Access to sysctl on FreeBSD?

[Skye]

How do I access the sysctl(3) call from Python on BSD?

Specifically I want to retrieve:

$ sysctl -d net.inet.ip.stats
net.inet.ip.stats: IP statistics (struct ipstat, netinet/ip_var.h)

So I'll need some way of getting to struct ipstat from Python as well

Thanks,
Skye

 

[Martin v. Löwis]

How do I access the sysctl(3) call from Python on BSD?

Specifically I want to retrieve:

$ sysctl -d net.inet.ip.stats
net.inet.ip.stats: IP statistics (struct ipstat, netinet/ip_var.h)

So I'll need some way of getting to struct ipstat from Python as well

At the moment, only through ctypes - if you need to use sysctl(3),
and you'll have to manufacture the layout of struct ipstat yourself.

Regards,
Martin

 

[Skye]

Great, thanks for the help (I'm fairly new to Python, didn't know
about ctypes)

from ctypes import *
libc = CDLL("libc.so.7")
size = c_uint(0)
libc.sysctlbyname("net.inet.ip.stats", None, byref(size), None, 0)
buf = c_char_p(" " * size.value)
libc.sysctlbyname("net.inet.ip.stats", buf, byref(size), None, 0)

So now that I've got the data, can you point me towards docs
explaining how to layout struct ipstat?

Thanks,
Skye

 

[Skye]

Nevermind, I seem to have found it on my own =]

http://python.org/doc/2.5/lib/module-struct.html

This module performs conversions between Python values and C structs
represented as Python strings. It uses format strings (explained
below) as compact descriptions of the lay-out of the C structs and the
intended conversion to/from Python values. This can be used in
handling binary data stored in files or from network connections,
among other sources.

I freakin' love Python!!

Skye

 

[Thomas Heller]

Great, thanks for the help (I'm fairly new to Python, didn't know
about ctypes)

from ctypes import *
libc = CDLL("libc.so.7")
size = c_uint(0)
libc.sysctlbyname("net.inet.ip.stats", None, byref(size), None, 0)
buf = c_char_p(" " * size.value)
libc.sysctlbyname("net.inet.ip.stats", buf, byref(size), None, 0)

IIUC, sysctlbyname writes the information you requested into the buffer supplied
as second argument. If this is true, then the above code is incorrect.
c_char_p(src) creates an object that shares the buffer with the Python string 'src';
but Python strings are immutable and you must not write ctypes code that modifies them.

Instead, you should code:

from ctypes import *
libc = CDLL("libc.so.7")
size = c_uint(0)
libc.sysctlbyname("net.inet.ip.stats", None, byref(size), None, 0)
buf = create_string_buffer(size.value)
libc.sysctlbyname("net.inet.ip.stats", buf, byref(size), None, 0)

Thomas