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[lttv.git] / trunk / lttv / doc / developer / ust.txt

LTTng User-space Tracing Design

A user-space application may contain static instrumentation, and be linked with
an associated runtime library, in order to produce an execution trace. User
space Tracepoints and Markers, analogous to Kernel Tracepoints and Markers,
define program locations and arguments provided. Probes may then be connected to
each Tracepoint and Marker before they are activated. The probes will typically
call LTTng user-space event writing functions.

When the instrumented application starts, a pipe is opened to allow external
tracing control, asynchronous notification is requested when commands arrive in
the pipe, and a signal handler is installed for SIGIO (or a carefully chosen
chainable signal number). Every time such signal is received, the runtime
library checks for commands received on the external tracing control pipe. The
recognized commands are:

- list the available Markers and Tracepoints;
- dynamically load a library (presumably containing probes to connect);
- connect a probe to a Tracepoint or Marker;
- activate a Tracepoint or Marker;
- initialize the tracing buffers;
- flush the tracing buffers;
- finalize the tracing buffers;

The application may also spontaneously provide information to the pipe:

- a buffer is full;
- new tracepoints or markers are appearing or disappearing, because modules are
  dynamically loaded or unloaded;

In addition, the tracing control application should be notified when the
application exits (to save the content of buffers if the application is
crashing). Such notification may be obtained through utrace.