Re: something about reading kenel information ?

2008/4/25 qugjie <[email protected]>:
>  #ifdef UDPSTAT_SYMBOL
>         (this macro is define in file include/net-snmp/system/generic.h )
>     auto_nlist(UDPSTAT_SYMBOL, 0, 0);
>  #endif
>
>  what has done this routine ?

This locates the UDPSTAT_SYMBOL within the running kernel,
ready for when it will be needed later.   It does not retreive the
contents of this structure - just finds where the data is.


>  UDP_STAT_STRUCTURE udpstat;
>     here define a variable udpstat, this variable is used below.
>  but what type is it?

It is of type 'UDP_STAT_STRUCTURE'
What type this actually is depends on the O/S

On HP-UX, this is a simple integer.
On Linux, it's an internally defined structure
On Solaris it's "mib2_udp_t"
On Windows it's MIB_UDPSTATS
On Irix (?and some other) systems it's "struct kna"
On BSD-like systems it's  "struct udpstat"

Not all of these will actually use the auto_nlist mechanism.
This is probably mainly used for "struct udpstat".

That's one of the problems of having a single code base
that runs on a wide variety of kit, and delves deep into
the (O/S-specific) internals.   This was an attempt to
simplify the code - ultimately unsuccessful.

  More recently, we've tended to move away from this
#define / #ifdef  approach, and have a separate code
file for each architecture - loading the underlying data
into a single common format.
   (See 'mibgroup/hardware' for one set of examples)



>  The macro UDP_STAT_STRUCTURE is defined above,
>  #define UDP_STAT_STRUCTURE  struct udpstat
>  I cann't fine the struct udpstat where defined.

That will be defined in the system header files.
Assuming that you're using a relevant O/S.



>  If i want to get the disk size, which step i must to do?
>  In the linex kenel the struct statfs can know this, but
>  i don't know how to relate net-snmp to kernel.

That's by far the hardest part of implementing a MIB module.
In general, you're better off avoiding the kernel whenever
possible.   Linux in particular provides most information via
the /proc filesystem.   Very little of the Linux support in the
Net-SNMP agent will dig directly into the kernel.   I'd
forget all about auto_nlist if I were you.




>  I know the disk size can also get form /proc/ file system.
>  if i get this info from the /proc/file, can it be or not?

That's a better approach - yes.
Although the size of disks wouldn't typically be read from
either the kernel or /proc.   The current Net-SNMP code
for reporting on disks uses 'ioctl()' calls on the disk devices
themselves.


>  Is there some example?

See
        mibgroup/ucd-snmp/disk.c
   (which actually works with filesystems, not disks)
and
        mibgroup/host/hr_disk.c

Dave

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