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(python2.1-ext.info)Reference Counts

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Reference Counts
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In languages like C or C++, the programmer is responsible for dynamic
allocation and deallocation of memory on the heap.  In C, this is done
using the functions `malloc()' and `free()'.  In C++, the operators
`new' and `delete' are used with essentially the same meaning; they are
actually implemented using `malloc()' and `free()', so we'll restrict
the following discussion to the latter.

Every block of memory allocated with `malloc()' should eventually be
returned to the pool of available memory by exactly one call to
`free()'.  It is important to call `free()' at the right time.  If a
block's address is forgotten but `free()' is not called for it, the
memory it occupies cannot be reused until the program terminates.  This
is called a "memory leak".  On the other hand, if a program calls
`free()' for a block and then continues to use the block, it creates a
conflict with re-use of the block through another `malloc()' call.
This is called "using freed memory".  It has the same bad consequences
as referencing uninitialized data -- core dumps, wrong results,
mysterious crashes.

Common causes of memory leaks are unusual paths through the code.  For
instance, a function may allocate a block of memory, do some
calculation, and then free the block again.  Now a change in the
requirements for the function may add a test to the calculation that
detects an error condition and can return prematurely from the
function.  It's easy to forget to free the allocated memory block when
taking this premature exit, especially when it is added later to the
code.  Such leaks, once introduced, often go undetected for a long
time: the error exit is taken only in a small fraction of all calls,
and most modern machines have plenty of virtual memory, so the leak
only becomes apparent in a long-running process that uses the leaking
function frequently.  Therefore, it's important to prevent leaks from
happening by having a coding convention or strategy that minimizes this
kind of errors.

Since Python makes heavy use of `malloc()' and `free()', it needs a
strategy to avoid memory leaks as well as the use of freed memory.  The
chosen method is called "reference counting".  The principle is simple:
every object contains a counter, which is incremented when a reference
to the object is stored somewhere, and which is decremented when a
reference to it is deleted.  When the counter reaches zero, the last
reference to the object has been deleted and the object is freed.

An alternative strategy is called "automatic garbage collection".
(Sometimes, reference counting is also referred to as a garbage
collection strategy, hence my use of "automatic" to distinguish the
two.)  The big advantage of automatic garbage collection is that the
user doesn't need to call `free()' explicitly.  (Another claimed
advantage is an improvement in speed or memory usage -- this is no hard
fact however.)  The disadvantage is that for C, there is no truly
portable automatic garbage collector, while reference counting can be
implemented portably (as long as the functions `malloc()' and `free()'
are available -- which the C Standard guarantees).  Maybe some day a
sufficiently portable automatic garbage collector will be available for
C.  Until then, we'll have to live with reference counts.

Reference Counting in Python

Ownership Rules

Thin Ice

NULL Pointers