Add memory allocation from wiki.

docs/src/tutorial/index.rst

@@ -12,6 +12,7 @@ Tutorials
    caveats
    profiling_tutorial
    strings
+   memory_allocation
    pure
    numpy
    readings

docs/src/tutorial/memory_allocation.rst

+.. highlight:: cython
+
+.. _memory_allocation:
+
+*****************
+Memory Allocation
+*****************
+
+Dynamic memory allocation is mostly a non-issue in Python.
+Everything is an object, and the reference counting system and garbage collector
+automatically return memory to the system when it is no longer being used.
+
+Objects can be used in Cython as well, but sometimes this incurs a certain
+amount of overhead.  In C, simple values and structs
+(such as a local variable ``cdef double x``) are allocated on the stack and
+passed by value, but for larger more complicated objects
+(e.g. a dynamically-sized list of doubles) memory must be
+manually requested and released.
+C provides the functions ``malloc``, ``realloc``, and ``free`` for this purpose,
+which can be imported in cython from ``clibc.stdlib``. Their signatures are::
+
+    void* malloc(size_t size)
+    void* realloc(void* ptr, size_t size)
+    void free(void* ptr)
+
+A very simple example of malloc usage is the following::
+
+    import random
+    from libc.stdlib cimport malloc, free
+
+    def random_noise(int number=1):
+        cdef int i
+        # allocate number * sizeof(double) bytes of memory
+        cdef double *my_array = <double *>malloc(number * sizeof(double))
+        if not my_array:
+            raise MemoryError()
+
+        try:
+            ran = random.normalvariate
+            for i in range(number):
+                my_array[i] = ran(0,1)
+
+            return [ my_array[i] for i in range(number) ]
+        finally:
+            # return the previously allocated memory to the system
+            free(my_array)
+
+One important thing to remember is that blocks of memory obtained with malloc
+*must* be manually released with free when one is done with them or it won't
+be reclaimed until the python process exits. This is called a memory leak.
+If a chuck of memory needs a larger lifetime then can be managed by a
+``try..finally`` block, another helpful idiom is to tie its lifetime to a
+Python object to leverage the Python runtime's memory management, e.g.::
+
+  cdef class SomeMemory:
+  
+      cdef doube* data
+      
+      def __init__(self, number):
+          # allocate some memory (filled with random data)
+          self.data = <double*> malloc(number * sizeof(double))
+          if self.data == NULL:
+              raise MemoryError()
+    
+      def resize(self, new_number):
+          # Allocates new_number * sizeof(double) bytes,
+          # preserving the contents and making a best-effort to
+          # re-use the original data location.
+          self.data = <double*> realloc(self.data, new_number * sizeof(double))
+          
+      def __dealloc__(self, number):
+          if self.data != NULL:
+              free(self.data)
+
+It should be noted that Cython has special support for (multi-dimensional)
+arrays of simple types via NumPy and memory views which are more full featured
+and easier to work with than pointers while still retaining the speed/static
+typing benefits. 
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