参照这篇博客: http://www.geekpage.jp/programming/linux-network/book/04/4-21.php

* 查看主机当前网卡，哪块在使用.

ifconfig

lo0: flags=8049 mtu 16384
options=3
inet6 ::1 prefixlen 128
inet 127.0.0.1 netmask 0xff000000
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1
nd6 options=1
gif0: flags=8010 mtu 1280
stf0: flags=0<> mtu 1280
en0: flags=8863 mtu 1500
ether a4:5e:60:d6:57:17
inet6 fe80::a65e:60ff:fed6:5717%en0 prefixlen 64 scopeid 0x4
inet 192.168.16.103 netmask 0xffffff00 broadcast 192.168.16.255
nd6 options=1
media: autoselect
status: active
en1: flags=963 mtu 1500
options=60
ether 4a:00:01:33:73:10
media: autoselect
status: inactive
en2: flags=963 mtu 1500
options=60
ether 4a:00:01:33:73:11
media: autoselect
status: inactive
p2p0: flags=8843 mtu 2304
ether 06:5e:60:d6:57:17
media: autoselect
status: inactive
awdl0: flags=8943 mtu 1484
ether 7a:00:b6:6c:ea:a0
inet6 fe80::7800:b6ff:fe6c:eaa0%awdl0 prefixlen 64 scopeid 0x8
nd6 options=1
media: autoselect
status: active
bridge0: flags=8822 mtu 1500
options=63
ether a6:5e:60:6d:62:00
Configuration:
id 0:0:0:0:0:0 priority 0 hellotime 0 fwddelay 0
maxage 0 holdcnt 0 proto stp maxaddr 100 timeout 1200
root id 0:0:0:0:0:0 priority 0 ifcost 0 port 0
ipfilter disabled flags 0x2
member: en1 flags=3
ifmaxaddr 0 port 5 priority 0 path cost 0
member: en2 flags=3
ifmaxaddr 0 port 6 priority 0 path cost 0
media:
status: inactive

ifconfig 输出

ifconfig en0

en0: flags=8863 mtu 1500
ether a4:5e:60:d6:57:17
inet6 fe80::a65e:60ff:fed6:5717%en0 prefixlen 64 scopeid 0x4
inet 192.168.16.103 netmask 0xffffff00 broadcast 192.168.16.255
nd6 options=1
media: autoselect
status: active

可以看到en0网卡有有效的ip地址net 192.168.16.103, 说明这块网卡在使用, Linux环境通常是eth0.

* 创建文件main.c

#include
#include
#include
#include
#include
#include
#include
#include

int main(int argc, const char * argv[]) {
int fd;
struct ifreq ifr;

fd = socket(AF\_INET, SOCK\_DGRAM, 0);

strncpy(ifr.ifr\_name, "en0", IFNAMSIZ-1);   // eth0  
if (ioctl(fd, SIOCGIFMTU, &ifr)) {  
    perror("ioctl");  
    return 1;  
}  
close(fd);

printf("%d\\n", ifr.ifr\_mtu);

return 0;  

}

* 编译运行

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1500

Program ended with exit code: 0

得出MTU为1500

* 一些函数原型

#include

int socket(int domain, int type, int protocol);

socket.h

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/*

* Types

*/

#define SOCK_STREAM 1 /* stream socket */

#define SOCK_DGRAM 2 /* datagram socket */

#define SOCK_RAW 3 /* raw-protocol interface */

// …

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/*

* [XSI] Structure used by kernel to store most addresses.

*/

struct sockaddr {

__uint8_t sa_len; /* total length */

sa_family_t sa_family; /* [XSI] address family */

char sa_data[14]; /* [XSI] addr value (actually larger) */

};

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int accept(int, struct sockaddr * __restrict, socklen_t * __restrict)

__DARWIN_ALIAS_C(accept);

int bind(int, const struct sockaddr *, socklen_t) __DARWIN_ALIAS(bind);

int connect(int, const struct sockaddr *, socklen_t) __DARWIN_ALIAS_C(connect);

////////////////////////////////////////////////////////////////

if.h

////////////////////////////////////////////////////////////////

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/*

* Interface request structure used for socket

* ioctl's.  All interface ioctl's must have parameter

* definitions which begin with ifr_name.  The

* remainder may be interface specific.

*/

struct ifreq {

#ifndef IFNAMSIZ

#define IFNAMSIZ IF_NAMESIZE

#endif

char ifr_name[IFNAMSIZ]; /* if name, e.g. "en0" */

union {

struct sockaddr ifru_addr;

struct sockaddr ifru_dstaddr;

struct sockaddr ifru_broadaddr;

short ifru_flags;

int ifru_metric;

int ifru_mtu;

int ifru_phys;

int ifru_media;

int ifru_intval;

caddr_t ifru_data;

struct ifdevmtu ifru_devmtu;

struct ifkpi ifru_kpi;

u_int32_t ifru_wake_flags;

u_int32_t ifru_route_refcnt;

int ifru_cap[2];

} ifr_ifru;

#define ifr_addr ifr_ifru.ifru_addr /* address */

#define ifr_dstaddr ifr_ifru.ifru_dstaddr /* other end of p-to-p link */

#define ifr_broadaddr ifr_ifru.ifru_broadaddr /* broadcast address */

/////////////////////////////////////////////////

ioctl -- control device

SYNOPSIS
#include

int ioctl(int fildes, unsigned long request, …);

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