Horace papa's life

started by Ingo Molnar <mingo@redhat.com>, 2001.09.17
2.6 port and netpoll api by Matt Mackall <mpm@selenic.com>, Sep 9 2003

Please send bug reports to Matt Mackall <mpm@selenic.com>
and Satyam Sharma <satyam.sharma@gmail.com>

Introduction:
=============

This module logs kernel printk messages over UDP allowing debugging of
problem where disk logging fails and serial consoles are impractical.

It can be used either built-in or as a module. As a built-in,
netconsole initializes immediately after NIC cards and will bring up
the specified interface as soon as possible. While this doesn't allow
capture of early kernel panics, it does capture most of the boot
process.

Sender and receiver configuration:
==================================

It takes a string configuration parameter "netconsole" in the
following format:

 netconsole=[src-port]@[src-ip]/[<dev>],[tgt-port]@<tgt-ip>/[tgt-macaddr]

   where
        src-port      source for UDP packets (defaults to 6665)
        src-ip        source IP to use (interface address)
        dev           network interface (eth0)
        tgt-port      port for logging agent (6666)
        tgt-ip        IP address for logging agent
        tgt-macaddr   ethernet MAC address for logging agent (broadcast)

Examples:

 linux netconsole=4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc

  or

 insmod netconsole netconsole=@/,@10.0.0.2/

It also supports logging to multiple remote agents by specifying
parameters for the multiple agents separated by semicolons and the
complete string enclosed in "quotes", thusly:

 modprobe netconsole netconsole="@/,@10.0.0.2/;@/eth1,6892@10.0.0.3/"

Built-in netconsole starts immediately after the TCP stack is
initialized and attempts to bring up the supplied dev at the supplied
address.

The remote host can run either 'netcat -u -l -p <port>',
'nc -l -u <port>' or syslogd.

Dynamic reconfiguration:
========================

Dynamic reconfigurability is a useful addition to netconsole that enables
remote logging targets to be dynamically added, removed, or have their
parameters reconfigured at runtime from a configfs-based userspace interface.
[ Note that the parameters of netconsole targets that were specified/created
from the boot/module option are not exposed via this interface, and hence
cannot be modified dynamically. ]

To include this feature, select CONFIG_NETCONSOLE_DYNAMIC when building the
netconsole module (or kernel, if netconsole is built-in).

Some examples follow (where configfs is mounted at the /sys/kernel/config
mountpoint).

To add a remote logging target (target names can be arbitrary):

 cd /sys/kernel/config/netconsole/
 mkdir target1

Note that newly created targets have default parameter values (as mentioned
above) and are disabled by default -- they must first be enabled by writing
"1" to the "enabled" attribute (usually after setting parameters accordingly)
as described below.

To remove a target:

 rmdir /sys/kernel/config/netconsole/othertarget/

The interface exposes these parameters of a netconsole target to userspace:

        enabled         Is this target currently enabled?       (read-write)
        dev_name        Local network interface name            (read-write)
        local_port      Source UDP port to use                  (read-write)
        remote_port     Remote agent's UDP port                 (read-write)
        local_ip        Source IP address to use                (read-write)
        remote_ip       Remote agent's IP address               (read-write)
        local_mac       Local interface's MAC address           (read-only)
        remote_mac      Remote agent's MAC address              (read-write)

The "enabled" attribute is also used to control whether the parameters of
a target can be updated or not -- you can modify the parameters of only
disabled targets (i.e. if "enabled" is 0).

To update a target's parameters:

 cat enabled                            # check if enabled is 1
 echo 0 > enabled                    # disable the target (if required)
 echo eth2 > dev_name                        # set local interface
 echo 10.0.0.4 > remote_ip           # update some parameter
 echo cb:a9:87:65:43:21 > remote_mac # update more parameters
 echo 1 > enabled                    # enable target again

You can also update the local interface dynamically. This is especially
useful if you want to use interfaces that have newly come up (and may not
have existed when netconsole was loaded / initialized).

Miscellaneous notes:
====================

WARNING: the default target ethernet setting uses the broadcast
ethernet address to send packets, which can cause increased load on
other systems on the same ethernet segment.

TIP: some LAN switches may be configured to suppress ethernet broadcasts
so it is advised to explicitly specify the remote agents' MAC addresses
from the config parameters passed to netconsole.

TIP: to find out the MAC address of, say, 10.0.0.2, you may try using:

 ping -c 1 10.0.0.2 ; /sbin/arp -n | grep 10.0.0.2

TIP: in case the remote logging agent is on a separate LAN subnet than
the sender, it is suggested to try specifying the MAC address of the
default gateway (you may use /sbin/route -n to find it out) as the
remote MAC address instead.

NOTE: the network device (eth1 in the above case) can run any kind
of other network traffic, netconsole is not intrusive. Netconsole
might cause slight delays in other traffic if the volume of kernel
messages is high, but should have no other impact.

NOTE: if you find that the remote logging agent is not receiving or
printing all messages from the sender, it is likely that you have set
the "console_loglevel" parameter (on the sender) to only send high
priority messages to the console. You can change this at runtime using:

 dmesg -n 8

or by specifying "debug" on the kernel command line at boot, to send
all kernel messages to the console. A specific value for this parameter
can also be set using the "loglevel" kernel boot option. See the
dmesg(8) man page and Documentation/kernel-parameters.txt for details.

Netconsole was designed to be as instantaneous as possible, to
enable the logging of even the most critical kernel bugs. It works
from IRQ contexts as well, and does not enable interrupts while
sending packets. Due to these unique needs, configuration cannot
be more automatic, and some fundamental limitations will remain:
only IP networks, UDP packets and ethernet devices are supported.

Here is the good introduction for igmp iptv.

https://www.apac-juniper.net/juniper_public/campaign/pdf/iptv/sc/200188.pdf

copy from

http://www.yolinux.com/TUTORIALS/LibraryArchives-StaticAndDynamic.html

Why libraries are used:

This methodology, also known as "shared components" or "archive libraries", groups together multiple compiled object code files into a single file known as a library. Typically C functions/C++ classes and methods which can be shared by more than one application are broken out of the application's source code, compiled and bundled into a library. The C standard libraries and C++ STL are examples of shared components which can be linked with your code. The benefit is that each and every object file need not be stated when linking because the developer can reference the individual library. This simplifies the multiple use and sharing of software components between applications. It also allows application vendors a way to simply release an API to interface with an application. Components which are large can be created for dynamic use, thus the library remain separate from the executable reducing it's size and thus disk space used. The library components are then called by various applications for use when needed.

There are two Linux C/C++ library types which can be created:

0. Static libraries (.a): Library of object code which is linked with, and becomes part of the application.
0. Dynamically linked shared object libraries (.so): There is only one form of this library but it can be used in two ways.
   0. Dynamically linked at run time but statically aware. The libraries must be available during compile/link phase. The shared objects are not included into the executable component but are tied to the execution.
   0. Dynamically loaded/unloaded and linked during execution (i.e. browser plug-in) using the dynamic linking loader system functions.

Library naming conventions:

Libraries are typically names with the prefix "lib". This is true for all the C standard libraries. When linking, the command line reference to the library will not contain the library prefix or suffix.

Thus the following link command: gcc src-file.c -lm -lpthread
The libraries referenced in this example for inclusion during linking are the math library and the thread library. They are found in /usr/lib/libm.a and /usr/lib/libpthread.a.

How to generate a library:

• Compile: cc -Wall -c ctest1.c ctest2.c
  Compiler options:
  • -Wall: include warnings. See man page for warnings specified.
• Create library "libctest.a": ar -cvq libctest.a ctest1.o ctest2.o
• List files in library: ar -t libctest.a
• Linking with the library:
  • cc -o executable-name prog.c libctest.a
  • cc -o executable-name prog.c -L/path/to/library-directory -lctest
• Example files:
  • ctest1.c

    void ctest1(int *i) { *i=5; }

  • ctest2.c

    void ctest2(int *i)

fflush就是將output buffer的內容寫到file裡.那如果是fflush()就是寫到stdout.

Syntax:

#include <stdio.h>
int fflush( FILE *stream );

Description:
If the given file stream is an output stream, then fflush() causes the output buffer to be written to the file. If the given stream is of the input type, then fflush() causes the input buffer to be cleared. fflush() is useful when debugging, if a program segfaults before it has a chance to write output to the screen. Calling fflush( STDOUT ) directly after debugging output will ensure that your output is displayed at the correct time.

#include<stdio.h>
#include<unistd.h>

int main()
{
#if 1
        int i;
        for (i=0;i<10;i++)
        {
          printf("%d",i);
           fflush(stdout);
          sleep(1);