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>>> libcatof("345") -1073746168 >>>
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To address this problem, the type signature and handling of any foreign function func can be set by changing the following attributes:
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funcargtypes
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A tuple of ctypes datatypes (described here) describing the input arguments to func
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funcrestype
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A ctypes datatype describing the return value of func None is used for functions returning void
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A Python callable object taking three parameters (result, func, args) where result is the value returned by a foreign function, func is a reference to the foreign function itself, and args is a tuple of the input argumentsThis function is called after a foreign function call and can be used to perform error checking and other actions Here is an example of fixing the atof() function interface, as shown in the previous example:
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>>> libcatofrestype=ctypesc_double >>> libcatof("345") 345 >>>
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The ctypesd_double is a reference to a predefined datatypeThe next section describes these datatypes
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Datatypes
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Table 265 shows the ctypes datatypes that can be used in the argtypes and restype attributes of foreign functionsThe Python Value column describes the type of Python data that is accepted for the given data type
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26 Extending and Embedding Python
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Table 265
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ctypes Datatypes C Datatype
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bool signed char char char * double long double float int signed char short int long long long long long short size_t unsigned char unsigned int unsigned char unsigned short unsigned int unsigned long long unsigned long unsigned long long unsigned short void * wchar_t wchar_t *
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c_bool c_bytes c_char c_char_p c_double c_longdouble c_float c_int c_int8 c_int16 c_int32 c_int64 c_long c_longlong c_short c_size_t c_ubyte c_uint c_uint8 c_uint16 c_uint32 c_uint64 c_ulong c_ulonglong c_ushort c_void_p c_wchar c_wchar_p
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Python Value
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True or False Small integer Single character
Null-terminated string or bytes Floating point Floating point Floating point Integer 8-bit integer 16-bit integer 32-bit integer 64-bit integer Integer Integer Integer Integer Unsigned integer Unsigned integer 8-bit unsigned integer 16-bit unsigned integer 32-bit unsigned integer 64-bit unsigned integer Unsigned integer Unsigned integer Unsigned integer Integer Single Unicode character Null-terminated Unicode
To create a type representing a C pointer, apply the following function to one of the other types:
POINTER(type)
Defines a type that is a pointer to type type For example, POINTER(c_int) represents the C type int * To define a type representing a fixed-size C array, multiply an existing type by the number of array dimensions For example, c_int*4 represents the C datatype int[4] To define a type representing a C structure or union, you inherit from one of the base classes Structure or UnionWithin each derived class, you define a class variable _fields_ that describes the contents _fields_ is a list of 2 or 3 tuples of the form (name, ctype) or (name, ctype, width), where name is an identifier for the
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structure field, ctype is a ctype class describing the type, and width is an integer bitfield width For example, consider the following C structure:
struct Point { double x, y; };
The ctypes description of this structure is
class Point(Structure): _fields_ = [ ("x", c_double), ("y", c_double) ]
Calling Foreign Functions
To call functions in a library, you simply call the appropriate function with a set of arguments that are compatible with its type signature For simple datatypes such as c_int, c_double, and so forth, you can just pass compatible Python types as input (integers, floats, and so on) It is also possible to pass instances of c_int, c_double and similar types as input For arrays, you can just pass a Python sequence of compatible types To pass a pointer to a foreign function, you must first create a ctypes instance that represents the value that will be pointed at and then create a pointer object using one of the following functions:
byref(cvalue [, offset])
Represents a lightweight pointer to cvalue cvalue must be an instance of a ctypes datatype offset is a byte offset to add to the pointer valueThe value returned by the function can only be used in function calls
pointer(cvalue)
Creates a pointer instance pointing to cvalue cvalue must be an instance of a ctypes datatypeThis creates an instance of the POINTER type described earlier Here is an example showing how you would pass a parameter of type double * into a C function:
dval = c_double(00) r = foo(byref(dval)) p_dval = pointer(dval) r = foo(p_dval) # Create a double instance # Calls foo(&dval) # Creates a pointer variable # Calls foo(p_dval)
# Inspect the value of dval afterwards print (dvalvalue)
It should be noted that you cannot create pointers to built-in types such as int or float Passing pointers to such types would violate mutability if the underlying C function changed the value The cobjvalue attribute of a ctypes instance cobj contains the internal data For example, the reference to dvalvalue in the previous code returns the floating-point value stored inside the ctypes c_double instance dval