Make syscall in Python

Question

I want to make syscall in Python and the function is not in libc, is there a way to do it in Python?

More specifically, I want to call getdents, whose manpage says

Note: There are no glibc wrappers for these system calls;

All existing related solutions I found on the web uses ctypes with libc.so: for example.

Please don't question why I want to use getdents directly, I have a very specific reason to do that, and it would be distracting to discuss in this question. Thank you.

Kijewski · Accepted Answer · 2016-05-04 16:48:37Z

46

Libc exposes a function to invoke "custom" syscalls: long syscall(long number, ...);

syscall() is a small library function that invokes the system call whose assembly language interface has the specified number with the specified arguments. Employing syscall() is useful, for example, when invoking a system call that has no wrapper function in the C library.

Just access this function like any foreign function:

import ctypes

libc = ctypes.CDLL(None)
syscall = libc.syscall

e.g.

syscall(39)  # 39 = getpid, but you get the gist

Or to translate the example in the man page:

import os, ctypes

off_t = ctypes.c_long  # YMMV
__NR_getdents = 78  # YMMV

class linux_dirent(ctypes.Structure):
    _fields_ = [
        ('d_ino', ctypes.c_long),
        ('d_off', off_t),
        ('d_reclen', ctypes.c_ushort),
        ('d_name', ctypes.c_char)
    ]

_getdents = ctypes.CDLL(None).syscall
_getdents.restype = ctypes.c_int
_getdents.argtypes = ctypes.c_long, ctypes.c_uint, ctypes.POINTER(ctypes.c_char), ctypes.c_uint

fd = os.open('/tmp/', os.O_RDONLY | os.O_DIRECTORY)

buf = ctypes.ARRAY(ctypes.c_char, 1024)()
while True:
    nread = _getdents(__NR_getdents, fd, buf, len(buf))
    if nread == -1:
        raise OSError('getdents')
    elif nread == 0:
        break

    pos = 0
    while pos < nread:
        d = linux_dirent.from_buffer(buf, pos)

        name = buf[pos + linux_dirent.d_name.offset : pos + d.d_reclen]
        name = name[:name.index('\0')]
        print 'name:', name

        pos += d.d_reclen

edited May 4, 2016 at 16:48

answered May 4, 2016 at 15:55

Kijewski

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2 Comments

cheshirekow Over a year ago

Is this strategy _getdents = ctypes.CDLL(None).syscall reusable. Let's say you wanted both getdents and some other syscall that takes a different set of arguments. Can you create another copy of syscall and then modify argtypes to something else? Or does anotherfun = ctypes.CDLL(None).syscall refer to the same object as _getdents and so modifying argtypes also modifies argtypes of _getdents?

Nathaniel J. Smith Over a year ago

@cheshirekow What a strange coincidence... I was just looking that up myself now, 10 hours after you asked it on a 3 year old question... AFAICT the answer is that for a single call to CDLL you can only set the argtypes once. But if you call CDLL(None) twice, you can set dll.syscall.argtypes separately on each copy.

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