On Fri, Sep 1, 2017 at 3:23 AM, Peter Otten <__pete...@web.de> wrote:
>
> I think you have to specify the types yourself:
>
>>>> import ctypes
>>>> libm = ctypes.cdll.LoadLibrary("libm.so")
>>>> libm.sqrt(42)
> 0
>>>> libm.sqrt.argtypes = [ctypes.c_double]
>>>> libm.sqrt.restype = ctypes.c_double
>>>> libm.sqrt(42)
> 6.48074069840786
On POSIX systems, use ctypes.util.find_library('m'), which, for
example, resolves to "libm.so.6" in Ubuntu Linux 16.04. On the same
system, "libm.so" is an ld script that's used by the compile-time
linker.
$ cat /usr/lib/x86_64-linux-gnu/libm.so
/* GNU ld script
*/
OUTPUT_FORMAT(elf64-x86-64)
GROUP ( /lib/x86_64-linux-gnu/libm.so.6
AS_NEEDED (
/usr/lib/x86_64-linux-gnu/libmvec_nonshared.a
/lib/x86_64-linux-gnu/libmvec.so.1 ) )
The runtime linker doesn't know how to handle this script.
>>> ctypes.CDLL('libm.so')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib/python3.5/ctypes/__init__.py",
line 347, in __init__
self._handle = _dlopen(self._name, mode)
OSError: /usr/lib/x86_64-linux-gnu/libm.so: invalid ELF header
On Windows, Python 3.5+ uses the Universal C Runtime. You can
reference it directly as follows:
ucrt = ctypes.CDLL('ucrtbase', use_errno=True)
But in practice you should use the math API set [1]. For example:
>>> crt_math = ctypes.CDLL('api-ms-win-crt-math-l1-1-0',
... use_errno=True)
>>> crt_math.sqrt.restype = ctypes.c_double
>>> crt_math.sqrt.argtypes = [ctypes.c_double]
>>> crt_math.sqrt(1.3)
1.140175425099138
>>> ctypes.get_errno()
0
Note that find_library('c') and find_library('m') both return None
under Windows in Python 3.5+.
>>> ctypes.util.find_library('c') is None
True
>>> ctypes.util.find_library('m') is None
True
[1]:
https://msdn.microsoft.com/en-us/library/mt656782#_api-ms-win-crt-math-l1-1-0
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