NAME

http - Client-side implementation of the HTTP/1.1 protocol

SYNOPSIS

package require http ?2.7?

::http::config ?options?

::http::geturl url ?options?

::http::formatQuery key value ?key value ...?

::http::reset token ?why?

::http::wait token

::http::status token

::http::size token

::http::code token

::http::ncode token

::http::meta 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/1.1 protocol. The package implements the GET, POST, and HEAD operations of HTTP/1.1. 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 extended 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 contains 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 completes. 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:

::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 several options:

proc httpCallback {token} {
    upvar #0 $token state
    # Access state as a Tcl array
}

proc httpHandlerCallback {socket token} {
    upvar #0 $token state
    # Access socket, and state as a Tcl array
    # For example...
    ...
    set data [read $socket 1000]
    set nbytes [string length $data]
    ...
    return $nbytes
}

Pragma: no-cache

proc httpProgress {token total current} {
    upvar #0 $token state
}

::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 is not useful for the case where ::http::geturl is called without the -command option because in this case the ::http::geturl call does not return until the HTTP transaction is complete, and thus there is 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::meta token

This is a convenience procedure that returns the meta element of the state array which contains the HTTP response headers. See below for an explanation of this element.

::http::cleanup token

This procedure cleans up the state associated with the connection identified by token. After this call, the procedures like ::http::data cannot be used to get information about the operation. It is strongly recommended that you call this function after you are 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 performance 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 previously 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 reading the HTTP reply headers or data. Because ::http::geturl does not return a token in these cases, it does all the required cleanup and there is no issue of your app having to call ::http::cleanup.

For asynchronous ::http::geturl calls, all of the above error situations apply, except that if there is 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 its 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 are 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 procedure 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

HTTP/1.1 code string

array set meta $state(meta)

EXAMPLE

# Copy a URL to a file and print meta-data
proc httpcopy { url file {chunk 4096} } {
   set out [open $file w]
   set token [::http::geturl $url -channel $out \
          -progress httpCopyProgress -blocksize $chunk]
   close $out
   # This ends the line started by httpCopyProgress
   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 [httpcopy [string trim $value] $file $chunk]
      }
   }
   incr max
   foreach {name value} $state(meta) {
      puts [format "%-*s %s" $max $name: $value]
   }
   return $token
}
proc httpCopyProgress {args} {
   puts -nonewline stderr .
   flush stderr
}

SEE ALSO

safe(n), socket(n), safesock(n)

KEYWORDS

security policy, socket