dev_alloc_skb(len+16) skb_reserve(skb,2) skb

本文介绍了Linux内核中skb缓冲区管理的重要函数，包括dev_alloc_skb用于分配skb缓冲区并内置未指定头部空间，skb_reserve用于调整头部空间，以及skb_put和skb_push如何增加数据区长度或预留头部空间。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

/**
*      dev_alloc_skb - allocate an skbuff for receiving
*      @length: length to allocate
*
*      Allocate a new &sk_buff and assign it a usage count of one. The
*      buffer has unspecified headroom built in. Users should allocate
*      the headroom they think they need without accounting for the
*      built in space. The built in space is used for optimisations.
*
*      %NULL is returned if there is no free memory. Although this function
*      allocates memory it can be called from an interrupt.
*/
static inline struct sk_buff *dev_alloc_skb(unsigned int length)
{
        return __dev_alloc_skb(length, GFP_ATOMIC);
}

首先L2的地址一共是14(6+6+2)个BYTE:

L2+L3(IP addr)+L4

按照常理L2地址是在L3后加, 所以在ALLOCSKB的时候要留14个BYTE,为了以后给L2用.
但是计算机一般是4字节对齐的, 如果留14个BYTE那么IP只能排在15个byte位子, ip头要经常访问所以这样效率似乎不好.
于是在预留2个(14+2 = 16) 正好让IP头4字节对齐.

/**
*      skb_reserve - adjust headroom
*      @skb: buffer to alter
*      @len: bytes to move
*
*      Increase the headroom of an empty &sk_buff by reducing the tail
*      room. This is only allowed for an empty buffer.
*/
static inline void skb_reserve(struct sk_buff *skb, int len)
{
        skb->data += len;
        skb->tail += len;
}

skb_reserve(skb,2);

The networking layer currently aligns IP headers in rx packets. It does

this via skb_reserve(,2).



skb_put 于 skb_push:

skb_put() 增长数据区的长度来为memcpy准备空间. 许多的网络操作需要加入一些桢头, 这可以使用skb_push


来将数据区向后推, 为头留出空间.  



----------------------------------------  

| head | data |             |  

----------------------------------------  

skb_put  

-----------------------------------------  

| head | data | put_data |       |  

-----------------------------------------  

skb_push


  ------------------------------------------  

| head | push_data | data | put_data | |  

------------------------------------------  




/**

 *      skb_put - add data to a buffer                             

 *      @skb: buffer to use

 *      @len: amount of data to add                                

 *

 *      This function extends the used data area of the buffer. If this would

 *      exceed the total buffer size the kernel will panic. A pointer to the

 *      first byte of the extra data is returned.                  

 */



static inline unsigned char *skb_put(struct sk_buff *skb, unsigned int len)

{                                                                  

        unsigned char *tmp = skb->tail;                            

        SKB_LINEAR_ASSERT(skb);                                    

        skb->tail += len;                                          

        skb->len  += len;                                          

        if (unlikely(skb->tail>skb->end))                          

                skb_over_panic(skb, len, current_text_addr());     

        return tmp;                                                

}