xtrace

xtrace [options] [command [arguments ...]]

xtrace is a debugging tool that inspects the X11 wire protocol traffic between an X client (such as a GUI application) and an X server (such as Xorg or Xwayland). It starts a fake X server on a chosen display number, forwards every connection to the real X server pointed to by $DISPLAY (or --display), and prints a human-readable log of each request, reply, event, and error that flows across the wire.The launched program has its $DISPLAY environment variable rewritten to the fake display, so it transparently connects through xtrace. This makes it possible to see, for example, which X requests an application issues on startup, what events the server delivers, and where protocol errors occur — without modifying the application itself.When no command is given (typically combined with --waitforclient), xtrace simply listens on the fake display and logs any client that later connects to it, which is useful for attaching to applications that are started by other means.

-d, --display DISPLAY

Real X server to forward connections to (defaults to $DISPLAY).

Fake display name xtrace listens on (default :9).

Keep running after the launched client disconnects.

Terminate when all clients have disconnected (default behavior).

Do not exit even if the launched program terminates without connecting.

Copy X authentication cookies from the real display (default).

Do not copy authentication data to the fake display.

Read authentication data from file.

Write generated authentication data to file.

Write trace output to FILE instead of stdout.

Buffer trace output for higher performance.

Include byte counts for every read and write operation.

Prefix each entry with an absolute wall-clock timestamp.

Prefix each entry with time elapsed since the previous entry.

Prefix each entry with a monotonic timestamp source.

xtrace traces only the X11 protocol; it does not see GLX, DRI3, shared-memory (MIT-SHM) buffers, or other channels that bypass the wire. Output can be very verbose — buffering (-b) and targeted runs are recommended. Under Wayland, only XWayland clients are traceable.

xtrace was written by Bernhard R. Link and is packaged in Debian and Ubuntu as the xtrace package. It is commonly used by X11 developers and package maintainers to debug protocol-level issues in clients and toolkits.

xauth(1), Xorg(1), xdpyinfo(1), xev(1), strace(1)