#labels Featured
How to compress when both the source and the target are streams, named pipes, etc.

= Introduction =

It is possible to read the *source file* from a FIFO or a named pipe. The *encoder* is designed not to seek backward, but this has implications for the *decoder* memory budget.

== Details ==

The source window size (set by the -B flag) used for the decoder must be *greater than or equal to* the source window size used for the encoder.

This mode is the default for ExternalCompression, if you pass a compressed source file at the command-line interface, xdelta3 forks the decompression command and reads the source file through a pipe.

You will see

{{{
xdelta3: non-seekable source: copy is too far back (try raising -B): XD3_INTERNAL
}}}

Xdelta _should_ transmit this information.  See issue 117.