使用ctypes时，C函数中的printf（）在Python中输出错误的结果

Question

I have the following problem. Any help would be appreciated :)

I am trying to call a C function from Python using ctypes. I have been successful in sharing the shared library (.dll on Windows with MS Visual Studio 2017) to Python 3.6.3.

There is a problem when I try to call the following function:

__declspec(dllexport) void printFunc()
{
    printf("hello world !!");
    //fflush(stdout);
}

I would like to see the output at Python interpreter as 'hello world !!' when I execute

mydll = cdll.LoadLibrary('path\\to\\sharedLibrary.dll')
mydll.printFunc.restype = None
mydll.printFunc()

Currently I see no output (because, the restype is None) when I execute the above code.

Expected output at Python interpreter after running the script:

>>> hello world !!

Any ideas please ??

Answer 1

• Your "hello world !!" should be printed in any case. Maybe stdout is redirected to somewhere, where you can't see it? Or line buffering is an issue, try fflush(stdout) after your printf() call.

• The default return type for those functions is int . You do not explicitely return an int, so just some value which happened to be in some cpu register is taken as return value. Chances are, that it is the return value of the printf() , which is 14 in this case (the number of characters printed)

• You can change the return type to void by issuing: mydll.printFunc.restype = None , then you shouldn't observe any integer as return value of the (python) function call.

• If you want to have the output at your python interpreter instead of stdout, you will have to return the string from your function instead of passing it to printf() and adjust the return type accordingly:

.

 __declspec(dllexport) char *printFunc() {
     return "hello world !!";
 }

And in your python interpreter:

>>> from ctypes import *
>>> mydll = cdll.LoadLibrary('path\\to\\sharedLibrary.dll')
>>> mydll.printFunc.restype = c_char_p
>>> mydll.printFunc()
'hello world !!'