Netchannels is one of the basic modules of this library, because it
provides some very basic abstractions needed for many other functions
of the library. The key abstractions Netchannels defines are the types
in_obj_channel and out_obj_channel. Both are class types providing
sequential access to byte streams, one for input, one for output.
They are comparable to the types in_channel and out_channel of the
standard library that
allow access to files. However, there is one fundamental difference:
in_channel and out_channel are restricted to resources that are
available through file descriptors, whereas in_obj_channel and
out_obj_channel are just class types, and by providing implementations
for them any kind of resources can be accessed.
In some respect, Netchannels fixes a deficiency of the standard
library. Look at the module Printf which defines six variants
of the printf function:
val fprintf : out_channel -> ('a, out_channel, unit) format -> 'a
val printf : ('a, out_channel, unit) format -> 'a
val eprintf : ('a, out_channel, unit) format -> 'a
val sprintf : ('a, unit, string) format -> 'a
val bprintf : Buffer.t -> ('a, Buffer.t, unit) format -> 'a
val kprintf : (string -> string) -> ('a, unit, string) format -> 'a
It is possible to write into six different kinds of print targets.
The basic problem of this style is that the provider of a service
function like printf must define it for every commonly used
print target. The other solution is that the provider defines only
one version of the service function, but that the caller of the
function arranges the polymorphism. A Netchannels-aware Printf
would have only one variant of printf:
val printf : out_obj_channel -> ('a, out_obj_channel, unit) format -> 'a
The caller would create the right out_obj_channel object for the
real print target:
let file_ch = new output_file (file : out_channel) in
printf file_ch ...
(printing into files), or:
let buffer_ch = new output_buffer (buf : Buffer.t) in
printf buffer_ch ...
(printing into buffers). Of course, this is only a hypothetical example. The point is that this library defines many parsers and printers, and that it is really a simplification for both the library and the user of the library to have this object encapsulation of I/O resources.
in_obj_channel For example, let us program a function reading a data source
line by line, and returning the sum of all lines which must be integer
numbers. The argument ch is an open Netchannels.in_obj_channel,
and the return value is the sum:
let sum_up (ch : in_obj_channel) =
let sum = ref 0 in
try
while true do
let line = ch # input_line() in
sum := !sum + int_of_string line
done;
assert false
with
End_of_file ->
!sum
The interesting point is that the data source can be anything: a channel,
a string, or any other class that implements the class type
in_obj_channel.
This expression opens the file "data" and returns the sum of this file:
let ch = new input_channel (open_in "data") in
sum_up ch
The class Netchannels.input_channel is an implementation of the type
in_obj_channel where every method of the class simply calls the
corresponding function of the module Pervasives. (By the way, it would
be a good idea to close the channel afterwards: ch#close_in().
We will discuss that below.)
This expression sums up the contents of a constant string:
let s = "1\n2\n3\n4" in
let ch = new input_string s in
sum_up ch
The class Netchannels.input_string is an implementation of the type
in_obj_channel that reads from a string that is treated
like a channel.
The effect of using the Netchannels module is that the same
implementation sum_up can be used to read from multiple
data sources, as it is sufficient to call the function with different
implementations of in_obj_channel.
in_obj_channel The properties of any class that implements in_obj_channel
can be summarized as follows:
new), the
netchannel is open. The netchannel remains open until it is
explicitly closed (method close_in : unit -> unit). When you call a
method of a closed netchannel, the exception
Closed_channel is raised (even if you try to close the channel again).
really_input : string -> int -> int -> unit
input_char : unit -> char
input_byte : unit -> int
input_line : unit -> string
End_of_file. input : string -> int -> int -> int
End_of_file, and not by the
return value 0.pos_in : int returns the current byte position of
the channel in a way that is logically consistent with the
input methods: After reading n bytes, the method
must return a position that is increased by n. Usually the
position is zero after the object has been created, but this
is not specified. Positions are available even for file
descriptors that are not seekable.seek_in method. Seekable channels are
currently out of scope, as netstring focuses on non-seekable channels.out_obj_channel The following function outputs the numbers of an int list
sequentially on the passed netchannel:
let print_int_list (ch : out_obj_channel) l =
List.iter
(fun n ->
ch # output_string (string_of_int n);
ch # output_char '\n';
)
l;
ch # flush()
The following statements write the output into a file:
let ch = new output_channel (open_out "data") in
print_int_list ch [1;2;3]
And these statements write the output into a buffer:
let b = Buffer.create 16 in
let ch = new output_buffer b in
print_int_list ch [1;2;3]
Again, the caller of the function print_int_list determines the
type of the output destination, and you do not need several functions
for several types of destination.
out_obj_channel The properties of any class that implements out_obj_channel
can be summarized as follows:
new), the
netchannel is open. The netchannel remains open until it is
explicitly closed (method close_out : unit -> unit). When you call a
method of a closed netchannel, the exception
Closed_channel is raised (even if you try to close the channel again). output : string -> int -> int -> int
really_output : string -> int -> int -> unit
output_char : char -> unit
output_byte : int -> unit
output_string : string -> unit
flush : unit -> unit, the contents of the output buffer are
forced to be written to the destination. output_buffer : Buffer.t -> unit
Buffer.output_channel, i.e. the contents of the buffer
are printed to the channel. output_channel : ?len:int -> in_obj_channel -> unit
in_obj_channel and prints them to
the output channel. By default, the input channel is read until the
EOF position. If the len argument is passed, at
most this number of bytes are copied from the input
channel to the output channel. The input channel remains
open in all cases.pos_out : int returns byte positions
that are logically consistent: After writing n bytes, the method
must return a position that is increased by n. Usually the
position is zero after the object has been created, but this
is not specified. Positions are available even for file
descriptors that are not seekable.seek_out method.
Seekable channels are currently out of scope, as netstring
focuses on non-seekable channels.As channels may use file descriptors for their implementation, it is very important that all open channels are closed after they have been used; otherwise the operating system will certainly get out of file descriptors. The simple way,
let ch = new <channel_class> args ... in
... do something ...
ch # close_in() or close_out()
is dangerous because an exception may be raised between channel creation
and the close_* invocation. An elegant solution is to use
with_in_obj_channel and with_out_obj_channel, as in:
with_in_obj_channel (* or with_out_obj_channel *)
(new <channel_class> ...)
(fun ch ->
... do something ...
)
This programming idiom ensures that the channel is always closed after usage, even in the case of exceptions.
Complete examples:
let sum = with_in_obj_channel
(new input_channel (open_in "data"))
sum_up ;;
with_out_obj_channel
(new output_channel (open_out "data"))
(fun ch -> print_int_list ch ["1";"2";"3"]) ;;
In the Netstring library there are lots of parsers and printers
that accept netchannels as data sources and destinations, respectively. One
of them is the Nethtml module providing an HTML parser and printer. A
few code snippets how to call them, just to get used to netchannels:
let html_document =
with_in_obj_channel
(new input_channel (open_in "myfile.html"))
Nethtml.parse ;;
with_out_obj_channel
(new output_channel (open_out "otherfile.html"))
(fun ch -> Nethtml.write ch html_document) ;;
Sometimes you do not want that generated output is directly sent to the underlying file descriptor, but rather buffered until you know that everything worked fine. Imagine you program a network service, and you want to return the result only when the computations are successful, and an error message otherwise. One way to achieve this effect is to manually program a buffer:
let network_service ch =
try
let b = Buffer.create 16 in
let ch' = new output_buffer b in
... computations, write results into ch' ...
ch' # close_out;
ch # output_buffer b
with
error ->
... write error message to ch ...
There is a better way to do this, as there are transactional output
channels. This type of netchannels provide a buffer for all written
data like the above example, and only if data is explicitly committed
it is copied to the real destination. Alternatively, you can also
rollback the channel, i.e. delete the internal buffer. The signature
of the type trans_out_obj_channel is:
class type trans_out_obj_channel = object
inherit out_obj_channel
method commit_work : unit -> unit
method rollback_work : unit -> unit
end
They have the same methods as out_obj_channel plus
commit_work and rollback_work. There are two
implementations, one of them keeping the buffer in memory, and the
other using a temporary file:
let ch' = new buffered_trans_channel ch
And:
let ch' = new tempfile_trans_channel ch
In the latter case, there are optional arguments specifiying where the temporary file is created.
Now the network service would look like:
let network_service transaction_provider ch =
try
let ch' = transaction_provider ch in
... computations, write results into ch' ...
ch' # commit_work();
ch' # close_out() (* implies ch # close_out() *)
with
error ->
ch' # rollback_work();
... write error message to ch' ...
ch' # commit_work();
ch' # close_out() (* implies ch # close_out() *)
You can program this function without specifying which of the two implementations is used. Just call this function as
network_service (new buffered_trans_channel) ch
or
network_service (new tempfile_trans_channel) ch
to determine the type of transaction buffer.
Some details:
commit_work copies all uncommitted data
to the underlying channel, and flushes all buffers.rollback_work is called the uncommitted data are deleted.flush does not have any effect.rollback_work resets the position
to the value of the last commit_work call.The class pipe is an in_obj_channel and an
out_obj_channel at the same time (i.e. the class has
the type io_obj_channel). A pipe has two endpoints, one
for reading and one for writing (similar in concept to the pipes provided
by the operating system, but note that our pipes have nothing to do
with the OS pipes). Of course, you cannot read and write
at the same time, so
there must be an internal buffer storing the data that have
been written but not yet read. How can such a construction be
useful? Imagine you have two routines that run alternately,
and one is capable of writing into netchannels, and the other
can read from a netchannel. Pipes are the missing
communication link in this situation, because the writer
routine can output into the pipe, and the reader routine can
read from the buffer of the pipe. In the following example,
the writer outputs numbers from 1 to 100, and the reader sums
them up:
let pipe = new pipe() ;;
let k = ref 1 ;;
let writer() =
if !k <= 100 then (
pipe # output_string (string_of_int !k);
incr k;
if !k > 100 then pipe # close_out() else pipe # output_char '\n';
) ;;
let sum = ref 0 ;;
let reader() =
let line = pipe # input_line() in
sum := !sum + int_of_string line ;;
try
while true do
writer();
reader()
done
with
End_of_file ->
() ;;
The writer function prints the numbers into the pipe, and the
reader function reads them in. By closing only the output end
Of the pipe the writer signals the end of the stream, and the
input_line method raises the exception End_of_file.
Of course, this example is very simple. What does happen
when more is printed into the pipe than read? The internal
buffer grows. What does happen when more is tried to read from
the pipe than available? The input methods signal this by
raising the special exception
Buffer_underrun. Unfortunately, handling this exception
can be very complicated, as the reader must be able to deal
with partial reads.
This could be solved by using the Netstream module. A
netstream is another extension of in_obj_channel that
allows one to look ahead, i.e. you can look at the bytes that
will be read next, and use this information to decide whether
enough data are available or not. Netstreams are explained in
another chapter of this manual.
Pipes have another feature that makes them useful even for
"normal" programming. You can specify a conversion function
that is called when data is to be transferred from the writing
end to the reading end of the pipe. The module
Netencoding.Base64 defines such a pipe that converts data: The
class encoding_pipe automatically encodes all bytes
written into it by the Base64 scheme:
let pipe = new Netencoding.Base64.encoding_pipe() ;;
pipe # output_string "Hello World";
pipe # close_out() ;;
let s = pipe # input_line() ;;
s has now the value "SGVsbG8gV29ybGQ=", the encoded
form of the input. This kind of pipe has the same interface
as the basic pipe class, and the same problems to use it.
Fortunately, the Netstring library has another facility
simplifying the usage of pipes, namely filters.
There are two kinds of filters: The class
Netchannels.output_filter redirects data written to an
out_obj_channel through a pipe, and the class
Netchannels.input_filter arranges that data read from an
in_obj_channel flows through a pipe. An example makes
that clearer. Imagine you have a function write_results
that writes the results of a computation into an
out_obj_channel. Normally, this channel is simply a
file:
with_out_obj_channel
(new output_channel (open_out "results"))
write_results
Now you want that the file is Base64-encoded. This can be
arranged by calling write_results differently:
let pipe = new Netencoding.Base64.encoding_pipe() in
with_out_obj_channel
(new output_channel (open_out "results"))
(fun ch ->
let ch' = new output_filter pipe ch in
write_results ch';
ch' # close_out()
)
Now any invocation of an output method for ch'
actually prints into the filter, which redirects the data
through the pipe, thus encoding them, and finally
passing the encoded data to the underlying channel
ch. Note that you must close ch' to ensure
that all data are filtered, it is not sufficient to flush
output.
It is important to understand why filters must be closed to work properly. The problem is that the Base64 encoding converts triples of three bytes into quadruples of four bytes. Because not every string to convert is a multiple of three, there are special rules how to handle the exceeding one or two bytes at the end. The pipe must know the end of the input data in order to apply these rules correctly. If you only flush the filter, the exceeding bytes would simply remain in the internal buffer, because it is possible that more bytes follow. By closing the filter, you indicate that the definite end is reached, and the special rules for trailing data must be performed. \- Many conversions have similar problems, and because of this it is a good advice to always close output filters after usage.
There is not only the class output_filter but also
input_filter. This class can be used to perform
conversions while reading from a file. Note that you often do
not need to close input filters, because input channels can
signal the end by raising End_of_file, so the mentioned
problems usually do not occur.
There are a number of predefined conversion pipes:
Netencoding.Base64.encoding_pipe: Performs Base64 encodingNetencoding.Base64.decoding_pipe: Performs Base64 decodingNetencoding.QuotedPrintable.encoding_pipe: Performs
QuotedPrintable encodingNetencoding.QuotedPrintable.decoding_pipe: Performs
QuotedPrintable decodingNetconversion.conversion_pipe: Converts the character encoding
form charset A to charset BAs subtyping and inheritance are orthogonal in O'Caml, you can
simply create your own netchannels by defining classes that match the
in_obj_channel or out_obj_channel types. E.g.
class my_in_channel : in_obj_channel =
object (self)
method input s pos len = ...
method close_in() = ...
method pos_in = ...
method really_input s pos len = ...
method input_char() = ...
method input_line() = ...
method input_byte() = ...
end
Of course, this is non-trivial, especially for the in_obj_channel
case. Fortunately, the Netchannels module includes a "construction kit"
that allows one to define a channel class from only a few methods.
A closer look at in_obj_channel and out_obj_channel
shows that some methods can be derived from more fundamental methods.
The following class types include only the fundamental methods:
class type raw_in_channel = object
method input : string -> int -> int -> int
method close_in : unit -> unit
method pos_in : int
end
class type raw_out_channel = object
method output : string -> int -> int -> int
method close_out : unit -> unit
method pos_out : int
method flush : unit -> unit
end
In order to define a new class, it is sufficient to define this
raw version of the class, and to lift it to the full functionality.
For example, to define my_in_channel:
class my_raw_in_channel : raw_in_channel =
object (self)
method input s pos len = ...
method close_in() = ...
method pos_in = ...
end
class my_in_channel =
in_obj_channel_delegation (lift_in (`Raw(new my_raw_in_channel)))
The function Netchannels.lift_in can lift several forms of incomplete
channel objects to the full class type in_obj_channel. There is also
the corresponding function Netchannels.lift_out. Note that lifting
adds by default another internal buffer to the channel that must be
explicitly turned off when it is not wanted. The rationale for this
buffer is that it avoids some cases with extremely poor performance
which might be surprising for many users.
The class in_obj_channel_delegation is just an auxiliary construction
to turn the in_obj_channel object returned by lift_in again
into a class.
printf and scanf?
In principle, methods for printf and scanf could be
added to out_obj_channel and in_obj_channel, respectively,
as recent versions of O'Caml added the necessary language
means (polymorphic methods, kprintf, kscanf). However,
polymorphic methods work only well when the type of the
channel object is always annotated (e.g. as
(ch : out_obj_channel) # printf ...), so this is not
that much better than
ch # output_string (sprintf ...).in_obj_channel to an ocamllex-generated
lexer?
Yes, just call Netchannels.lexbuf_of_in_obj_channel to turn the
in_obj_channel into a lexbuf.input and output methods return 0
to indicate this. However, the non-raw methods cannot cope
with these situations.Unix.select on the
level of netchannels.netencoding.ml, it includes several
examples of conversion pipes.