Static Probe Points - Debugging with GDB
Description: Debugging with GDB

Next: Error in Breakpoints, Previous: Save Breakpoints, Up: Breakpoints

#### 5.1.10 Static Probe Points

gdb supports SDT probes in the code. SDT stands for Statically Defined Tracing, and the probes are designed to have a tiny runtime code and data footprint, and no dynamic relocations. They are usable from assembly, C and C`++` languages. See http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation for a good reference on how the SDT probes are implemented.

Currently, `SystemTap` (http://sourceware.org/systemtap/) SDT probes are supported on ELF-compatible systems. See http://sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps for more information on how to add `SystemTap` SDT probes in your applications.

Some probes have an associated semaphore variable; for instance, this happens automatically if you defined your probe using a DTrace-style `.d` file. If your probe has a semaphore, gdb will automatically enable it when you specify a breakpoint using the ‘`-probe-stap`’ notation. But, if you put a breakpoint at a probe's location by some other method (e.g., `break file:line`), then gdb will not automatically set the semaphore.

You can examine the available static static probes using `info probes`, with optional arguments:

`info probes stap` [provider [name [objfile]]]
  If given, provider is a regular expression used to match against provider names when selecting which probes to list. If omitted, probes by all probes from all providers are listed.

  If given, name is a regular expression to match against probe names when selecting which probes to list. If omitted, probe names are not considered when deciding whether to display them.

  If given, objfile is a regular expression used to select which object files (executable or shared libraries) to examine. If not given, all object files are considered.

`info probes all`
  List the available static probes, from all types.

A probe may specify up to twelve arguments. These are available at the point at which the probe is defined—that is, when the current PC is at the probe's location. The arguments are available using the convenience variables (see Convenience Vars) `$_probe_arg0`...`$_probe_arg11`. Each probe argument is an integer of the appropriate size; types are not preserved. The convenience variable `$_probe_argc` holds the number of arguments at the current probe point.

These variables are always available, but attempts to access them at any location other than a probe point will cause gdb to give an error message.