Linux udhcp client (udhcpc) get IP at anytime

/***************************************************************************************
 *                      Linux udhcp client (udhcpc) get IP at anytime
 * 声明：
 *     对于ARM产品，之前在想如何能够做到像PC机那样，当网线插入RJ45座的时候，就能自动获
 * 取到IP，这样就无需另外去设置IP等相关信息，主要考虑到嵌入式产品有时候可能根本没有图形
 * 界面，如果不能自动连接网络，只能通过调试口去设置网络，不合情理，直觉认为可能需要依赖
 * udhcpc来做这件事，于是发现了udhcpc命令的-b参数。
 *
 *                                                    2015-10-21 晴 深圳 南山平山村 曾剑锋
 **************************************************************************************/

一、参考文章：
    . Busybox Command Help
        http://busybox.net/downloads/BusyBox.html
    . udhcpc 后台运行的方法【总结】
        http://www.360doc.com/content/12/0323/09/9106558_196861856.shtml
    . udhcpc running in the background [Summary]
        http://www.programmershare.com/2365338/

二、busybox udhcpc help:
    udhcpc [-Cfbnqtvo] [-c CID] [-V VCLS] [-H HOSTNAME] [-i INTERFACE]
    [-p pidfile] [-r IP] [-s script] [-O dhcp-option]...
        -V,--vendorclass=CLASSID        Vendor class identifier
        -i,--interface=INTERFACE        Interface to use (default eth0)
        -H,-h,--hostname=HOSTNAME       Client hostname
        -c,--clientid=CLIENTID  Client identifier
        -C,--clientid-none      Suppress default client identifier
        -p,--pidfile=file       Create pidfile
        -r,--request=IP         IP address to request
        -s,--script=file        Run file at DHCP events (default /usr/share/udhcpc/default.script)
        -t,--retries=N          Send up to N request packets
        -T,--timeout=N          Try to get a lease for N seconds (default )
        -A,--tryagain=N         Wait N seconds (default ) after failure
        -O,--request-option=OPT Request DHCP option OPT (cumulative)
        -o,--no-default-options Do not request any options (unless -O is also given)
        -f,--foreground Run in foreground
        -b,--background Background if lease is not immediately obtained
        -S,--syslog     Log to syslog too
        -n,--now        Exit with failure if lease is not immediately obtained
        -q,--quit       Quit after obtaining lease
        -R,--release    Release IP on quit
        -a,--arping     Use arping to validate offered address

三、udhcpc -b test：
    root@freescale ~$ ifconfig
    eth0      Link encap:Ethernet  HWaddr 1E:ED:::1A:B3
              UP BROADCAST RUNNING MULTICAST  MTU:  Metric:
              RX packets: errors: dropped: overruns: frame:
              TX packets: errors: dropped: overruns: carrier:
              collisions: txqueuelen:
              RX bytes: (0.0 B)  TX bytes: (0.0 B)

    lo        Link encap:Local Loopback
              inet addr:127.0.0.1  Mask:255.0.0.0
              UP LOOPBACK RUNNING  MTU:  Metric:
              RX packets: errors: dropped: overruns: frame:
              TX packets: errors: dropped: overruns: carrier:
              collisions: txqueuelen:
              RX bytes: (390.9 KiB)  TX bytes: (390.9 KiB)
    root@freescale ~$ udhcpc -b （这里我并没有用udhcpc -b -i eth0 -p /var/run/udhcpc.pid -R &）
    udhcpc (v1.20.2) started
    Sending discover...
    Sending discover...
    Sending discover...
    No lease, forking to background
    root@freescale ~$ PHY: : - Link is Up - /Full
    root@freescale ~$ ifconfig   （插入网线之后，最好等上2-4秒才能获取到IP）
    eth0      Link encap:Ethernet  HWaddr 1E:ED:::1A:B3
              inet addr:10.0.1.52  Bcast:10.0.1.255  Mask:255.255.255.0
              UP BROADCAST RUNNING MULTICAST  MTU:  Metric:
              RX packets: errors: dropped: overruns: frame:
              TX packets: errors: dropped: overruns: carrier:
              collisions: txqueuelen:
              RX bytes: (1.2 KiB)  TX bytes: (656.0 B)

    lo        Link encap:Local Loopback
              inet addr:127.0.0.1  Mask:255.0.0.0
              UP LOOPBACK RUNNING  MTU:  Metric:
              RX packets: errors: dropped: overruns: frame:
              TX packets: errors: dropped: overruns: carrier:
              collisions: txqueuelen:
              RX bytes: (390.9 KiB)  TX bytes: (390.9 KiB)