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180 lines
7.6 KiB
Plaintext
180 lines
7.6 KiB
Plaintext
This file describes some special Python build types enabled via
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compile-time preprocessor defines.
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---------------------------------------------------------------------------
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Py_REF_DEBUG
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Turn on aggregate reference counting. This arranges that extern
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_Py_RefTotal hold a count of all references, the sum of ob_refcnt across
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all objects. In a debug-mode build, this is where the "8288" comes from
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in
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>>> 23
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23
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[8288 refs]
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>>>
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Note that if this count increases when you're not storing away new objects,
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there's probably a leak. Remember, though, that in interactive mode the
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special name "_" holds a reference to the last result displayed!
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Py_REF_DEBUG also checks after every decref to verify that the refcount
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hasn't gone negative, and causes an immediate fatal error if it has.
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Special gimmicks:
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sys.gettotalrefcount()
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Return current total of all refcounts.
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Available under Py_REF_DEBUG in Python 2.3.
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Before 2.3, Py_TRACE_REFS was required to enable this function.
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---------------------------------------------------------------------------
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Py_TRACE_REFS
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Turn on heavy reference debugging. This is major surgery. Every PyObject
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grows two more pointers, to maintain a doubly-linked list of all live
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heap-allocated objects (note that, e.g., most builtin type objects are not
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in this list, as they're statically allocated). Note that because the
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fundamental PyObject layout changes, Python modules compiled with
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Py_TRACE_REFS are incompatible with modules compiled without it.
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Py_TRACE_REFS implies Py_REF_DEBUG.
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Special gimmicks:
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sys.getobjects(max[, type])
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Return list of the (no more than) max most-recently allocated objects,
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most recently allocated first in the list, least-recently allocated
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last in the list. max=0 means no limit on list length.
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If an optional type object is passed, the list is also restricted to
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objects of that type.
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The return list itself, and some temp objects created just to call
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sys.getobjects(), are excluded from the return list. Note that the
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list returned is just another object, though, so may appear in the
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return list the next time you call getobjects(); note that every
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object in the list is kept alive too, simply by virtue of being in
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the list.
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envar PYTHONDUMPREFS
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If this envar exists, Py_Finalize() arranges to print a list of
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all still-live heap objects.
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---------------------------------------------------------------------------
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PYMALLOC_DEBUG
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When pymalloc is enabled (WITH_PYMALLOC is defined), calls to the PyObject_
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memory routines are handled by Python's own small-object allocator, while
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calls to the PyMem_ memory routines are directed to the system malloc/
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realloc/free. If PYMALLOC_DEBUG is also defined, calls to both PyObject_
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and PyMem_ memory routines are directed to a special debugging mode of
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Python's small-object allocator.
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This mode fills dynamically allocated memory blocks with special,
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recognizable bit patterns, and adds debugging info on each end of
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dynamically allocated memory blocks. The special bit patterns are:
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#define CLEANBYTE 0xCB /* clean (newly allocated) memory */
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#define DEADBYTE 0xDB /* dead (newly freed) memory */
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#define FORBIDDENBYTE 0xFB /* fordidden -- untouchable bytes */
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Strings of these bytes are unlikely to be valid addresses, floats, or 7-bit
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ASCII strings.
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8 bytes are added at each end of each block of N bytes requested. The
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memory layout is like so, where p represents the address returned by a
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malloc-like or realloc-like function (p[i:j] means the slice of bytes
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from *(p+i) inclusive up to *(p+j) exclusive; note that the treatment
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of negative indices differs from a Python slice):
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p[-8:-4]
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Number of bytes originally asked for. 4-byte unsigned integer,
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big-endian (easier to read in a memory dump).
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p[-4:0]
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Copies of FORBIDDENBYTE. Used to catch under- writes and reads.
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p[0:N]
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The requested memory, filled with copies of CLEANBYTE.
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Used to catch reference to uninitialized memory.
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When a realloc-like function is called requesting a larger memory
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block, the new excess bytes are also filled with CLEANBYTE.
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When a free-like function is called, these are overwritten with
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DEADBYTE, to catch reference to free()ed memory. When a realloc-
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like function is called requesting a smaller memory block, the excess
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old bytes are also filled with DEADBYTE.
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p[N:N+4]
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Copies of FORBIDDENBYTE. Used to catch over- writes and reads.
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p[N+4:N+8]
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A serial number, incremented by 1 on each call to a malloc-like or
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realloc-like function.
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4-byte unsigned integer, big-endian.
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If "bad memory" is detected later, the serial number gives an
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excellent way to set a breakpoint on the next run, to capture the
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instant at which this block was passed out. The static function
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bumpserialno() in obmalloc.c is the only place the serial number
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is incremented, and exists so you can set such a breakpoint easily.
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A malloc-like or free-like function first checks that the FORBIDDENBYTEs
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at each end are intact. If they've been altered, diagnostic output is
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written to stderr, and the program is aborted by Py_FatalError().
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Note that PYMALLOC_DEBUG requires WITH_PYMALLOC.
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Special gimmicks:
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envar PYTHONMALLOCSTATS
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If this envar exists, a report of pymalloc summary statistics is
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printed to stderr whenever a new arena is allocated, and also
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by Py_Finalize().
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---------------------------------------------------------------------------
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Py_DEBUG
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This is what is generally meant by "a debug build" of Python.
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Py_DEBUG implies Py_REF_DEBUG, Py_TRACE_REFS, and PYMALLOC_DEBUG (if
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WITH_PYMALLOC is enabled). In addition, C assert()s are enabled (via
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the C way: by not defining NDEBUG), and some routines do additional
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sanity checks inside "#ifdef Py_DEBUG" blocks.
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---------------------------------------------------------------------------
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COUNT_ALLOCS
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Each type object grows three new members:
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/* Number of times an object of this type was allocated. */
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int tp_allocs;
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/* Number of times an object of this type was deallocated. */
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int tp_frees;
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/* Highwater mark: the maximum value of tp_allocs - tp_frees so
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* far; or, IOW, the largest number of objects of this type alive at
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* the same time.
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*/
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int tp_maxalloc;
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Allocation and deallocation code keeps these counts up to date.
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Py_Finalize() displays a summary of the info returned by sys.getcounts()
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(see below), along with assorted other special allocation counts (like
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the number of tuple allocations satisfied by a tuple free-list, the number
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of 1-character strings allocated, etc).
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Before Python 2.2, type objects were immortal, and the COUNT_ALLOCS
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implementation relies on that. As of Python 2.2, heap-allocated type/
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class objects can go away. COUNT_ALLOCS can blow up in 2.2 and 2.2.1
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because of this; this was fixed in 2.2.2. Use of COUNT_ALLOCS makes
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all heap-allocated type objects immortal, except for those for which no
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object of that type is ever allocated.
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Special gimmicks:
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sys.getcounts()
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Return a list of 4-tuples, one entry for each type object for which
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at least one object of that type was allocated. Each tuple is of
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the form:
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(tp_name, tp_allocs, tp_frees, tp_maxalloc)
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Each distinct type object gets a distinct entry in this list, even
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if two or more type objects have the same tp_name (in which case
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there's no way to distinguish them by looking at this list). The
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list is ordered by time of first object allocation: the type object
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for which the first allocation of an object of that type occurred
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most recently is at the front of the list.
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---------------------------------------------------------------------------
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