lwip中pbuf_alloc()内存申请函数

1. PBUF_RAM一次性分配size大小的连续内存

2. RBUF_ROM只需要分配小小的管理pbuf的控制管理内存

3. PBUF_ROOL分配一个链表,链表上每个元素所管理的内存最大不超过PBUF_POOL_BUFSIZE,它更像linux

中的kmem_alloc内存高速缓存机制,所以它也更适合在网卡驱动irq中断中为刚刚到来的网络数据包申请存储空间

/*-----------------------------------------------------------------------------------*/

/* pbuf_alloc():

*

* Allocates a pbuf at protocol layer l. 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). The

* flag 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.

* * RBUF_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.

* * PBUF_ROOL: the pbuf is allocated as a pbuf chain, with pbufs from

* the pbuf pool that is allocated during pbuf_init().

*/

/*-----------------------------------------------------------------------------------*/

struct pbuf *

pbuf_alloc(pbuf_layer l, u16_t size, pbuf_flag flag)

{

struct pbuf *p, *q, *r;

u16_t offset;

s32_t rsize;

offset = 0;

switch(l) {

case PBUF_TRANSPORT: // 如果为传输层申请pbuf,那么有效数据的偏移位置为PBUF_TRANSPORT_HLEN[luther.gliethttp]

offset += PBUF_TRANSPORT_HLEN;

/* FALLTHROUGH */

case PBUF_IP: // 如果为ip层申请pbuf,那么有效数据的偏移位置为PBUF_IP_HLEN+PBUF_LINK_HLEN

offset += PBUF_IP_HLEN;

offset += PBUF_LINK_HLEN;

/* FALLTHROUGH */

case PBUF_LINK: // 如果是链路层申请pbuf内存,那么数据偏移位置就是0

break;

case PBUF_RAW:

break;

default:

ASSERT("pbuf_alloc: bad pbuf layer", 0);

return NULL;

}

switch(flag) {

case PBUF_POOL:

/* Allocate head of pbuf chain into p. */

p = pbuf_pool_alloc(); // 为PBUF_POOL类型,那么需要多申请pbuf的head控制头部所需内存[luther.gliethttp]

if(p == NULL) {

#ifdef PBUF_STATS

++stats.pbuf.err;

#endif /* PBUF_STATS */

return NULL;

}

p->next = NULL;

/* Set the payload pointer so that it points offset bytes into

pbuf data memory. */

p->payload = MEM_ALIGN((void *)((u8_t *)p + (sizeof(struct pbuf) + offset))); // 以offset为基准,登记有效数据存储的起始偏移位置到p->payload[luther.gliethttp]

/* The total length of the pbuf is the requested size. */

p->tot_len = size; // pbuf链表上有效数据总大小

/* Set the length of the first pbuf is the chain. */

// pbuf链表上每个元素所能存储的最大数据为PBUF_POOL_BUFSIZE,如果超过该值,那么就会

// 使用链表方式,链接其很多个pbuf,直到申请的size数据全部能够正常存储为止[luther.gliethttp]

p->len = size > PBUF_POOL_BUFSIZE - offset? PBUF_POOL_BUFSIZE - offset: size;

p->flags = PBUF_FLAG_POOL;

/* Allocate the tail of the pbuf chain. */

r = p;

rsize = size - p->len; // 第1个pbuf所能存储数据大小为p->len,这里计算还需要多少存储空间存储剩下的数据.

while(rsize > 0) { // 构成数据缓存链表,每个链表元素所能存储的最大数据量为PBUF_POOL_BUFSIZE个字节[luther.gliethttp]

q = pbuf_pool_alloc();

if(q == NULL) {

DEBUGF(PBUF_DEBUG, ("pbuf_alloc: Out of pbufs in pool,\n"));

#ifdef PBUF_STATS

++stats.pbuf.err;

#endif /* PBUF_STATS */

pbuf_pool_free(p);

return NULL;

}

q->next = NULL;

r->next = q;

q->len = rsize > PBUF_POOL_BUFSIZE? PBUF_POOL_BUFSIZE: rsize;

q->flags = PBUF_FLAG_POOL;

q->payload = (void *)((u8_t *)q + sizeof(struct pbuf));

r = q;

q->ref = 1;

q = q->next;

rsize -= PBUF_POOL_BUFSIZE;

}

r->next = NULL;

ASSERT("pbuf_alloc: pbuf->payload properly aligned",

((u32_t)p->payload % MEM_ALIGNMENT) == 0);

break;

case PBUF_RAM:

/* If pbuf is to be allocated in RAM, allocate memory for it. */

// PBUF_RAM类型内存,那么一次性申请size大小的连续内存

p = mem_malloc(MEM_ALIGN_SIZE(sizeof(struct pbuf) + size + offset));

if(p == NULL) {

return NULL;

}

/* Set up internal structure of the pbuf. */

p->payload = MEM_ALIGN((void *)((u8_t *)p + sizeof(struct pbuf) + offset));

p->len = p->tot_len = size;

p->next = NULL;

p->flags = PBUF_FLAG_RAM;

ASSERT("pbuf_alloc: pbuf->payload properly aligned",

((u32_t)p->payload % MEM_ALIGNMENT) == 0);

break;

case PBUF_ROM:

/* If the pbuf should point to ROM, we only need to allocate

memory for the pbuf structure. */

// PBUF_ROM类型内存,那么只需要申请pbufs头部控制结构体所需内存即可[luther.gliethttp]

p = memp_mallocp(MEMP_PBUF);

if(p == NULL) {

return NULL;

}

p->payload = NULL;

p->len = p->tot_len = size;

p->next = NULL;

p->flags = PBUF_FLAG_ROM;

break;

default:

ASSERT("pbuf_alloc: erroneous flag", 0);

return NULL;

}

p->ref = 1;

return p;

}

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