Fundamental data types

ctypes defines a number of primitive C compatible data types :

ctypes type C type Python type
c_char char 1-character string
c_wchar wchar_t 1-character unicode string
c_byte char int/long
c_ubyte unsigned char int/long
c_short short int/long
c_ushort unsigned short int/long
c_int int int/long
c_uint unsigned int int/long
c_long long int/long
c_ulong unsigned long int/long
c_longlong __int64 or long long int/long
c_ulonglong unsigned __int64 or unsigned long long int/long
c_float float float
c_double double float
c_char_p char * (NUL terminated) string or None
c_wchar_p wchar_t * (NUL terminated) unicode or None
c_void_p void * int/long or None

All these types can be created by calling them with an optional initializer of the correct type and value:

>>> c_int()
>>> c_char_p("Hello, World")
c_char_p('Hello, World')
>>> c_ushort(-3)

Since these types are mutable, their value can also be changed afterwards:

>>> i = c_int(42)
>>> print i
>>> print i.value
>>> i.value = -99
>>> print i.value

Assigning a new value to instances of the pointer types c_char_p, c_wchar_p, and c_void_p changes the memory location they point to, not the contents of the memory block (of course not, because Python strings are immutable):

>>> s = "Hello, World"
>>> c_s = c_char_p(s)
>>> print c_s
c_char_p('Hello, World')
>>> c_s.value = "Hi, there"
>>> print c_s
c_char_p('Hi, there')
>>> print s                 # first string is unchanged
Hello, World

You should be careful, however, not to pass them to functions expecting pointers to mutable memory. If you need mutable memory blocks, ctypes has a create_string_buffer function which creates these in various ways. The current memory block contents can be accessed (or changed) with the raw property; if you want to access it as NUL terminated string, use the value property:

>>> from ctypes import *
>>> p = create_string_buffer(3)      # create a 3 byte buffer, initialized to NUL bytes
>>> print sizeof(p), repr(p.raw)
3 '\x00\x00\x00'
>>> p = create_string_buffer("Hello")      # create a buffer containing a NUL terminated string
>>> print sizeof(p), repr(p.raw)
6 'Hello\x00'
>>> print repr(p.value)
>>> p = create_string_buffer("Hello", 10)  # create a 10 byte buffer
>>> print sizeof(p), repr(p.raw)
10 'Hello\x00\x00\x00\x00\x00'
>>> p.value = "Hi"      
>>> print sizeof(p), repr(p.raw)
10 'Hi\x00lo\x00\x00\x00\x00\x00'

The create_string_buffer function replaces the c_buffer function (which is still available as an alias), as well as the c_string function from earlier ctypes releases. To create a mutable memory block containing unicode characters of the C type wchar_t use the create_unicode_buffer function.

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