Passing pointers (or: passing parameters by reference)

Sometimes a C api function expects a pointer to a data type as parameter, probably to write into the corresponding location, or if the data is too large to be passed by value. This is also known as passing parameters by reference.

ctypes exports the byref function which is used to pass parameters by reference. The same effect can be achieved with the pointer function, although pointer does a lot more work since it constructs a real pointer object, so it is faster to use byref if you don't need the pointer object in Python itself:

>>> i = c_int()
>>> f = c_float()
>>> s = create_string_buffer('\000' * 32)
>>> print i.value, f.value, repr(s.value)
0 0.0 ''
>>> libc.sscanf("1 3.14 Hello", "%d %f %s",
...             byref(i), byref(f), s)
>>> print i.value, f.value, repr(s.value)
1 3.1400001049 'Hello'

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