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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. Structures and unions must derive from the Structure and Union base classes which are defined in the ctypes module. Here is a simple example of a POINT structure, which contains two integers named x and y , and also shows how to initialize a structure in the constructor:. You can, however, build much more complicated structures. A structure can itself contain other structures by using a structure as a field type. Field descriptor s can be retrieved from the class , they are useful for debugging because they can provide useful information:.
Unions and structures with bit-fields should always be passed to functions by pointer. By default, Structure and Union fields are aligned in the same way the C compiler does it. This must be set to a positive integer and specifies the maximum alignment for the fields. This is what pragma pack n also does in MSVC. These classes cannot contain pointer fields.
It is possible to create structures and unions containing bit fields. Here is an example of a somewhat artificial data type, a structure containing 4 POINTs among other stuff:. The above code print a series of 0 0 lines, because the array contents is initialized to zeros.
Pointer instances are created by calling the pointer function on a ctypes type:. Pointer instances have a contents attribute which returns the object to which the pointer points, the i object above:. Note that ctypes does not have OOR original object return , it constructs a new, equivalent object each time you retrieve an attribute:.
Generally you only use this feature if you receive a pointer from a C function, and you know that the pointer actually points to an array instead of a single item. Behind the scenes, the pointer function does more than simply create pointer instances, it has to create pointer types first. Calling the pointer type without an argument creates a NULL pointer.
NULL pointers have a False boolean value:. Usually, ctypes does strict type checking. There are some exceptions to this rule, where ctypes accepts other objects. For example, you can pass compatible array instances instead of pointer types. Sometimes you have instances of incompatible types. In C, you can cast one type into another type. For these cases, the cast function is handy. The cast function can be used to cast a ctypes instance into a pointer to a different ctypes data type.
It returns an instance of the second argument, which references the same memory block as the first argument:. So, cast can be used to assign to the values field of Bar the structure:. Incomplete Types are structures, unions or arrays whose members are not yet specified. In C, they are specified by forward declarations, which are defined later:. We create two instances of cell , and let them point to each other, and finally follow the pointer chain a few times:. These are sometimes called callback functions.
First, you must create a class for the callback function. The class knows the calling convention, the return type, and the number and types of arguments this function will receive. Both of these factory functions are called with the result type as first argument, and the callback functions expected argument types as the remaining arguments. The callback will then be called with two pointers to items, and it must return a negative integer if the first item is smaller than the second, a zero if they are equal, and a positive integer otherwise.
So our callback function receives pointers to integers, and must return an integer. First we create the type for the callback function:. This behavior is correct for most purposes, but it means that values stored with threading. Some shared libraries not only export functions, they also export variables. When a frozen module is imported, it is searched in this table. Third-party code could play tricks with this to provide a dynamically created collection of frozen modules. So manipulating this pointer could even prove useful.
To restrict the example size, we show only how this table can be read with ctypes :. The fact that standard Python has a frozen module and a frozen package indicated by the negative size member is not well known, it is only used for testing. There are some edges in ctypes where you might expect something other than what actually happens.
We certainly expected the last statement to print 3 4 1 2. What happened? Here are the steps of the rc. Note that temp0 and temp1 are objects still using the internal buffer of the rc object above. So executing rc. This, in turn, changes the contents of temp1.
So, the last assignment rc. Why is it printing False? Storing a Python object in the memory block does not store the object itself, instead the contents of the object is stored. Accessing the contents again constructs a new Python object each time!
The resize function can be used to resize the memory buffer of an existing ctypes object. The function takes the object as first argument, and the requested size in bytes as the second argument. The memory block cannot be made smaller than the natural memory block specified by the objects type, a ValueError is raised if this is tried:.
This is nice and fine, but how would one access the additional elements contained in this array? Since the type still only knows about 4 elements, we get errors accessing other elements:. Another way to use variable-sized data types with ctypes is to use the dynamic nature of Python, and re- define the data type after the required size is already known, on a case by case basis.
The ctypes. Try to find a library and return a pathname. If no library can be found, returns None. It returns the filename of the library file. You don't need to install ctypes at all ; it is part of the Python standard library, as of Python 2. See the module documentation. It was written for Python 2. You got the errors with Python 2 because you are missing the Python development headers provided by the python-devel RPM. Stack Overflow for Teams — Start collaborating and sharing organizational knowledge.
Create a free Team Why Teams? Learn more about Collectives. Learn more about Teams. Could not load branches. Could not load tags. A tag already exists with the provided branch name.
Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Are you sure you want to create this branch? Local Codespaces. But it gave me an error message that. I got ctypes Anyone have some ideas or a solution? If it doesn't work on Python3. You shouldn't use Python 3. There are some caveats, and it depends on what the functionality of that code is.
In some cases it might be very time consuming to convert it. So, depending on your use case, you may want to keep a legacy version of Python 2. This is included by default in Python, so you don't need to install anything. Assuming you have the correct Python 2.
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