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The Hypertext Transfer Protocol (HTTP) is the most prevalent
application protocol used on the Internet today. It was originally
used for document retrieval, and has grown into a complex protocol
which supports file uploading, script execution, file management
and distributed web authoring through extensions such as WebDAV.
The SocketTools Hypertext Transfer Protocol component implements
version 0.9, 1.0 and 1.1 of the protocol, including features such
as support for proxy servers, persistent connections, user-defined
header fields and chunked data.
File Transfers
Similar to the interface used with the File Transfer Protocol
component, you can use HTTP to upload and download files. In
addition to the standard method for downloading files, the
component supports two methods for uploading files, using either
the PUT or the POST command. When downloading a file from the
server, you can either store the contents in a local file, or you
can copy the data into a memory buffer that you allocate.
Similarly, when uploading files, you can either specify a local
file to upload, or you can provide a memory buffer that contains
the file data to send to the server. High level methods such as
PutFile and GetFile can be used to transfer files in a single
method call. There are also methods such as OpenFile and CreateFile
which provide lower level file I/O interfaces.
Script Execution
Another common use for HTTP is to execute scripts on the web
server. The application can pass additional data to the script,
which is similar in concept to how arguments are passed to a
command that is entered from the command prompt. This uses the
standard POST command, and the resulting output from the script is
returned back to the application where it can be displayed or
processed. An application can use the Command method to execute the
script and then process the output in code, or can use the higher
level method PostData which will execute the script and return the
output from that script in a single method call.
Uniform Resource Locators
Anyone who has used a web browser is familiar with the Uniform
Resource Locator (URL); it is the value that is entered as the
address of a website. URLs have a specific format which provides
information about the remote host, the port number and the name of
the resource that is being accessed:
http://[username :
[password] @] remotehost [:remoteport] / resource [?
parameters]
The first part of the URL identifies the protocol, also known as
the scheme, which is to be used. With web servers, this will be
either http or https for secure connections. If a username and
password is required for authentication, then this will be included
in the URL before the name of the remote host. Next, there is the
name of the remote host to connect to, optionally followed by a
port number. If no port number is given, then the default port for
the protocol will be used. This is followed by the resource, which
is usually a path to a file or script on the server. Parameters to
the resource may also be specified, called the query string, which
are typically used as arguments to a script that is executed on the
server.
Understanding how a URL is constructed will help in
understanding how the different methods in the component work
together. For example, the remote host name and port number portion
of the URL are the values passed to the Connect method to establish
the connection. The user name and password values are assigned to
the UserName and Password properties to authenticate the client
session. And the resource name is passed to the GetData or GetFile
methods to transfer it to the local system.
The following properties, methods and events are available for
use by your application:
Initialize
Initialize the component and load the Windows Sockets library for
the current process. The application must call this method to
initialize the component before setting any properties or calling
any other methods.
Connect
Establish a connection to the server. Once the connection has been
established, the other methods in the component may be used to
access the resources on the server.
Disconnect
Disconnect from the server and release any resources that have been
allocated for the client session. After this method is called, the
client session is no longer valid.
Uninitialize
Unload the Windows Sockets library and release any resources that
have been allocated for the current process. This is the last
method call that the application should make prior to
terminating.
Using an interface similar to the File Transfer Protocol
component, this component provides several methods which can be
used to transfer files between the local and remote host. This
group of methods is high level, meaning that it is not necessary to
actually write the code to read and/or write the file data. The
component automatically handles the lower level file I/O and
notifies your application of the status of the transfer by
periodically generating progress events.
GetData
This method transfers a file from the remote host to the local
system, storing the file data in memory. This can be useful if your
application needs to perform some operation based on the contents
of the file, but does not need to store the file locally.
GetFile
This method transfers a file from the remote host and stores it in
a file on the local system.
PutData
This method creates a file on the remote host containing the data
that you provide. This can be useful if your application wants to
upload dynamically created content without having to create a
temporary file on the local system.
PutFile
This method uploads a file from the local system to the remote host
using the PUT command. Not all servers support this command, and
some may require that the client authenticate prior to calling this
method.
PostFile
This method uploads a file from the local system to the remote host
using the POST command. This enables your application to upload a
file in the same way that a user would when using a form in a web
browser.
File Management
The component can also perform some basic file management
methods as well as send custom commands to the server. Some web
servers also provide more advanced document management methods
using WebDAV, an extension to HTTP for distributed document
authoring.
GetFileSize
Return the size of a file on the server without actually
downloading the contents of the file. It is important to note that
most servers will only return file size information for actual
documents stored on the server, not for dynamically created content
generated by scripts or web pages which use server-side
includes.
GetFileTime
Return the modification time for the specified file on the server.
This can be used by your application to determine if the file has
been changed since the time that you last uploaded or downloaded
the contents.
DeleteFile
Remove a file from the server. This operation requires that the
current user have the appropriate permissions to delete the file.
Not all servers support the use of this command, and it would
typically require that the client authenticate prior to calling
this method.
Command
This method enables the client to send any command directly to the
server. This is commonly used to issue custom commands to servers
that are configured to use extensions to the standard protocol.
The component also provides methods to execute scripts on the
web server and return the output from those scripts back to your
application. Your program can pass additional data to the script,
typically either as a query string or as form data, which is
similar in concept to how arguments are passed to a command that is
entered from the command prompt.
GetData
In addition to being used to simply return the contents of a file,
this method can also be used to execute a script on the server and
return the output of that script to your program. Arguments to the
script can be specified by passing them as a query string. For
example, consider the following resource name:
/cgi-bin/test.cgi?data1=value1&data2=value2
This would specify that the script /cgi-bin/test.cgi is to be
executed, and two arguments will be passed to that script:
data1=value and data2=value2. The ampersand is used to separate the
arguments, and they are grouped as pairs of values separated by an
equal sign. Note that the actual format and value of the query
string depends on how the script is written.
PostData
An alternative method of providing information to a script is to
post data to the script. Instead of the data being part of the
resource name itself, posted data is sent separately and is
provided as input to the script. This is the same method that is
typically used when a user clicks the Submit button on a web-based
form. This method requires the name of the script and the address
of a buffer that contains the data that is to be posted. The
resulting output from the script is returned to the caller in the
same way that the GetData method works.
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