lwip 协议栈发送数据包的一个疑问。

[zheng_wen_fei]

lwip 协议栈发送数据包时，会把一个包分解成若干个buffer，然后用这些buffer组成一个列表。但是不清楚它分解的原则是什么？是按照什么来分解这个包的？？着急呀，各位高手解答一下。

[fpcc]

这个东西你下他的协议来看不就有了.呵呵.

[zheng_wen_fei]

[Quote=引用 1 楼 fpcc 的回复:]
这个东西你下他的协议来看不就有了.呵呵.
[/Quote]

协议栈那玩意看起来困难，找不到北，但貌似固定大小的。

[zbychhaozeng]

协议buffer是有固定大小，这个因系统不同而不同，在lwip里面pbuf_alloc，就是用来分配空间的，如果你的数据很大，它就会分成如干成分MEMP_PBUF_POOL，这个就是具体的大小！代码如下：
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) {
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 = memp_malloc(MEMP_PBUF_POOL);
LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_alloc: allocated pbuf %p\n", (void *)p));
if (p == NULL) {
PBUF_POOL_IS_EMPTY();
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)));
LWIP_ASSERT("pbuf_alloc: pbuf p->payload properly aligned",
((mem_ptr_t)p->payload % MEM_ALIGNMENT) == 0);
/* the total length of the pbuf chain is the requested size */
p->tot_len = length;
/* set the length of the first pbuf in the chain */
p->len = LWIP_MIN(length, PBUF_POOL_BUFSIZE_ALIGNED - LWIP_MEM_ALIGN_SIZE(offset));
LWIP_ASSERT("check p->payload + p->len does not overflow pbuf",
((u8_t*)p->payload + p->len <=
(u8_t*)p + SIZEOF_STRUCT_PBUF + PBUF_POOL_BUFSIZE_ALIGNED));
LWIP_ASSERT("PBUF_POOL_BUFSIZE must be bigger than MEM_ALIGNMENT",
(PBUF_POOL_BUFSIZE_ALIGNED - LWIP_MEM_ALIGN_SIZE(offset)) > 0 );
/* set reference count (needed here in case we fail) */
p->ref = 1;

/* now allocate the tail of the pbuf chain */

/* 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 = memp_malloc(MEMP_PBUF_POOL);
if (q == NULL) {
PBUF_POOL_IS_EMPTY();
/* free chain so far allocated */
pbuf_free(p);
/* 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 */
LWIP_ASSERT("rem_len < max_u16_t", rem_len < 0xffff);
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);
LWIP_ASSERT("pbuf_alloc: pbuf q->payload properly aligned",
((mem_ptr_t)q->payload % MEM_ALIGNMENT) == 0);
LWIP_ASSERT("check p->payload + p->len does not overflow pbuf",
((u8_t*)p->payload + p->len <=
(u8_t*)p + SIZEOF_STRUCT_PBUF + PBUF_POOL_BUFSIZE_ALIGNED));
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;

LWIP_ASSERT("pbuf_alloc: pbuf->payload properly aligned",
((mem_ptr_t)p->payload % MEM_ALIGNMENT) == 0);
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 = memp_malloc(MEMP_PBUF);
if (p == NULL) {
LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_LEVEL_SERIOUS,
("pbuf_alloc: Could not allocate MEMP_PBUF for PBUF_%s.\n",
(type == PBUF_ROM) ? "ROM" : "REF"));
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:
LWIP_ASSERT("pbuf_alloc: erroneous type", 0);
return NULL;
}
/* set reference count */
p->ref = 1;
/* set flags */
p->flags = 0;
LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_alloc(length=%"U16_F") == %p\n", length, (void *)p));
return p;
}