http
Http(n) Tcl Built-In Commands Http(n)
______________________________________________________________________________
NAME
Http - Client-side implementation of the HTTP/1.0 protocol.
SYNOPSIS
package require http ?2.4?
::http::config ?options?
::http::geturl url ?options?
::http::formatQuery list
::http::reset token
::http::wait token
::http::status token
::http::size token
::http::code token
::http::ncode token
::http::data token
::http::error token
::http::cleanup token
::http::register proto port command
::http::unregister proto
_________________________________________________________________
DESCRIPTION
The http package provides the client side of the HTTP/1.0 protocol.
The package implements the GET, POST, and HEAD operations of HTTP/1.0.
It allows configuration of a proxy host to get through firewalls. The
package is compatible with the Safesock security policy, so it can be
used by untrusted applets to do URL fetching from a restricted set of
hosts. This package can be extened to support additional HTTP transport
protocols, such as HTTPS, by providing a custom socket command, via
http::register.
The ::http::geturl procedure does a HTTP transaction. Its options
determine whether a GET, POST, or HEAD transaction is performed. The
return value of ::http::geturl is a token for the transaction. The
value is also the name of an array in the ::http namespace that con-
tains state information about the transaction. The elements of this
array are described in the STATE ARRAY section.
If the -command option is specified, then the HTTP operation is done in
the background. ::http::geturl returns immediately after generating
the HTTP request and the callback is invoked when the transaction com-
pletes. For this to work, the Tcl event loop must be active. In Tk
applications this is always true. For pure-Tcl applications, the
caller can use ::http::wait after calling ::http::geturl to start the
event loop.
COMMANDS
::http::config ?options?
The ::http::config command is used to set and query the name of
the proxy server and port, and the User-Agent name used in the
HTTP requests. If no options are specified, then the current
configuration is returned. If a single argument is specified,
then it should be one of the flags described below. In this
case the current value of that setting is returned. Otherwise,
the options should be a set of flags and values that define the
configuration:
-accept mimetypes
The Accept header of the request. The default is */*,
which means that all types of documents are accepted.
Otherwise you can supply a comma separated list of mime
type patterns that you are willing to receive. For exam-
ple, "image/gif, image/jpeg, text/*".
-proxyhost hostname
The name of the proxy host, if any. If this value is the
empty string, the URL host is contacted directly.
-proxyport number
The proxy port number.
-proxyfilter command
The command is a callback that is made during
::http::geturl to determine if a proxy is required for a
given host. One argument, a host name, is added to com-
mand when it is invoked. If a proxy is required, the
callback should return a two element list containing the
proxy server and proxy port. Otherwise the filter should
return an empty list. The default filter returns the
values of the -proxyhost and -proxyport settings if they
are non-empty.
-useragent string
The value of the User-Agent header in the HTTP request.
The default is "Tcl http client package 2.2."
::http::geturl url ?options?
The ::http::geturl command is the main procedure in the package.
The -query option causes a POST operation and the -validate
option causes a HEAD operation; otherwise, a GET operation is
performed. The ::http::geturl command returns a token value
that can be used to get information about the transaction. See
the STATE ARRAY and ERRORS section for details. The
::http::geturl command blocks until the operation completes,
unless the -command option specifies a callback that is invoked
when the HTTP transaction completes. ::http::geturl takes sev-
eral options:
-binary boolean
Specifies whether to force interpreting the url data as
binary. Normally this is auto-detected (anything not
beginning with a text content type or whose content
encoding is gzip or compress is considered binary data).
-blocksize size
The blocksize used when reading the URL. At most size
bytes are read at once. After each block, a call to the
-progress callback is made (if that option is specified).
-channel name
Copy the URL contents to channel name instead of saving
it in state(body).
-command callback
Invoke callback after the HTTP transaction completes.
This option causes ::http::geturl to return immediately.
The callback gets an additional argument that is the
token returned from ::http::geturl. This token is the
name of an array that is described in the STATE ARRAY
section. Here is a template for the callback:
proc httpCallback {token} {
upvar #0 $token state
# Access state as a Tcl array
}
-handler callback
Invoke callback whenever HTTP data is available; if
present, nothing else will be done with the HTTP data.
This procedure gets two additional arguments: the socket
for the HTTP data and the token returned from
::http::geturl. The token is the name of a global array
that is described in the STATE ARRAY section. The proce-
dure is expected to return the number of bytes read from
the socket. Here is a template for the callback:
proc httpHandlerCallback {socket token} {
upvar #0 $token state
# Access socket, and state as a Tcl array
...
(example: set data [read $socket 1000];set nbytes [string length $data])
...
return nbytes
}
-headers keyvaluelist
This option is used to add extra headers to the HTTP
request. The keyvaluelist argument must be a list with
an even number of elements that alternate between keys
and values. The keys become header field names. New-
lines are stripped from the values so the header cannot
be corrupted. For example, if keyvaluelist is Pragma no-
cache then the following header is included in the HTTP
request:
Pragma: no-cache
-progress callback
The callback is made after each transfer of data from the
URL. The callback gets three additional arguments: the
token from ::http::geturl, the expected total size of the
contents from the Content-Length meta-data, and the cur-
rent number of bytes transferred so far. The expected
total size may be unknown, in which case zero is passed
to the callback. Here is a template for the progress
callback:
proc httpProgress {token total current} {
upvar #0 $token state
}
-query query
This flag causes ::http::geturl to do a POST request that
passes the query to the server. The query must be a x-
url-encoding formatted query. The ::http::formatQuery
procedure can be used to do the formatting.
-queryblocksize size
The blocksize used when posting query data to the URL.
At most size bytes are written at once. After each
block, a call to the -queryprogress callback is made (if
that option is specified).
-querychannel channelID
This flag causes ::http::geturl to do a POST request that
passes the data contained in channelID to the server. The
data contained in channelID must be a x-url-encoding for-
matted query unless the -type option below is used. If a
Content-Length header is not specified via the -headers
options, ::http::geturl attempts to determine the size of
the post data in order to create that header. If it is
unable to determine the size, it returns an error.
-queryprogress callback
The callback is made after each transfer of data to the
URL (i.e. POST) and acts exactly like the -progress
option (the callback format is the same).
-timeout milliseconds
If milliseconds is non-zero, then ::http::geturl sets up
a timeout to occur after the specified number of mil-
liseconds. A timeout results in a call to ::http::reset
and to the -command callback, if specified. The return
value of ::http::status is timeout after a timeout has
occurred.
-type mime-type
Use mime-type as the Content-Type value, instead of the
default value (application/x-www-form-urlencoded) during
a POST operation.
-validate boolean
If boolean is non-zero, then ::http::geturl does an HTTP
HEAD request. This request returns meta information
about the URL, but the contents are not returned. The
meta information is available in the state(meta) vari-
able after the transaction. See the STATE ARRAY section
for details.
::http::formatQuery key value ?key value ...?
This procedure does x-url-encoding of query data. It takes an
even number of arguments that are the keys and values of the
query. It encodes the keys and values, and generates one string
that has the proper & and = separators. The result is suitable
for the -query value passed to ::http::geturl.
::http::reset token ?why?
This command resets the HTTP transaction identified by token, if
any. This sets the state(status) value to why, which defaults
to reset, and then calls the registered -command callback.
::http::wait token
This is a convenience procedure that blocks and waits for the
transaction to complete. This only works in trusted code
because it uses vwait. Also, it's not useful for the case where
::http::geturl is called without the -command option because in
this case the ::http::geturl call doesn't return until the HTTP
transaction is complete, and thus there's nothing to wait for.
::http::data token
This is a convenience procedure that returns the body element
(i.e., the URL data) of the state array.
::http::error token
This is a convenience procedure that returns the error element
of the state array.
::http::status token
This is a convenience procedure that returns the status element
of the state array.
::http::code token
This is a convenience procedure that returns the http element of
the state array.
::http::ncode token
This is a convenience procedure that returns just the numeric
return code (200, 404, etc.) from the http element of the state
array.
::http::size token
This is a convenience procedure that returns the currentsize
element of the state array, which represents the number of bytes
received from the URL in the ::http::geturl call.
::http::cleanup token
This procedure cleans up the state associated with the connec-
tion identified by token. After this call, the procedures like
::http::data cannot be used to get information about the opera-
tion. It is strongly recommended that you call this function
after you're done with a given HTTP request. Not doing so will
result in memory not being freed, and if your app calls
::http::geturl enough times, the memory leak could cause a per-
formance hit...or worse.
::http::register proto port command
This procedure allows one to provide custom HTTP transport types
such as HTTPS, by registering a prefix, the default port, and
the command to execute to create the Tcl channel. E.g.:
package require http
package require tls
http::register https 443 ::tls::socket
set token [http::geturl https://my.secure.site/]
::http::unregister proto
This procedure unregisters a protocol handler that was previ-
ously registered via http::register.
ERRORS
The http::geturl procedure will raise errors in the following cases:
invalid command line options, an invalid URL, a URL on a non-existent
host, or a URL at a bad port on an existing host. These errors mean
that it cannot even start the network transaction. It will also raise
an error if it gets an I/O error while writing out the HTTP request
header. For synchronous ::http::geturl calls (where -command is not
specified), it will raise an error if it gets an I/O error while read-
ing the HTTP reply headers or data. Because ::http::geturl doesn't
return a token in these cases, it does all the required cleanup and
there's no issue of your app having to call ::http::cleanup.
For asynchronous ::http::geturl calls, all of the above error situa-
tions apply, except that if there's any error while reading the HTTP
reply headers or data, no exception is thrown. This is because after
writing the HTTP headers, ::http::geturl returns, and the rest of the
HTTP transaction occurs in the background. The command callback can
check if any error occurred during the read by calling ::http::status
to check the status and if it's error, calling ::http::error to get the
error message.
Alternatively, if the main program flow reaches a point where it needs
to know the result of the asynchronous HTTP request, it can call
::http::wait and then check status and error, just as the callback
does.
In any case, you must still call http::cleanup to delete the state
array when you're done.
There are other possible results of the HTTP transaction determined by
examining the status from http::status. These are described below.
ok If the HTTP transaction completes entirely, then status will be
ok. However, you should still check the http::code value to get
the HTTP status. The http::ncode procedure provides just the
numeric error (e.g., 200, 404 or 500) while the http::code pro-
cedure returns a value like "HTTP 404 File not found".
eof If the server closes the socket without replying, then no error
is raised, but the status of the transaction will be eof.
error The error message will also be stored in the error status array
element, accessible via ::http::error.
Another error possibility is that http::geturl is unable to write all
the post query data to the server before the server responds and closes
the socket. The error message is saved in the posterror status array
element and then http::geturl attempts to complete the transaction.
If it can read the server's response it will end up with an ok status,
otherwise it will have an eof status.
STATE ARRAY
The ::http::geturl procedure returns a token that can be used to get to
the state of the HTTP transaction in the form of a Tcl array. Use this
construct to create an easy-to-use array variable:
upvar #0 $token state
Once the data associated with the url is no longer needed, the state
array should be unset to free up storage. The http::cleanup procedure
is provided for that purpose. The following elements of the array are
supported:
body The contents of the URL. This will be empty if the
-channel option has been specified. This value is
returned by the ::http::data command.
charset
The value of the charset attribute from the Content-Type
meta-data value. If none was specified, this defaults to
the RFC standard iso8859-1, or the value of
$::http::defaultCharset. Incoming text data will be
automatically converted from this charset to utf-8.
coding A copy of the Content-Encoding meta-data value.
currentsize
The current number of bytes fetched from the URL. This
value is returned by the ::http::size command.
error If defined, this is the error string seen when the HTTP
transaction was aborted.
http The HTTP status reply from the server. This value is
returned by the ::http::code command. The format of this
value is:
HTTP/1.0 code string
The code is a three-digit number defined in the HTTP
standard. A code of 200 is OK. Codes beginning with 4
or 5 indicate errors. Codes beginning with 3 are redi-
rection errors. In this case the Location meta-data
specifies a new URL that contains the requested informa-
tion.
meta The HTTP protocol returns meta-data that describes the
URL contents. The meta element of the state array is a
list of the keys and values of the meta-data. This is in
a format useful for initializing an array that just con-
tains the meta-data:
array set meta $state(meta)
Some of the meta-data keys are listed below, but the HTTP
standard defines more, and servers are free to add their
own.
Content-Type
The type of the URL contents. Examples include
text/html, image/gif, application/postscript and
application/x-tcl.
Content-Length
The advertised size of the contents. The actual
size obtained by ::http::geturl is available as
state(size).
Location
An alternate URL that contains the requested
data.
posterror
The error, if any, that occurred while writing the post
query data to the server.
status Either ok, for successful completion, reset for user-
reset, timeout if a timeout occurred before the transac-
tion could complete, or error for an error condition.
During the transaction this value is the empty string.
totalsize
A copy of the Content-Length meta-data value.
type A copy of the Content-Type meta-data value.
url The requested URL.
EXAMPLE
# Copy a URL to a file and print meta-data
proc ::http::copy { url file {chunk 4096} } {
set out [open $file w]
set token [geturl $url -channel $out -progress ::http::Progress \
-blocksize $chunk]
close $out
# This ends the line started by http::Progress
puts stderr ""
upvar #0 $token state
set max 0
foreach {name value} $state(meta) {
if {[string length $name] > $max} {
set max [string length $name]
}
if {[regexp -nocase ^location$ $name]} {
# Handle URL redirects
puts stderr "Location:$value"
return [copy [string trim $value] $file $chunk]
}
}
incr max
foreach {name value} $state(meta) {
puts [format "%-*s %s" $max $name: $value]
}
return $token
}
proc ::http::Progress {args} {
puts -nonewline stderr . ; flush stderr
}
SEE ALSO
safe(n), socket(n), safesock(n)
KEYWORDS
security policy, socket
Tcl 8.3 Http(n)
