Non-seekable source files

How to use a source supplied through standard input, a named pipe, or a FIFO rather than a regular file.

Overview

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

Use -s - to select standard input as the source:

source-command | xdelta3 -d -s - delta.vcdiff target

The source and the primary input cannot both use standard input. Use ./- to refer to a literal file named -.

Details

The source window size (set by -B) used by the decoder must be greater than or equal to the source window size used by the encoder. If a copy references source data further back than the decoder's window allows, you will see:

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

Raise -B (see Tuning the memory budget) so the decoder keeps enough of the source in memory.

Run the encoder with -v to print the minimum -B required by the delta it produces.

This non-seekable mode is the default for external compression: when you pass a compressed source file on the command line, xdelta3 forks the decompression command and reads the source through a pipe.

Armor mode requires a seekable source

Armor mode (on by default) reads the source in full to compute and verify its BLAKE3 digest, which requires a seekable (regular) file. With a non-seekable source the decoder cannot verify it up front, so it warns and proceeds; pass -a to disable armor for fully streaming use.