转自：https://blog.****.net/kzq_qmi/article/details/46900589
数据包pbuf：
LwIP采用数据结构 pbuf 来描述数据包，其结构如下：
struct pbuf {
/** next pbuf in singly linked pbuf chain */
struct pbuf *next;
/** pointer to the actual data in the buffer */
void *payload;
/**
* total length of this buffer and all next buffers in chain
* belonging to the same packet.
*
* For non-queue packet chains this is the invariant:
* p->tot_len == p->len + (p->next? p->next->tot_len: 0)
*/
u16_t tot_len;
/** length of this buffer */
u16_t len;
/** pbuf_type as u8_t instead of enum to save space */
u8_t /*pbuf_type*/ type;
/** misc flags */
u8_t flags;
/**
* the reference count always equals the number of pointers
* that refer to this pbuf. This can be pointers from an application,
* the stack itself, or pbuf->next pointers from a chain.
*/
u16_t ref;
}; 各成员含义上面的注释已经说得很清楚了。
关于采用链表结构，是因为实际发送或接收的数据包可能很大，而每个 pbuf 能够管理的数据可能很少，所以，往往需要多个 pbuf 结构才能完全描述一个数据包。
另外，最后的 ref 字段表示该 pbuf 被引用的次数。这里又是一个纠结的地方啊。初始化一个 pbuf 的时候， ref 字段值被设置为 1，当有其他 pbuf 的 next 指针指向该 pbuf 时，该 pbuf 的 ref 字段值加一。所以，要删除一个 pbuf 时， ref 的值必须为 1 才能删除成功，否则删除失败。
上图中注意 payload 并没有指向 ref 字段之后，而是隔了一定的区域。这段区域就是offset 的大小，这段区域用来存储数据的包头，如 TCP 包头， IP 包头等。当然， offset 也可以是 0。
来看代码：
/**
* Allocates a pbuf of the given type (possibly a chain for PBUF_POOL type).
*
* The actual memory allocated for the pbuf is determined by the
* layer at which the pbuf is allocated and the requested size
* (from the size parameter).
*
* @param layer flag to define header size
* @param length size of the pbuf's payload
* @param type this parameter decides how and where the pbuf
* should be allocated as follows:
*
* - PBUF_RAM: buffer memory for pbuf is allocated as one large
* chunk. This includes protocol headers as well.
* - PBUF_ROM: no buffer memory is allocated for the pbuf, even for
* protocol headers. Additional headers must be prepended
* by allocating another pbuf and chain in to the front of
* the ROM pbuf. It is assumed that the memory used is really
* similar to ROM in that it is immutable and will not be
* changed. Memory which is dynamic should generally not
* be attached to PBUF_ROM pbufs. Use PBUF_REF instead.
* - PBUF_REF: no buffer memory is allocated for the pbuf, even for
* protocol headers. It is assumed that the pbuf is only
* being used in a single thread. If the pbuf gets queued,
* then pbuf_take should be called to copy the buffer.
* - PBUF_POOL: the pbuf is allocated as a pbuf chain, with pbufs from
* the pbuf pool that is allocated during pbuf_init().
*
* @return the allocated pbuf. If multiple pbufs where allocated, this
* is the first pbuf of a pbuf chain.
*/
struct pbuf *
pbuf_alloc(pbuf_layer layer, u16_t length, pbuf_type type)
{
struct pbuf *p, *q, *r;
u16_t offset;
s32_t rem_len; /* remaining length */
LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_alloc(length=%"U16_F")\n", length));
/* determine header offset */
offset = 0;
switch (layer) { //注意这里从协议栈上层开始，方便offset从上层往下叠加，因此也没加 break
case PBUF_TRANSPORT:
/* add room for transport (often TCP) layer header */
offset += PBUF_TRANSPORT_HLEN;
/* FALLTHROUGH */
case PBUF_IP:
/* add room for IP layer header */
offset += PBUF_IP_HLEN;
/* FALLTHROUGH */
case PBUF_LINK:
/* add room for link layer header */
offset += PBUF_LINK_HLEN;
break;
case PBUF_RAW:
break;
default:
LWIP_ASSERT("pbuf_alloc: bad pbuf layer", 0);
return NULL;
}
switch (type) {
case PBUF_POOL:
/* allocate head of pbuf chain into p */
p = (struct pbuf *)memp_malloc(MEMP_PBUF_POOL); //分配第一个pbuf
if (p == NULL) {
return NULL;
}
p->type = type;
p->next = NULL;
/* make the payload pointer point 'offset' bytes into pbuf data memory */
p->payload = LWIP_MEM_ALIGN((void *)((u8_t *)p + (SIZEOF_STRUCT_PBUF + offset)));
/* the total length of the pbuf chain is the requested size */
p->tot_len = length; //该pbuf及其以后pbuf的负载数据总长度
/* set the length of the first pbuf in the chain */
p->len = LWIP_MIN(length, PBUF_POOL_BUFSIZE_ALIGNED - LWIP_MEM_ALIGN_SIZE(offset)); //负载数据可能大于分配空间长度，也有可能小于，取当前pbuf实际的负载长度
/* set reference count (needed here in case we fail) */
p->ref = 1;
/* now allocate the tail of the pbuf chain */
//如果一个pbuf不够的话，接着分配
/* remember first pbuf for linkage in next iteration */
r = p;
/* remaining length to be allocated */
rem_len = length - p->len;
/* any remaining pbufs to be allocated? */
while (rem_len > 0) {
q = (struct pbuf *)memp_malloc(MEMP_PBUF_POOL); //从第二个pbuf开始，不再需要TCP/IP之类的头，所以没有offset
if (q == NULL) {
/* free chain so far allocated */
pbuf_free(p); //注意这里，如果当前pbuf分配不成功，要把之前分配的所有pbuf都释放掉
/* bail out unsuccesfully */
return NULL;
}
q->type = type;
q->flags = 0;
q->next = NULL;
/* make previous pbuf point to this pbuf */
r->next = q;
/* set total length of this pbuf and next in chain */
q->tot_len = (u16_t)rem_len;
/* this pbuf length is pool size, unless smaller sized tail */
q->len = LWIP_MIN((u16_t)rem_len, PBUF_POOL_BUFSIZE_ALIGNED);
q->payload = (void *)((u8_t *)q + SIZEOF_STRUCT_PBUF);
q->ref = 1;
/* calculate remaining length to be allocated */
rem_len -= q->len;
/* remember this pbuf for linkage in next iteration */
r = q;
}
/* end of chain */
/*r->next = NULL;*/
break;
case PBUF_RAM:
/* If pbuf is to be allocated in RAM, allocate memory for it. */
p = (struct pbuf*)mem_malloc(LWIP_MEM_ALIGN_SIZE(SIZEOF_STRUCT_PBUF + offset) + LWIP_MEM_ALIGN_SIZE(length));
if (p == NULL) {
return NULL;
}
/* Set up internal structure of the pbuf. */
p->payload = LWIP_MEM_ALIGN((void *)((u8_t *)p + SIZEOF_STRUCT_PBUF + offset));
p->len = p->tot_len = length;
p->next = NULL;
p->type = type;
break;
/* pbuf references existing (non-volatile static constant) ROM payload? */
case PBUF_ROM:
/* pbuf references existing (externally allocated) RAM payload? */
case PBUF_REF:
/* only allocate memory for the pbuf structure */
p = (struct pbuf *)memp_malloc(MEMP_PBUF);
if (p == NULL) {
return NULL;
}
/* caller must set this field properly, afterwards */
p->payload = NULL;
p->len = p->tot_len = length;
p->next = NULL;
p->type = type;
break;
default:
return NULL;
}
/* set reference count */
p->ref = 1;
/* set flags */
p->flags = 0;
return p;
}
/**
* Dereference a pbuf chain or queue and deallocate any no-longer-used
* pbufs at the head of this chain or queue.
*
* Decrements the pbuf reference count. If it reaches zero, the pbuf is
* deallocated.
*
* For a pbuf chain, this is repeated for each pbuf in the chain,
* up to the first pbuf which has a non-zero reference count after
* decrementing. So, when all reference counts are one, the whole
* chain is free'd.
*
* @param p The pbuf (chain) to be dereferenced.
*
* @return the number of pbufs that were de-allocated
* from the head of the chain.
*
* @note MUST NOT be called on a packet queue (Not verified to work yet).
* @note the reference counter of a pbuf equals the number of pointers
* that refer to the pbuf (or into the pbuf).
*
* @internal examples:
*
* Assuming existing chains a->b->c with the following reference
* counts, calling pbuf_free(a) results in:
*
* 1->2->3 becomes ...1->3
* 3->3->3 becomes 2->3->3
* 1->1->2 becomes ......1
* 2->1->1 becomes 1->1->1
* 1->1->1 becomes .......
*
*/
u8_t
pbuf_free(struct pbuf *p)
{
u16_t type;
struct pbuf *q;
u8_t count;
if (p == NULL) {
return 0;
}
count = 0;
/* de-allocate all consecutive pbufs from the head of the chain that
* obtain a zero reference count after decrementing*/
while (p != NULL) {
u16_t ref;
SYS_ARCH_DECL_PROTECT(old_level); //申请临界变量保护
/* Since decrementing ref cannot be guaranteed to be a single machine operation
* we must protect it. We put the new ref into a local variable to prevent
* further protection. */
SYS_ARCH_PROTECT(old_level); //进入临界区
/* all pbufs in a chain are referenced at least once */
LWIP_ASSERT("pbuf_free: p->ref > 0", p->ref > 0);
/* decrease reference count (number of pointers to pbuf) */
ref = --(p->ref);
SYS_ARCH_UNPROTECT(old_level); //退出临界区
/* this pbuf is no longer referenced to? */
if (ref == 0) {
/* remember next pbuf in chain for next iteration */
q = p->next;
type = p->type;
/* is this a pbuf from the pool? */
if (type == PBUF_POOL) {
memp_free(MEMP_PBUF_POOL, p);
/* is this a ROM or RAM referencing pbuf? */
} else if (type == PBUF_ROM || type == PBUF_REF) {
memp_free(MEMP_PBUF, p);
/* type == PBUF_RAM */
} else {
mem_free(p);
}
count++;
/* proceed to next pbuf */
p = q;
/* p->ref > 0, this pbuf is still referenced to */
/* (and so the remaining pbufs in chain as well) */
} else {
/* stop walking through the chain */
p = NULL;
}
}
/* return number of de-allocated pbufs */
return count;
}
/**
*
* Create PBUF_RAM copies of pbufs.
*
* Used to queue packets on behalf of the lwIP stack, such as
* ARP based queueing.
*
* @note You MUST explicitly use p = pbuf_take(p);
*
* @note Only one packet is copied, no packet queue!
*
* @param p_to pbuf destination of the copy
* @param p_from pbuf source of the copy
*
* @return ERR_OK if pbuf was copied
* ERR_ARG if one of the pbufs is NULL or p_to is not big
* enough to hold p_from
*/
err_t
pbuf_copy(struct pbuf *p_to, struct pbuf *p_from)
{
u16_t offset_to=0, offset_from=0, len;
/* is the target big enough to hold the source? */
LWIP_ERROR("pbuf_copy: target not big enough to hold source", ((p_to != NULL) &&
(p_from != NULL) && (p_to->tot_len >= p_from->tot_len)), return ERR_ARG;);
/* iterate through pbuf chain */
do
{
LWIP_ASSERT("p_to != NULL", p_to != NULL);
/* copy one part of the original chain */
if ((p_to->len - offset_to) >= (p_from->len - offset_from)) { //每次拷贝的长度是源端和目标端当前pbuf所剩空间的较小值，offset为当前pbuf拷贝数据的偏移量
/* complete current p_from fits into current p_to */
len = p_from->len - offset_from;
} else {
/* current p_from does not fit into current p_to */
len = p_to->len - offset_to;
}
MEMCPY((u8_t*)p_to->payload + offset_to, (u8_t*)p_from->payload + offset_from, len);
offset_to += len;
offset_from += len;
if (offset_to == p_to->len) { //目标端当前pbuf空间已满，转向下一个pbuf,记得offset清零
/* on to next p_to (if any) */
offset_to = 0;
p_to = p_to->next;
}
if (offset_from >= p_from->len) { //源端当前pbuf数据已拷贝完，转向下一个pbuf,记得offset清零
/* on to next p_from (if any) */
offset_from = 0;
p_from = p_from->next;
}
if((p_from != NULL) && (p_from->len == p_from->tot_len)) {
/* don't copy more than one packet! */
LWIP_ERROR("pbuf_copy() does not allow packet queues!\n",
(p_from->next == NULL), return ERR_VAL;);
}
if((p_to != NULL) && (p_to->len == p_to->tot_len)) {
/* don't copy more than one packet! */
LWIP_ERROR("pbuf_copy() does not allow packet queues!\n",
(p_to->next == NULL), return ERR_VAL;);
}
} while (p_from);
LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_copy: end of chain reached.\n"));
return ERR_OK;
}可以看到，回收 pbuf 使用pbuf_free()函数，该函数首先要减少 pbuf 索引计数（reference count）。如果引用计数已经减为 0，这个 pbuf 被回收。对于一个pbuf链来说，只有前一个pbuf被回收，才会考虑回收后面的pbuf，如果前面pbuf计数还不为0，则直接返回。