lwip的内存管理

lwip可以不用malloc，而完全用pool，全用全局变量，没看明白怎么实现的。

#if LWIP_NETCONN || LWIP_SOCKET
LWIP_MEMPOOL(NETBUF,         MEMP_NUM_NETBUF,          sizeof(struct netbuf),         "NETBUF")
LWIP_MEMPOOL(NETCONN,        MEMP_NUM_NETCONN,         sizeof(struct netconn),        "NETCONN")
#endif /* LWIP_NETCONN || LWIP_SOCKET */

 #define LWIP_MEMPOOL(name,num,size,desc) LWIP_MEMPOOL_DECLARE(name,num,size,desc)

 /**
  * @ingroup mempool
  * Declare a private memory pool
  * Private mempools example:
  * .h: only when pool is used in multiple .c files: LWIP_MEMPOOL_PROTOTYPE(my_private_pool);
  * .c:
  *   - in global variables section: LWIP_MEMPOOL_DECLARE(my_private_pool, 10, sizeof(foo), "Some description")
  *   - call ONCE before using pool (e.g. in some init() function): LWIP_MEMPOOL_INIT(my_private_pool);
  *   - allocate: void* my_new_mem = LWIP_MEMPOOL_ALLOC(my_private_pool);
  *   - free: LWIP_MEMPOOL_FREE(my_private_pool, my_new_mem);
  *
  * To relocate a pool, declare it as extern in cc.h. Example for GCC:
  *   extern u8_t __attribute__((section(".onchip_mem"))) memp_memory_my_private_pool[];
  */
 #define LWIP_MEMPOOL_DECLARE(name,num,size,desc) \
   LWIP_DECLARE_MEMORY_ALIGNED(memp_memory_ ## name ## _base, ((num) * (MEMP_SIZE + MEMP_ALIGN_SIZE(size)))); \
     \
   LWIP_MEMPOOL_DECLARE_STATS_INSTANCE(memp_stats_ ## name) \
     \
   static struct memp *memp_tab_ ## name; \
     \
   const struct memp_desc memp_ ## name = { \
     DECLARE_LWIP_MEMPOOL_DESC(desc) \
     LWIP_MEMPOOL_DECLARE_STATS_REFERENCE(memp_stats_ ## name) \
     LWIP_MEM_ALIGN_SIZE(size), \
     (num), \
     memp_memory_ ## name ## _base, \
     &memp_tab_ ## name \
   };
 
 #endif /* MEMP_MEM_MALLOC */

1 #ifndef LWIP_DECLARE_MEMORY_ALIGNED
2 #define LWIP_DECLARE_MEMORY_ALIGNED(variable_name, size) u8_t variable_name[LWIP_MEM_ALIGN_BUFFER(size)]
3 #endif