interpret following C statement?

[linux lover]

hello,
how to interpret following C statement?
#define TCP_SKB_CB(__skb)       ((struct tcp_skb_cb 
*)&((__skb)->cb[0]))
what is use of tcp_skb_cb structure in kernel source
tcp.h? how 
TCP_SKB_CB(skb) is translated in its definition?
regards,
linux_lover






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