Working with Binary Data in Python

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Working with binary data. Purpose of this page: To collate our knowledge about the facilities provided by Tcl to work with binary working with binary data, for example to talk to other working with binary data using a working with binary data protocol for exchanging information and commands. Definition of binary data in Tcl "Everything is a string. The Tcl language views strings as sequences of Unicode characters.

Surrogate pairs can be used working with binary data express characters beyond this Basic Multilingual Plane, but Tcl itself gives them no special treatment. Not all of these characters are assigned by Unicode, and some of them are reserved and should working with binary data be used, but Tcl doesn't check the Unicode validity of characters.

Binary data in the Tcl language is just strings consisting of characters with code points anywhere in the more limited range 0 to All Tcl commands that operate on binary data working with binary data in principle be reimplemented using the ordinary string commands e. On the C level the Tcl library may maintain a string representationan internal representationor both, for a value.

The string representation uses UTF-8 encoded strings for compatibility with the pre For dealing with binary data on C level see also http: When writing data with puts do not forget to use the option -nonewline or else puts will write an additional end-of-line character after the data you actually wanted to send out.

Speaking of end-of-line characters I should note that another common error when creating a channel destined for exchange of binary information is forgetting to use [ fconfigure channel -translation binary].

Without this Tcl will treat them as end-of-line characters and mangle working with binary data during input and output. In most cases, binary data also needs to be input and output to and from channels in a raw form. Translating from presumed UTF-8 to your system's character set can be a disaster.

Note that setting the -translation to binary also sets the -encoding to binary, so you can usually ignore this one. When reading binary information from a channel only read should be used. The latter command will try to working with binary data end-of-line characters no matter how the channel is configured.

You can never be sure that such a character will not crop up in the middle of your packet. When working with binary data an application which returns binary data via stdout do not use execbut the [open " TIP [ 1 ] addresses this issue. When dealing with binary files you should always do: In particular, you should avoid typing binary data directly into strings.

While Tcl is able to handle binary data, there are places where you can run into problems. In particular, if you happen to have a Tcl script containing the literal character for a control-Z, you will find, as of Tcl 8.

This is because beginning with 8. See source in particular the reference page for more details. Please note that the issue with control-Z is just a special case of a more general bit of advice for writing portable Tcl scripts. System encodings may be different on different systems.

If your file of Tcl code is going to move from system to system, you should be sure that all characters in it are valid in all system encodings. That command measures how long the UTF-8 representation of a string is in bytes. For binary data you don't want conversion working with binary data UTF-8, so you don't want [string bytelength] either. Use [string length] instead. It's confusing but probably logical.

SEH The working with binary data quote appeared in the docs sometime between 8. Of course this seems to contradict the concept that "everything is a string," since sometimes the thing is a byte array. What would be a way that one could read and write C structures in Tcl in a manner that would make the intents appear obvious to the reader?

Even though the above decodes the binary data, it is still not quite in a natural Tcl format. Unsigned numbers may still have incorrect sign extensions, for one. OTOH, you don't need that concat. But I could be miss interpreting the internals The reason I was worried about efficiency is I need to use this in a tight, time bound loop. It seems to be the ticket.

I originally thought I would need to use Swig or Critcl to do this As for gaining more speed The format string would get constructed only once, so the run-time overhead should be minimal. You'd probably need to experiment to see whether upvar of specified variables or uplevel of the entire binary scan command is most efficient.

Can anyone what is future and option trading the above page's recommendations together to form a best practices example?

In the old working with binary data of Tcl pre-Tcl 8. Getting started What is Tcl? Tcl in the wild Major applications.

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Most scripts work with data in the form of text and numbers, and sometimes other types of values such as dates or colors. When needed, SenseTalk can also deal with data in its binary form -- the raw bits and bytes that are stored on a computer. The put before and put after forms of the put command can be used to insert additional binary data before or after an existing value.

When two known binary data values are compared for equality, they are compared byte for byte to see that they have exactly the same binary contents. The hexadecimalData must consist of an even number of hexadecimal digits 0 through 9 and A through F.

Spaces may be used to break the sequence up for readability. The asData function, most often called using the as data operator, converts any value to its binary representation. Use the asData function or as data operator when you want to tell SenseTalk to treat a value as binary data. This is especially useful for reading or writing a file or URL in its raw binary form as described later in this chapter , but can also be used at any time to work with or display a value in its binary form.

Use as data to ensure that such a binary comparison is made. The as data operator is usually more readable and natural to use than the asData function, but is otherwise identical in functionally. The byte chunk type extends SenseTalk's chunk expressions to provide all of the flexibility offered by chunk expressions to working with binary data. The byte chunk type can be used to access a single byte or a range of bytes within a data value:. A byte chunk expression is always treated as data in its immediate context so there is no need to specify as data with it.

The dataSource doesn't need to be specified as data. A non-data value will be converted to data automatically before the requested bytes are extracted. One of the most important uses of binary data in scripts is when reading and writing data in binary non-text files. There are several ways to work with binary data files, depending on your needs. The easiest way to access a text file is to treat the file directly as a container.

The same approach will work for binary data files, by simply using the as data operator to indicate that the bytes of the file should be read directly:. Accessing a remote file through a URL works exactly the same as a local file. When more sophisticated processing is needed, the standard set of file commands including open file , read from file , write to file , seek in file , and close file can be used.

The read and write commands have special options available for reading and writing numbers in binary data in a variety of formats. See the description of the read and write commands in Working with Files and File Systems.

In addition to those numeric data types, the byte chunk type can be used with the read command to read any given number of bytes as data:. To write binary data into an open file at the current location, just specify as data:. The as data operator can be omitted if the value being written is specifically data already, such as when writing selected bytes from a data value:.

Binary data values are automatically converted to text whenever needed. There are many contexts in which this may happen, including when writing a value to a file or when a value is displayed. To force a value to be temporarily treated as data and avoid this conversion, use the as data operator:.

Whenever a string value is converted from text to data, the current setting of the defaultStringEncoding global property is used to control how each character is encoded into the binary data.

Conversions in the other direction — from data to text — are controlled by the defaultDataFormat global property. Whenever a value is converted from data to text, the current setting of the defaultDataFormat global property controls how it will be represented. Valid formats include "text", "hex", "base64", "size", or "auto". The initial setting is "text". With the "text" setting, the defaultStringEncoding global property is used to control how the binary data is interpreted as text.

The defaultDataFormat may also be set to "text: Depending on the encoding that is used, and the particular characters that appear in the text, the resulting data may have exactly one byte for each character in the original text, or there may be two or more bytes for some or all of the characters. The "hex" or "hexadecimal" format will display two hexadecimal digits for every byte of data, enclosed in angle brackets. This format is recognized by SenseTalk within a script for literal data values, so the value function can be used to convert text in this format back to binary data.

The "base64" format is a more compact representation of the bytes, using 64 different text characters to encode the data. The base64Decode function can be used to read text in this format and convert it back to binary data. The "size" format is the most abbreviated format that simply indicates the size of the data e.

This is a useful representation for displaying potentially large chunks of binary data to the user, but cannot be used for storage as the data itself would be lost.

The "auto" format automatically chooses the "size" format for large binary data more than 64 bytes , and the "hex" format for smaller sizes.

Use a format of "auto: The base64Encode function converts binary data to a base64 text representation. This function may also be called using the as base64 operator. The base64Decode function takes text in the base64 format and converts it back to binary data. Use these functions or the as base64 operator for converting binary data to or from the base64 format. The as base64 operator is often more readable and natural to use than the base64Encode function, but is otherwise identical in functionally.

Working with Binary Data Working with Binary Data Most scripts work with data in the form of text and numbers, and sometimes other types of values such as dates or colors. When to Use It Use the asData function or as data operator when you want to tell SenseTalk to treat a value as binary data. Byte Chunks The byte chunk type extends SenseTalk's chunk expressions to provide all of the flexibility offered by chunk expressions to working with binary data. The byte chunk type can be used to access a single byte or a range of bytes within a data value: Binary Data Files One of the most important uses of binary data in scripts is when reading and writing data in binary non-text files.

Simple Data File Access The easiest way to access a text file is to treat the file directly as a container. The same approach will work for binary data files, by simply using the as data operator to indicate that the bytes of the file should be read directly: In addition to those numeric data types, the byte chunk type can be used with the read command to read any given number of bytes as data: To force a value to be temporarily treated as data and avoid this conversion, use the as data operator: The DefaultDataFormat Global Property Whenever a value is converted from data to text, the current setting of the defaultDataFormat global property controls how it will be represented.

When to Use It Use these functions or the as base64 operator for converting binary data to or from the base64 format.