Tcl 8.5 Network Programming: Learn Tcl and you’ll never look back when it comes to developing network-aware applications. This book is the perfect way in, taking you from the basics to more advanced topics in easy, logical steps.

The previous chapter introduced the basics of the Tcl language. We learned the basics of strings, lists, dictionaries, and integer and floating-point numbers. This section introduces some of the more advanced Tcl features— working with the time and date, data, namespaces, and stack frames.

Learning some of these features is required if you wish to understand and use Tcl and its features in a better way—especially meta programming, which can be used to create our own syntax and alter a program's flow control. This is one of Tcl's powerful features.

Object-oriented programming has been a feature Tcl offered for several years with additional packages. This was possible due to the fact that Tcl syntax is easily extendable. Those packages include Incr Tcl, which is available from http://incrtcl.sourceforge.net/itcl/, XOTcl available from http://media.wu-wien.ac.at/ and Snit which can be obtained from http://www.wjduquette.com/snit/.

However, with the release of version 8.5, Tcl now comes with an object-oriented programming functionality included called TclOO. When Tcl 8.6 is out, TclOO will soon be part of core language. In Tcl 8.5 it is an extension to the language, but leverages 8.5 extensions to support it.

This is a set of additions that allows us to build a class hierarchy on its own. TclOO is designed so that it is possible to build wrappers so that it works the same as Incr Tcl, XOTcl, or Snit. So, for those of you that use any OO extension, it might be the right time to switch. For those of you that...

Tcl allows us to read from and write to files by using channels. A channel can be an open file, but it can also be a network socket, a pipe, or any other channel type. Depending on the type of the channel, it can support reading from it, writing to it, or both.

Tcl comes with three default channels—stdin, stdout, and stderr. These channels correspond to the standard input, standard output, and standard error channels of operating systems. Standard input can be used to read information from the user, standard output should be used to write information to user, and standard error is used to write errors. Depending on how our application is run, these channels can be redirected from/to files.

In the case of Microsoft Windows and applications using the GUI version of Tcl, invoked using the wish or tclkit commands, standard channels are not available. This is because graphical applications in Microsoft Windows do not have standard consoles. In such cases, an equivalent of these...

Tcl offers a mechanism to look for and create packages—reusable pieces of code, either bundled with Tcl, created as additional projects or code you write and want to use in multiple applications. Packages are meant to provide an easy to use mechanism for using additional libraries—either your library or third-party libraries. Packages are identified using a package name and version.

Package name can be any string, but a good practice is to name the package using lowercase only and using the same name as the namespace used. For example, functionality to work with base64 encoded data is called base64 and all commands are in the base64 namespace.

Versions are one or more numbers separated by dots, where the first leftmost one indicates major versions, and remaining ones indicate additional versioning, with the leftmost being most significant, and the rightmost least significant. Tcl compares versions by separating the numbers using dots and comparing each part as numbers. version...

Tcl is designed to use an event-driven model. This means that everything that should happen is either scheduled for a particular time or when an event occurs, such as data being read, a new connection is made, or an external application or database sends a notification. This means that your files, networking, and timers work in the same thread and use events to perform actions when something occurs. For example, in an application with user interface, an action can be bound to user clicking on a button, which causes specific code to be run whenever the user clicks on the button. Similarly, a network server will set up a listening connection and run specific code when a new connection is established. Similarly, whenever application can read or write to a network connection, the particular code can be run.

It is common to develop an application that is event driven. This means that in most cases the application will only do things when an event takes place or will perform...

Even though Tcl is designed to work efficiently in a single-threaded environment, it is possible to create separate threads in Tcl. While it can be used for performing any action, it is usually used for operations that take a lot of time to complete.

Threads in Tcl require that Tcl is built with threading enabled and has the package Thread installed, which is true for all ActiveTcl installations. Tcl builds from various operating system distributions may or may not be built with thread support enabled—in this case, the Thread package may not be present.

package require Thread
set tid [thread::create...

In this chapter, we learned about some of Tcl features that are needed in order to progress to more advanced topics covered from next chapter on.

We've presented how Tcl handles dates and times, which is very similar to other programming languages. However, the ability to parse human readable date, time, and scheduling definitions is something specific to Tcl and is considered to be a very powerful feature.

We also talked about the internals of Tcl, and how variables and data types are managed, focusing on the fact that even though Tcl can store everything as a string, internally it optimizes objects based on the way we access them throughout our code.

We also discussed object oriented programming and how Tcl 8.5 standardizes the approach and provides mechanisms for compatibility with other implementations of object-oriented programming in the Tcl world.

This chapter also goes through file-related operations—from reading and writing files, through file operations such as up to getting...