get_ifi_info函数

http://www.cnblogs.com/s7vens/archive/2012/02/14/2351160.html

/* Our own header for the programs that need interface configuration info. include this file, instead of "unp.h" */
#ifndef __unp_ifi_h
#define __unp_ifi_h
#include "unp.h"
#include <net/if.h>
#define IFI_NAME    16  /* same as IFNAMSIZ in <net/if.h> */
#define IFI_HADDR  8    /* allow for 64-bit EUI-64 in future */
sturct ifi_info
{/* 我们函数返回一个这些结构的链表，其中每个结构的ifi_next成员指向下一个结构 */
  char ifi_name[IFI_NAME];   /* interface name, null terminated */
  u_char   ifi_haddr[IFI_HADDR];  /* hardware address */
  u_short  ifi_hlen;  /* #bytes in hardware address: 0, 6, 8 */
  short ifi_flags;  /* IFI_xxx constants from <net/if.h> */
  short ifi_myflags; /* our own IFI_xxx flags */
  struct sockaddr * ifi_addr; /* primary address */
  struct sockaddr * ifi_brdaddr; /* broadcast address */
  struct sockaddr * ifi_dstaddr; /* destination address */
  struct ifi_info * ifi_next; /* next of these structures */
};
#define IFI_ALIAS  1  /* ifi_addr is an alias */
/* function prototypes */
struct ifi_info * get_ifi_info(int, int);
struct ifi_info * Get_ifi_info(int, int);
void free_ifi_info(struct ifi_info *);
#endif /* __unp_ifi_h */


#include "unpifi.h"
struct ifi_info * get_ifi_info(int family, int doaliases)
{
  struct ifi_info * ifi, * ifihead, * * ifipnext;
  int sockfd, len, lastlen, flags, myflags;
  char * ptr, * buf, lastname[IFNAMSIZ], * cptr;
  struct ifconf ifc;
  struct ifreq * ifr, ifrcopy;
  struct sockaddr_in * sinptr;
  sockfd = Socket(AF_INET, SOCK_DGRAM, 0); /* 创建一个用于ioctl的UDP套接口 */
  lastlen = 0; /* 把lastlen初始化成0 */
  len = 100 * sizeof(struct ifreq); /* initial buffer size guess */
  for( ; ; )
  {
    buf = Malloc(len); /* 分配一个缓冲区，开始时为100个ifreq结构的空间 */
    ifc.ifc_len = len;
    ifc.ifc_buf = buf;
    if(ioctl(sockfd, SIOCGIFCONF, &ifc) < 0)
    {/* 如果ioctl返回一个EINVAL错误，而且还没有得到过一个成功的返回(也就是lastlen还是0)，那么我们还没有分配一个足够大的缓冲区，继续进行循环 */
      if(errno != EINVAL || lastlen != 0)
        err_sys("ioctl error");
    }
    else
    {/* 如果ioctl返回成功，而且返回的长度等于lastlen，那么这个长度没有改变(及缓冲区足够大)，于是break出这个循环，因为已经得到了所有信息 */
      if(ifc.ifc_len == lastlen)
        break;  /* success, len has not changed */
      lastlen = ifc.ifc_len;
    }
    /* 每循环一次，把缓冲区的大小增加能多存放10个ifreq结构的空间 */
    len += 10 * sizeof(struct ifreq); /* increment */
    free(buf);
  }
  ifihead = NULL; /* 因为将要返回一个指向ifi_info结构的链表头的指针，所以用两个变量ifihead和ifipnext在建立链表时保存指针 */
  ifipnext = &ifihead;
  lastname[0] = 0;
  for(ptr = buf; ptr < buf + ifc.ifc_len; ) /* 在遍历所有ifreq结构时，ifr指向每个结构，然后我们增加ptr指向下一个结构 */
  {
    ifr = (struct ifreq *)ptr;
#ifdef  HAVE_SOCKADDR_SA_LEN
    len = max(sizeof(struct sockaddr), ifr->ifr_addr.sa_len);
#else
    switch(ifr->ifr_addr.sa_family)
    {
#ifdef  IPV6
      case AF_INET6:
        len = sizeof(struct sockaddr_in6);
      break;
#endif
      case AF_INET;
      default:
        len = sizeof(struct sockaddr);
      break;
    }
#endif  /* HAVE_SOCKADDR_SA_LEN */
    ptr += sizeof(ifr->ifr_name) + len; /* for next one in buffer */
    if(ifr->ifr_addr.sa_family != family) /* 忽略所有不是调用者期望的地址族的地址 */
      continue; /* ignore if not desired address family */
    myflags = 0; /* 检测在该接口上可能存在的任何别名地址，也就是赋给这个接口的其它地址 */
    if((cptr = strchr(ifr->ifr_name, ':' )) != NULL)
      * cptr = 0; /* replace colon will null */
    if(strncmp(lastname, ifr->ifr_name, IFNAMSIZ) == 0)
    {
      if(doaliases == 0)
        continue; /* already processed this interface */
      myflags = IFI_ALIAS;
    }
    memcpy(lastname, ifr->ifr_name, IFNAMSIZ);
    ifrcopy = * ifr; /* 发出一个SIOCGIFFLAGS ioctl请求取接口标志。ioctl的第三个参数是一个指向ifreq结构的指针，结构中包含要获取标志的接口名。在发出ioctl之前对ifreq结构做一个拷贝，因为如果不这样做的话，这个请求会覆盖接口的IP地址，因为它们都同一联合的成员。如果接口不在工作，就忽略掉 */
    ioctl(sockfd, SIOCGIFFLAGS, &ifrcopy);
    flags = ifrcopy.ifr_flags;
    if((flags & IFF_UP) ==0)
      continue; /* ignore if interface not up */
    ifi = Calloc(1, sizeof(struct ifi_info)); /* 给ifi_info结构分配内存，并把它加到正在建立的链表的末尾 */
    * ifipnext = ifi; /* prev points to this new one */
    ifipnext = &ifi->ifi_next; /* pointer to next one goes here */
    ifi->ifi_flags = flags; /* IFF_xxx values */
    ifi->ifi_myflags = myflags; /* IFF_xxx values */
    memcpy(ifi->ifi_name, ifr->ifr_name, IFI_NAME);
    ifi->ifi_name[IFI_NAME-1] = '\0';
    switch(ifr->ifr_addr.sa_family)
    {
      case AF_INET:
        sinptr = (struct sockaddr_in *)&fir->ifr_addr;
        if(ifi->ifi_addr == NULL)
        {
          ifi->ifi_addr = Calloc(1, sizeof(struct sockaddr_in));
          memcpy(ifi->ifi_addr, sinptr, sizeof(struct sockaddr_in));
#ifdef SIOCGIFBRDADDR
            if(flags & IFF_BROADCAST)
            {
              ioctl(sockfd, SIOCGIFBRDADDR, &ifrcopy);
              sinptr = (struct sockaddr_in *)&ifrcopy.ifr_broadaddr;
              ifi->ifi_brdaddr = Calloc(1, sizeof(struct sockaddr_in));
              memcpy(ifi->ifi_brdaddr, sinptr, sizeof(struct sockaddr_in));
            }
#endif
#ifdef  SIOCGIFDSTADDR
            if(flags & IFF_POINTOPOINT)
            {
              ioctl(sockfd, SIOCGIFDSTADDR, &ifrcopy);
              sinptr = (struct sockaddr_in *)&ifrcopy.ifr_dstaddr;
              ifi->ifi_dstaddr = Calloc(1, sizeof(struct sockaddr_in));
              memcpy(ifi->ifi_dstaddr, sinptr, sizeof(struct sockaddr_in));
            }
#endif
        }
       break;
      default:
       break;
    }
  }
  free(buf);
  return(ifihead); /* pointer to first structure in linked list */
}

/* free_ifi_info函数，它以一个由get_ifi_info返回的指针做按数，释放所有动态分配的内存 */
void free_ifi_info(struct ifi_info * ifihead)
{
  struct ifi_info * ifi, * ifihead;
  for(ifi= ifihead; ifi != NULL; ifi = ifinext)
  {
    if(ifi->ifi_addr != NULL)
      free(ifi->ifi_addr);
    if(ifi->ifi_brdaddr != NULL)
      free(ifi->ifi_brdaddr);
    if(ifi->ifi_dstaddr != NULL)
      free(ifi->ifi_dstaddr);
    ifinext = ifi->ifi_next; /* cant fetch ifi_next after free() */
    free(ifi); /* the ifi_info{} itself */
  }
}