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"""Run-time calculation of offset into Python string structure
Does a scan to find the digits of pi in a string structure
in order to produce an offset that can be used to produce
data-pointers from Python strings.
Porting note:
Currently this uses id( str a ) to get the base address
of the Python string. Python implementations where id( a )
is *not* the memory address of the string will not work!
"""
import ctypes
PI_DIGITS = '31415926535897931'
def calculateOffset( ):
"""Calculates the data-pointer offset for strings
This does a sequential scan for 100 bytes from the id
of a string to find special data-value stored in the
string (the digits of PI). It produces a dataPointer
function which adds that offset to the id of the
passed strings.
"""
finalOffset = None
a = PI_DIGITS
# XXX NOT portable across Python implmentations!!!
initial = id(a)
targetType = ctypes.POINTER( ctypes.c_char )
for offset in range( 100 ):
vector = ctypes.cast( initial+offset,targetType )
allMatched = True
for index,digit in enumerate( a ):
if vector[index] != digit:
allMatched = False
break
if allMatched:
finalOffset = offset
break
if finalOffset is not None:
def dataPointer( data ):
"""Return the data-pointer from the array using calculated offset
data -- a Python string
Returns the raw data-pointer to the internal buffer of the passed string
"""
if not isinstance( data, str ):
raise TypeError(
"""This function can only handle Python strings! Got %s"""%(
type(data),
)
)
return id(data) + finalOffset
# just for later reference...
dataPointer.offset = finalOffset
return dataPointer
raise RuntimeError(
"""Unable to determine dataPointer offset for strings!"""
)
dataPointer = calculateOffset()
if __name__ == "__main__":
a = 'this'
print id(a), dataPointer( a ), dataPointer(a) - id(a)
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