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gdb: Static Probe Points

 
 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.
 
    Currently, the following types of probes are supported on
 ELF-compatible systems:
 
    * 'SystemTap' (<http://sourceware.org/systemtap/>) SDT probes(1).
      'SystemTap' probes are usable from assembly, C and C++
      languages(2).
 
    * 'DTrace' (<http://oss.oracle.com/projects/DTrace>) USDT probes.
      'DTrace' probes are usable from C and C++ languages.
 
    Some 'SystemTap' 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.  'DTrace' probes do not support
 semaphores.
 
    You can examine the available static static probes using 'info
 probes', with optional arguments:
 
 'info probes [TYPE] [PROVIDER [NAME [OBJFILE]]]'
      If given, TYPE is either 'stap' for listing 'SystemTap' probes or
      'dtrace' for listing 'DTrace' probes.  If omitted all probes are
      listed regardless of their types.
 
      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.
 
    Some probe points can be enabled and/or disabled.  The effect of
 enabling or disabling a probe depends on the type of probe being
 handled.  Some 'DTrace' probes can be enabled or disabled, but
 'SystemTap' probes cannot be disabled.
 
    You can enable (or disable) one or more probes using the following
 commands, with optional arguments:
 
 'enable probes [PROVIDER [NAME [OBJFILE]]]'
      If given, PROVIDER is a regular expression used to match against
      provider names when selecting which probes to enable.  If omitted,
      all probes from all providers are enabled.
 
      If given, NAME is a regular expression to match against probe names
      when selecting which probes to enable.  If omitted, probe names are
      not considered when deciding whether to enable 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.
 
 'disable probes [PROVIDER [NAME [OBJFILE]]]'
      See the 'enable probes' command above for a description of the
      optional arguments accepted by this command.
 
    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 (Convenience Vars)
 '$_probe_arg0'...'$_probe_arg11'.  In 'SystemTap' probes each probe
 argument is an integer of the appropriate size; types are not preserved.
 In 'DTrace' probes types are preserved provided that they are recognized
 as such by GDB; otherwise the value of the probe argument will be a long
 integer.  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.
 
    ---------- Footnotes ----------
 
    (1) See
 <http://sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps> for
 more information on how to add 'SystemTap' SDT probes in your
 applications.
 
    (2) See
 <http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation> for
 a good reference on how the SDT probes are implemented.
 

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