Access to errno from Python?

Question

I am stuck with a fairly complex Python module that does not return useful error codes (it actually fails disturbingly silently). However, the underlying C library it calls sets errno.

Normally errno comes in over OSError attributes, but since I don't have an exception, I can't get at it.

Using ctypes, libc.errno doesn't work because errno is a macro in GNU libc. Python 2.6 has some affordances but Debian still uses Python 2.5. Inserting a C module into my pure Python program just to read errno disgusts me.

Is there some way to access errno? A Linux-only solution is fine, since the library being wrapped is Linux-only. I also don't have to worry about threads, as I'm only running one thread during the time in which this can fail.

Answer 1

Update: On Python 2.6+, use ctypes.get_errno().

Python 2.5

Belowed code is not reliable (or comprehensive, there are a plefora of ways errno could be defined) but it should get you started (or reconsider your position on a tiny extension module (after all on Debian python setup.py install or easy_install should have no problem to build it)). From http://codespeak.net/pypy/dist/pypy/rpython/lltypesystem/ll2ctypes.py

if not hasattr(ctypes, 'get_errno'):
    # Python 2.5 or older
    if sys.platform == 'win32':
        standard_c_lib._errno.restype = ctypes.POINTER(ctypes.c_int)
        def _where_is_errno():
            return standard_c_lib._errno()

    elif sys.platform in ('linux2', 'freebsd6'):
        standard_c_lib.__errno_location.restype = ctypes.POINTER(ctypes.c_int)
        def _where_is_errno():
            return standard_c_lib.__errno_location()

    elif sys.platform in ('darwin', 'freebsd7'):
        standard_c_lib.__error.restype = ctypes.POINTER(ctypes.c_int)
        def _where_is_errno():
            return standard_c_lib.__error()
    ctypes.get_errno = lambda: _where_is_errno().contents.value 

Where standard_c_lib:

def get_libc_name():
    if sys.platform == 'win32':
        # Parses sys.version and deduces the version of the compiler
        import distutils.msvccompiler
        version = distutils.msvccompiler.get_build_version()
        if version is None:
            # This logic works with official builds of Python.
            if sys.version_info < (2, 4):
                clibname = 'msvcrt'
            else:
                clibname = 'msvcr71'
        else:
            if version <= 6:
                clibname = 'msvcrt'
            else:
                clibname = 'msvcr%d' % (version * 10)

        # If python was built with in debug mode
        import imp
        if imp.get_suffixes()[0][0] == '_d.pyd':
            clibname += 'd'

        return clibname+'.dll'
    else:
        return ctypes.util.find_library('c')

# Make sure the name is determined during import, not at runtime
libc_name = get_libc_name() 
standard_c_lib = ctypes.cdll.LoadLibrary(get_libc_name())