Purpose of this page: To collate our knowledge about the facilities provided by Tcl to work with binary data, for example to talk to other applications using a binary protocol for exchanging information and commands.
The main facility is the binary command with its subcommands to dissect (scan) and join (format) binary data into/from standard tcl values (strings, integers, lists, et cetera).
To exchange the binary information with other applications all of the facilities of the I/O system are at our fingertips and ready to be used. But note:
- 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]. This command reconfigures the channel to leave the characters \n and \r untouched. Without this Tcl will treat them as end-of-line characters and mangle them 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. You therefore almost certainly need [fconfigure $channel -encoding binary]. DKF sez: 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. Avoid gets! The latter command will try to recognize end-of-line characters no matter what the channel is configured too. You can never be sure that such a character will not crop up in the middle of your packet.
- When spawning an application which returns binary data via stdout do not use exec, but the [open "|..."] idiom as only the latter allows you to change the pipe channel to binary. exec hides the pipe channel and may use the wrong encoding and translation settings when reading the information from the external application.
On news:comp.lang.tcl , Mac Cody and Jeff David write:
Mac Cody wrote:
> Here is a simple example that
> first writes binary data to a file and then reads back the
> binary data:
>
> set outBinData [binary format s2Sa6B8 {100 -2} 100 foobar 01000001]
> puts "Format done: $outBinData"
> set fp [open binfile w]Important safety tip. When dealing with binary files you should always do:
fconfigure $fp -translation binary
I got bit hard on this one once when my \x0a and \x0d bytes got translated.
> puts -nonewline $fp $outBinData
> close $fp
> set fp [open binfile r]
fconfigure $fp -translation binary
> set inBinData [read $fp]
> close $fp
> binary scan $inBinData s2Sa6B8 val1 val2 val3 val4
> puts "Scan done: $val1 $val2 $val3 $val4"
>
Jeff David
See Binary representation of numbers and Dump a file in hex and ASCII for examples of usage.