Question

I have following code

import sys from ctypes import * from ctypes.util import find_library  libc = cdll.LoadLibrary(find_library('c')) CTL_KERN = 1 KERN_SHMMAX = 34 sysctl_names = {     'memory_shared_buffers' : (CTL_KERN, KERN_SHMMAX),     }  def posix_sysctl_long(name):     _mem = c_uint64(0)     _arr = c_int * 2     _name = _arr()     _name[0] = c_int(sysctl_names[name][0])     _name[1] = c_int(sysctl_names[name][1])     result = libc.sysctl(_name, byref(_mem), c_size_t(sizeof(_mem)), None, c_size_t(0))     if result != 0:         raise Exception('sysctl returned with error %s' % result)     return _mem.value  print posix_sysctl_long('memory_shared_buffers') 

which produces following result:

Traceback (most recent call last):   File 'test.py', line 23, in <module>     print posix_sysctl_long('memory_shared_buffers')   File 'test.py', line 20, in posix_sysctl_long     raise Exception('sysctl returned with error %s' % result) Exception: sysctl returned with error -1 

I gues I did something wrong. What would be the correct calling convention? How would I find out what exactly went wrong?

Answer 1

You are not providing the correct values to the sysctl function. Detailed information on the arguments of sysctl() can be found here.

Here are your errors:

• You have forgotten the nlen argument (second argument)
• The oldlenp argument is a pointer to the size, not directly the size

Here is the correct function (with minor improvement):

def posix_sysctl_long(name):     _mem = c_uint64(0)     _def = sysctl_names[name]     _arr = c_int * len(_def)     _name = _arr()     for i, v in enumerate(_def):         _name[i] = c_int(v)     _sz = c_size_t(sizeof(_mem))     result = libc.sysctl(_name, len(_def), byref(_mem), byref(_sz), None, c_size_t(0))     if result != 0:         raise Exception('sysctl returned with error %s' % result)     return _mem.value