spget_compare

✅ Verified · no code on this page ·

The badge reports the whole docs-verify suite, not this chapter alone.

Replay a captured PVAccess GET exchange and check spvirit's encoders against it byte for byte. A protocol-debugging tool, not something you reach for day to day.

spget_compare [OPTIONS]

Requires the client feature. It talks to no network — it reads a file.

Flags

One of the two is required:

$ spget_compare
Provide --dump-raw or --dump-file

The hex dump format

A --dump-file is what you get from copying frames out of a packet capture. Direction markers separate frames; blank lines end them; an optional four-character offset column is stripped:

C->S
0000  ca 02 00 00 08 00 00 00  01 00 00 00 ...
0010  00 00 00 00

S->C
0000  ca 02 40 01 ...

Anything that is not a two-character hex byte is ignored, so annotated captures usually paste in unchanged (spvirit-tools/src/bin/spvirit_get_compare.rs:265).

--dump-raw has no direction markers. The tool infers direction from the server bit in each frame's command flags (spvirit-tools/src/bin/spvirit_get_compare.rs:256).

What it checks

It picks the first connection-validation, create-channel, GET init, and GET data frame out of the capture, re-encodes each with spvirit's own encoder, and compares. One line per frame, in one of two shapes (spvirit-tools/src/bin/spvirit_get_compare.rs:160):

<LABEL>: OK (len=<n>)
<LABEL>: MISMATCH at offset <i> (actual len=<n>, expected len=<m>)
  actual:   <hh>  expected: <hh>

The offset of the first differing byte is usually enough to identify the field. This is how a wire-compatibility bug against EPICS Base gets localised: capture the exchange from a working pvget, replay it here, and read off where spvirit diverges.