Base-64

Base64 is a group of similar binary-to-text encoding schemes that represent binary data in an ASCII string format by translating it into a radix-64 representation.

Base64

Base64

base64 – Encoding « base64 »

SYNOPSIS

package require Tcl 8.2
package require Trf 2.1p2
base64 options… data

DESCRIPTION

The command base64 is one of several data encodings provided by the
package trf. See trf-intro for an overview of the whole package.

This encoding transforms every block of three bytes into a block of
four bytes, each of which is printable, i.e. 7bit ASCII. This implies
that the result is valid UTF-8 too. The command uses essentially the
same algorithm as for uuencode, except for a different mapping from
6-bit fragments to printable bytes.

base64 options… data

-mode encode|decode

This option has to be present and is always understood by
the encoding.

For immediate mode the argument value specifies the oper-
ation to use. For an attached encoding it specifies the
operation to use for writing. Reading will automatically
use the reverse operation. See section IMMEDIATE versus
ATTACHED for explanations of these two terms.

Beyond the argument values listed above all unique abbre-
viations are recognized too.

Encode converts from arbitrary (most likely binary) data
into the described representation, decode does the
reverse .

-attach channel

The presence/absence of this option determines the main
operation mode of the transformation.

If present the transformation will be stacked onto the
channel whose handle was given to the option and run in
attached mode. More about this in section IMMEDIATE ver-
sus ATTACHED.

If the option is absent the transformation is used in
immediate mode and the options listed below are recog-
nized. More about this in section IMMEDIATE versus
ATTACHED.

-in channel

This options is legal if and only if the transformation
is used in immediate mode. It provides the handle of the
channel the data to transform has to be read from.

If the transformation is in immediate mode and this
option is absent the data to transform is expected as the
last argument to the transformation.

-out channel

This options is legal if and only if the transformation
is used in immediate mode. It provides the handle of the
channel the generated transformation result is written
to.

If the transformation is in immediate mode and this
option is absent the generated data is returned as the
result of the command itself.

NOTES

[1] The encoding is equivalent to PGP’s ASCII armor and was also
accepted as one of the MIME encodings for encapsulation of
binary data. See RFC 2045
tor.org/rfc/rfc2045.txt) for details and the specification of
this encoding.

[2] The encoding buffers 2 bytes.

IMMEDIATE versus ATTACHED

The transformation distinguishes between two main ways of using it.
These are the immediate and attached operation modes.

For the attached mode the option -attach is used to associate the
transformation with an existing channel. During the execution of the
command no transformation is performed, instead the channel is changed
in such a way, that from then on all data written to or read from it
passes through the transformation and is modified by it according to
the definition above. This attachment can be revoked by executing the
command unstack for the chosen channel. This is the only way to do this
at the Tcl level.

In the second mode, which can be detected by the absence of option
-attach, the transformation immediately takes data from either its com-
mandline or a channel, transforms it, and returns the result either as
result of the command, or writes it into a channel. The mode is named
after the immediate nature of its execution.

Where the data is taken from, and delivered to, is governed by the
presence and absence of the options -in and -out. It should be noted
that this ability to immediately read from and/or write to a channel is
an historic artifact which was introduced at the beginning of Trf’s
life when Tcl version 7.6 was current as this and earlier versions have
trouble to deal with \0 characters embedded into either input or out-
put.

EOF

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