Question

I am trying to understand the Python “ctypes” module. I have put together a trivial example that — ideally — wraps the statvfs() function call. The code looks like this:

from ctypes import *

class struct_statvfs (Structure):
    _fields_ = [
            ('f_bsize', c_ulong),
            ('f_frsize', c_ulong),
            ('f_blocks', c_ulong),
            ('f_bfree', c_ulong),
            ('f_bavail', c_ulong),
            ('f_files', c_ulong),
            ('f_ffree', c_ulong),
            ('f_favail', c_ulong),
            ('f_fsid', c_ulong),
            ('f_flag', c_ulong),
            ('f_namemax', c_ulong),
            ]


libc = CDLL('libc.so.6')
libc.statvfs.argtypes = [c_char_p, POINTER(struct_statvfs)]
s = struct_statvfs()

res = libc.statvfs('/etc', byref(s))
print 'return = %d, f_bsize = %d, f_blocks = %d, f_bfree = %d' % (
    res, s.f_bsize, s.f_blocks, s.f_bfree)

Running this invariably returns:

return = 0, f_bsize = 4096, f_blocks = 10079070, f_bfree = 5048834
*** glibc detected *** python: free(): invalid next size (fast): 0x0000000001e51780 ***
*** glibc detected *** python: malloc(): memory corruption (fast): 0x0000000001e517e0 ***

I haven’t been able to find any examples of calling functions with complex types as parameters (there are lots of examples of functions that return complex types), but after staring at the ctypes documentation for a day or so I think my calling syntax is correct…and it is actually callling the statvfs() call and getting back correct results.

Am I misunderstanding the ctypes docs? Or is something else going on here?

Thanks!

Answer 1

Execute this command to get the exact definition of struct statvfs on your system:

echo '#include <sys/statvfs.h>' | gcc -E - | less

Then press /struct statvfs<enter> to skip to the definition and browse from there.

Also take a look at my patch to fusepy, and their definition.