transchan − command handler API of channel transforms
cmdPrefix option ?arg arg ...? ___________________________
The Tcl-level handler for a channel transformation has to be a command with subcommands (termed an ensemble despite not implying that it must be created with namespace ensemble create; this mechanism is not tied to namespace ensemble in any way). Note that cmdPrefix is whatever was specified in the call to chan push, and may consist of multiple arguments; this will be expanded to multiple words in place of the prefix.
Of all the possible subcommands, the handler must support initialize and finalize. Transformations for writable channels must also support write, and transformations for readable channels must also support read.
Note that in the descriptions below cmdPrefix may be more than one word, and handle is the value returned by the chan push call used to create the transformation.
The following subcommands are relevant to all types of channel.
cmdPrefix clear handle
This optional subcommand is called to signify to the transformation that any data stored in internal buffers (either incoming or outgoing) must be cleared. It is called when a chan seek is performed on the channel being transformed.
cmdPrefix finalize handle
This mandatory subcommand is called last for the given handle, and then never again, and it exists to allow for cleaning up any Tcl-level data structures associated with the transformation. Warning! Any errors thrown by this subcommand will be ignored. It is not guaranteed to be called if the interpreter is deleted.
cmdPrefix initialize handle mode
This mandatory subcommand is called first, and then never again (for the given handle). Its responsibility is to initialize all parts of the transformation at the Tcl level. The mode is a list containing any of read and write.
implies that the channel is writable.
implies that the channel is readable.
The return value of the subcommand should be a list containing the names of all subcommands supported by this handler. Any error thrown by the subcommand will prevent the creation of the transformation. The thrown error will appear as error thrown by chan push.
These subcommands are used for handling transformations applied to readable channels; though strictly read is optional, it must be supported if any of the others is or the channel will be made non-readable.
cmdPrefix drain handle
This optional subcommand is called whenever data in the transformation input (i.e. read) buffer has to be forced upward, i.e. towards the user or script. The result returned by the method is taken as the binary data to push upward to the level above this transformation (the reader or a higher-level transformation).
In other words, when this method is called the transformation cannot defer the actual transformation operation anymore and has to transform all data waiting in its internal read buffers and return the result of that action.
cmdPrefix limit? handle
This optional subcommand is called to allow the Tcl I/O engine to determine how far ahead it should read. If present, it should return an integer number greater than zero which indicates how many bytes ahead should be read, or an integer less than zero to indicate that the I/O engine may read as far ahead as it likes.
cmdPrefix read handle buffer
This subcommand, which must be present if the transformation is to work with readable channels, is called whenever the base channel, or a transformation below this transformation, pushes data upward. The buffer contains the binary data which has been given to us from below. It is the responsibility of this subcommand to actually transform the data. The result returned by the subcommand is taken as the binary data to push further upward to the transformation above this transformation. This can also be the user or script that originally read from the channel.
Note that the result is allowed to be empty, or even less than the data we received; the transformation is not required to transform everything given to it right now. It is allowed to store incoming data in internal buffers and to defer the actual transformation until it has more data.
These subcommands are used for handling transformations applied to writable channels; though strictly write is optional, it must be supported if any of the others is or the channel will be made non-writable.
cmdPrefix flush handle
This optional subcommand is called whenever data in the transformation ’write’ buffer has to be forced downward, i.e. towards the base channel. The result returned by the subcommand is taken as the binary data to write to the transformation below the current transformation. This can be the base channel as well.
In other words, when this subcommand is called the transformation cannot defer the actual transformation operation anymore and has to transform all data waiting in its internal write buffers and return the result of that action.
cmdPrefix write handle buffer
This subcommand, which must be present if the transformation is to work with writable channels, is called whenever the user, or a transformation above this transformation, writes data downward. The buffer contains the binary data which has been written to us. It is the responsibility of this subcommand to actually transform the data.
The result returned by the subcommand is taken as the binary data to write to the transformation below this transformation. This can be the base channel as well. Note that the result is allowed to be empty, or less than the data we got; the transformation is not required to transform everything which was written to it right now. It is allowed to store this data in internal buffers and to defer the actual transformation until it has more data.
API, channel, ensemble, prefix, transformation
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.