Copyright © 2000,2001,2002 Free Software Foundation, Inc.
     51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA
     
     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.
  1. PREAMBLE

    The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

    This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

    We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

  2. APPLICABILITY AND DEFINITIONS

    This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.

    A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

    A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.

    The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.

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    You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.

    You may also lend copies, under the same conditions stated above, and you may publicly display copies.

  4. COPYING IN QUANTITY

    If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

    If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

    If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

    It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.

  5. MODIFICATIONS

    You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

    1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
    2. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement.
    3. State on the Title page the name of the publisher of the Modified Version, as the publisher.
    4. Preserve all the copyright notices of the Document.
    5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
    6. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
    7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice.
    8. Include an unaltered copy of this License.
    9. Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
    10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the “History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
    11. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
    12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
    13. Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version.
    14. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title with any Invariant Section.
    15. Preserve any Warranty Disclaimers.

    If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.

    You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

    You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

    The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.

  6. COMBINING DOCUMENTS

    You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.

    The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

    In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”

  7. COLLECTIONS OF DOCUMENTS

    You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

    You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

  8. AGGREGATION WITH INDEPENDENT WORKS

    A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

    If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.

  9. TRANSLATION

    Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

    If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.

  10. TERMINATION

    You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.

  11. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

       Copyright (C)  year  your name.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.2
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:

         with the Invariant Sections being list their titles, with
         the Front-Cover Texts being list, and with the Back-Cover Texts
         being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.


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Footnotes

[1] The source code repository is located at http://repo.or.cz/w/guile-r6rs-libs.git. Hopefully, this package will eventually be integrated into core Guile.

[2] Guile-Lib 0.1.6 can be used but will yield inaccurate error reporting from the compiler in some cases.

[3] Guile 1.8 provides an implementation of the former in the (srfi srfi-34) module, while the latter is currently provided by the guile-lib package.

[4] This idea was inspired by Oleg Kiselyov's description of thunked parent pointers in SXML, which may be found at http://okmij.org/ftp/Scheme/parent-pointers.txt.

[5] Finding useful applications leveraging the flexibility offered by the run-time compiler back-end is left as an exercise to the reader.