Doc/lib/libgc.tex

   1 \section{\module{gc} ---
   2          Garbage Collector interface}
   3
   4 \declaremodule{extension}{gc}
   5 \modulesynopsis{Interface to the cycle-detecting garbage collector.}
   6 \moduleauthor{Neil Schemenauer}{nas@arctrix.com}
   7 \sectionauthor{Neil Schemenauer}{nas@arctrix.com}
   8
   9 The \module{gc} module is only available if the interpreter was built
  10 with the optional cyclic garbage detector (enabled by default).  If
  11 this was not enabled, an \exception{ImportError} is raised by attempts
  12 to import this module.
  13
  14 This module provides an interface to the optional garbage collector.  It
  15 provides the ability to disable the collector, tune the collection
  16 frequency, and set debugging options.  It also provides access to
  17 unreachable objects that the collector found but cannot free.  Since the
  18 collector supplements the reference counting already used in Python, you
  19 can disable the collector if you are sure your program does not create
  20 reference cycles.  Automatic collection can be disabled by calling
  21 \code{gc.disable()}.  To debug a leaking program call
  22 \code{gc.set_debug(gc.DEBUG_LEAK)}.
  23
  24 The \module{gc} module provides the following functions:
  25
  26 \begin{funcdesc}{enable}{}
  27 Enable automatic garbage collection.
  28 \end{funcdesc}
  29
  30 \begin{funcdesc}{disable}{}
  31 Disable automatic garbage collection.
  32 \end{funcdesc}
  33
  34 \begin{funcdesc}{isenabled}{}
  35 Returns true if automatic collection is enabled.
  36 \end{funcdesc}
  37
  38 \begin{funcdesc}{collect}{}
  39 Run a full collection.  All generations are examined and the
  40 number of unreachable objects found is returned.
  41 \end{funcdesc}
  42
  43 \begin{funcdesc}{set_debug}{flags}
  44 Set the garbage collection debugging flags.
  45 Debugging information will be written to \code{sys.stderr}.  See below
  46 for a list of debugging flags which can be combined using bit
  47 operations to control debugging.
  48 \end{funcdesc}
  49
  50 \begin{funcdesc}{get_debug}{}
  51 Return the debugging flags currently set.
  52 \end{funcdesc}
  53
  54 \begin{funcdesc}{set_threshold}{threshold0\optional{,
  55                                 threshold1\optional{, threshold2}}}
  56 Set the garbage collection thresholds (the collection frequency).
  57 Setting \var{threshold0} to zero disables collection.
  58
  59 The GC classifies objects into three generations depending on how many
  60 collection sweeps they have survived.  New objects are placed in the
  61 youngest generation (generation \code{0}).  If an object survives a
  62 collection it is moved into the next older generation.  Since
  63 generation \code{2} is the oldest generation, objects in that
  64 generation remain there after a collection.  In order to decide when
  65 to run, the collector keeps track of the number object allocations and
  66 deallocations since the last collection.  When the number of
  67 allocations minus the number of deallocations exceeds
  68 \var{threshold0}, collection starts.  Initially only generation
  69 \code{0} is examined.  If generation \code{0} has been examined more
  70 than \var{threshold1} times since generation \code{1} has been
  71 examined, then generation \code{1} is examined as well.  Similarly,
  72 \var{threshold2} controls the number of collections of generation
  73 \code{1} before collecting generation \code{2}.
  74 \end{funcdesc}
  75
  76 \begin{funcdesc}{get_threshold}{}
  77 Return the current collection thresholds as a tuple of
  78 \code{(\var{threshold0}, \var{threshold1}, \var{threshold2})}.
  79 \end{funcdesc}
  80
  81
  82 The following variable is provided for read-only access:
  83
  84 \begin{datadesc}{garbage}
  85 A list of objects which the collector found to be unreachable
  86 but could not be freed (uncollectable objects).  Objects that have
  87 \method{__del__()} methods and create part of a reference cycle cause
  88 the entire reference cycle to be uncollectable.  If
  89 \constant{DEBUG_SAVEALL} is set, then all unreachable objects will
  90 be added to this list rather than freed.
  91 \end{datadesc}
  92
  93
  94 The following constants are provided for use with
  95 \function{set_debug()}:
  96
  97 \begin{datadesc}{DEBUG_STATS}
  98 Print statistics during collection.  This information can
  99 be useful when tuning the collection frequency.
 100 \end{datadesc}
 101
 102 \begin{datadesc}{DEBUG_COLLECTABLE}
 103 Print information on collectable objects found.
 104 \end{datadesc}
 105
 106 \begin{datadesc}{DEBUG_UNCOLLECTABLE}
 107 Print information of uncollectable objects found (objects which are
 108 not reachable but cannot be freed by the collector).  These objects
 109 will be added to the \code{garbage} list.
 110 \end{datadesc}
 111
 112 \begin{datadesc}{DEBUG_INSTANCES}
 113 When \constant{DEBUG_COLLECTABLE} or \constant{DEBUG_UNCOLLECTABLE} is
 114 set, print information about instance objects found.
 115 \end{datadesc}
 116
 117 \begin{datadesc}{DEBUG_OBJECTS}
 118 When \constant{DEBUG_COLLECTABLE} or \constant{DEBUG_UNCOLLECTABLE} is
 119 set, print information about objects other than instance objects found.
 120 \end{datadesc}
 121
 122 \begin{datadesc}{DEBUG_SAVEALL}
 123 When set, all unreachable objects found will be appended to
 124 \var{garbage} rather than being freed.  This can be useful for debugging
 125 a leaking program.
 126 \end{datadesc}
 127
 128 \begin{datadesc}{DEBUG_LEAK}
 129 The debugging flags necessary for the collector to print
 130 information about a leaking program (equal to \code{DEBUG_COLLECTABLE |
 131 DEBUG_UNCOLLECTABLE | DEBUG_INSTANCES | DEBUG_OBJECTS | DEBUG_SAVEALL}).
 132 \end{datadesc}