if_nametoindex可以检查网卡名称是否有效

NAME
     if_nametoindex, if_indextoname, if_nameindex, if_freenameindex - convert
     interface index to name, and vice versa

SYNOPSIS
     #include <net/if.h>

     unsigned int
     if_nametoindex(const char *ifname);

     char *
     if_indextoname(unsigned int ifindex, char *ifname);

     struct if_nameindex *
     if_nameindex(void);

     void
     if_freenameindex(struct if_nameindex *ptr);