C语言编译时出现invalid application of 'sizeof' to an incomplete type 'struct tcphdr'怎么办?-灵析社区

Hello! 简而言之,所谓"struct tcphdr"是一个来自外部的被include的库文件的结构体. 而且它这个结构体的构造有点诡异.我不知道什么是__extension__ union,这会让它更难处理吗? struct tcphdr { __extension__ union { struct { uint16_t th_sport; /* source port */ uint16_t th_dport; /* destination port */ tcp_seq th_seq; /* sequence number */ tcp_seq th_ack; /* acknowledgement number */ # if __BYTE_ORDER == __LITTLE_ENDIAN uint8_t th_x2:4; /* (unused) */ uint8_t th_off:4; /* data offset */ # endif # if __BYTE_ORDER == __BIG_ENDIAN uint8_t th_off:4; /* data offset */ uint8_t th_x2:4; /* (unused) */ # endif uint8_t th_flags; # define TH_FIN 0x01 # define TH_SYN 0x02 # define TH_RST 0x04 # define TH_PUSH 0x08 # define TH_ACK 0x10 # define TH_URG 0x20 uint16_t th_win; /* window */ uint16_t th_sum; /* checksum */ uint16_t th_urp; /* urgent pointer */ }; struct { uint16_t source; uint16_t dest; uint32_t seq; uint32_t ack_seq; # if __BYTE_ORDER == __LITTLE_ENDIAN uint16_t res1:4; uint16_t doff:4; uint16_t fin:1; uint16_t syn:1; uint16_t rst:1; uint16_t psh:1; uint16_t ack:1; uint16_t urg:1; uint16_t res2:2; # elif __BYTE_ORDER == __BIG_ENDIAN uint16_t doff:4; uint16_t res1:4; uint16_t res2:2; uint16_t urg:1; uint16_t ack:1; uint16_t psh:1; uint16_t rst:1; uint16_t syn:1; uint16_t fin:1; # else # error "Adjust your defines" # endif uint16_t window; uint16_t check; uint16_t urg_ptr; }; }; }; 我不能在程序里对它进行sizeof,或者对指向它的类型的地址进行`->`操作,无法编译通过. 问题是这是官方给的源码,它肯定自己编译是通过的啊?是我的编译选项有问题吗?我是本地交叉编译. 而且,sizeof实在不行的话我可以直接把它全改成具体的数字,但`->`这个操作我不可能把它混过去. 怎么办好呢?