如何判断网线是否连接

使用ifconfigl命令能很方便的查看网卡与网线是否连通：

# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 00:25:35:68:CC:D6
inet addr:192.168.1.168 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::215:c5ff:fe18:ccd6/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:130722 errors:0 dropped:0 overruns:0 frame:0
TX packets:112560 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:104371099 (99.5 MiB) TX bytes:20518584 (19.5 MiB)
Interrupt:16

其中的RUNNING就表示网卡与网线正常链接，拔掉网线再运行此命令就会发现RUNNING不在了。

NTP-SERVER:/#mii-tool
eth0: negotiated 100baseTx-FD, link ok
eth1: no link
eth2: no link
eth3: no link

[NTP-Fedora20 ~]#mii-tool p32p1
p32p1: negotiated 100baseTx-FD, link ok
[NTP-Fedora20 ~]#ethtool p32p1
Settings for p32p1:
Supported ports: [ TP ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Supported pause frame use: No
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Advertised pause frame use: No
Advertised auto-negotiation: Yes
Speed: 100Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 1
Transceiver: internal
Auto-negotiation: on
MDI-X: on (auto)
Supports Wake-on: pumbg
Wake-on: g
Current message level: 0x00000007 (7)
drv probe link
Link detected: yes
[NTP-Fedora20 ~]#

方法一：使用ioctl向socket发送SIOCETHTOOL命令字


#include<stdio.h>
#include<stdlib.h>
#include<string.h>

#include <fcntl.h>
#include <errno.h>
#include <sys/ioctl.h>

#include <sys/types.h>
#include <sys/socket.h>
#include <linux/if.h>
#include <linux/sockios.h>
#include <linux/ethtool.h>

int get_netlink_status(constchar *if_name);

int main()
{
    if(getuid()!= 0)
    {
        fprintf(stderr,"Netlink Status Check Need Root Power.
");
        return 1;
    }

    printf("Net link status: %d
", get_netlink_status("eth0"));

    return 0;
}

// if_name like "ath0", "eth0". Notice: call this function
// need root privilege.
// return value:
// -1 -- error , details can check errno
// 1 -- interface link up
// 0 -- interface link down.
int get_netlink_status(constchar *if_name)
{
    int skfd;
    struct ifreq ifr;
    struct ethtool_value edata;

    edata.cmd = ETHTOOL_GLINK;
    edata.data = 0;

    memset(&ifr, 0,sizeof(ifr));
    strncpy(ifr.ifr_name, if_name,sizeof(ifr.ifr_name)- 1);
    ifr.ifr_data =(char *) &edata;

    if (( skfd= socket(AF_INET, SOCK_DGRAM, 0 )) < 0)
        return -1;

    if(ioctl( skfd, SIOCETHTOOL,&ifr ) == -1)
    {
        close(skfd);
        return -1;
    }

    close(skfd);
    return edata.data;

}

方法二： 使用ifconfigl命令能很方便的查看网卡与网线是否连通：

# ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 00:25:35:68:CC:D6  
          inet addr:192.168.1.168  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::215:c5ff:fe18:ccd6/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:130722 errors:0 dropped:0 overruns:0 frame:0
          TX packets:112560 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:104371099 (99.5 MiB)  TX bytes:20518584 (19.5 MiB)
          Interrupt:16 

其中的RUNNING就表示网卡与网线正常链接，拔掉网线再运行此命令就会发现RUNNING不在了。

    linux系统提供了popen/pclose进程管道让C和shell很方便的交互，下面C代码结合shell命令检测网卡与网线连通状况：

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

int GetNetStat( )
{
    char    buffer[BUFSIZ];
    FILE    *read_fp;
    int        chars_read;
    int        ret;

    memset( buffer, 0, BUFSIZ );
    read_fp = popen("ifconfig eth0 | grep RUNNING", "r");
    if ( read_fp != NULL )
    {
        chars_read = fread(buffer, sizeof(char), BUFSIZ-1, read_fp);
        if (chars_read > 0)
        {
            ret = 1;
        }
        else
        {
            ret = -1;
        }
        pclose(read_fp);
    }
    else
    {
        ret = -1;
    }

    return ret;
}


int main()
{
    int i=0;
    i = GetNetStat();
    printf( "
NetStat = %d
", i );
    return 0;
}