2008/4/25 qugjie :
> (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.

> 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

> Is there some example?

(which actually works with filesystems, not disks)


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