text: /**
   * @file
   * Dynamic memory manager
   *
   * This is a lightweight replacement for the standard C library malloc().
   *
   * If you want to use the standard C library malloc() instead, define
   * MEM_LIBC_MALLOC to 1 in your lwipopts.h
   *
   * To let mem_malloc() use pools (prevents fragmentation and is much faster than
   * a heap but might waste some memory), define MEM_USE_POOLS to 1, define
   * MEMP_USE_CUSTOM_POOLS to 1 and create a file "lwippools.h" that includes a list
   * of pools like this (more pools can be added between _START and _END):
   *
   * Define three pools with sizes 256, 512, and 1512 bytes
   * LWIP_MALLOC_MEMPOOL_START
   * LWIP_MALLOC_MEMPOOL(20, 256)
   * LWIP_MALLOC_MEMPOOL(10, 512)
   * LWIP_MALLOC_MEMPOOL(5, 1512)
   * LWIP_MALLOC_MEMPOOL_END
   */
  
  /*
   * Copyright (c) 2001-2004 Swedish Institute of Computer Science.
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without modification,
   * are permitted provided that the following conditions are met:
   *
   * 1. Redistributions of source code must retain the above copyright notice,
   *    this list of conditions and the following disclaimer.
   * 2. Redistributions in binary form must reproduce the above copyright notice,
   *    this list of conditions and the following disclaimer in the documentation
   *    and/or other materials provided with the distribution.
   * 3. The name of the author may not be used to endorse or promote products
   *    derived from this software without specific prior written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
   * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
   * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
   * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
   * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
   * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
   * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
   * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
   * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
   * OF SUCH DAMAGE.
   *
   * This file is part of the lwIP TCP/IP stack.
   *
   * Author: Adam Dunkels <adam@sics.se>
   *         Simon Goldschmidt
   *
   */
  
  #include "net/opt.h"
  #include "net/mem.h"
  #include "net/def.h"
  #include "net/sys.h"
  #include "net/stats.h"
  #include "net/err.h"
  
  #include <string.h>
  
  #if MEM_LIBC_MALLOC
  #include <stdlib.h> /* for malloc()/free() */
  #endif
  
  #if MEM_LIBC_MALLOC || MEM_USE_POOLS
  
  /** mem_init is not used when using pools instead of a heap or using
   * C library malloc().
   */
  void
  mem_init(void)
  {
  }
  
  /** mem_trim is not used when using pools instead of a heap or using
   * C library malloc(): we can't free part of a pool element and the stack
   * support mem_trim() to return a different pointer
   */
  void*
  mem_trim(void *mem, mem_size_t size)
  {
    LWIP_UNUSED_ARG(size);
    return mem;
  }
  #endif /* MEM_LIBC_MALLOC || MEM_USE_POOLS */
  
  #if MEM_LIBC_MALLOC
  /* lwIP heap implemented using C library malloc() */
  
  /* in case C library malloc() needs extra protection,
   * allow these defines to be overridden.
   */
  #ifndef mem_clib_free
  #define mem_clib_free free
  #endif
  #ifndef mem_clib_malloc
  #define mem_clib_malloc malloc
  #endif
  #ifndef mem_clib_calloc
  #define mem_clib_calloc calloc
  #endif
  
  #if LWIP_STATS && MEM_STATS
  #define MEM_LIBC_STATSHELPER_SIZE LWIP_MEM_ALIGN_SIZE(sizeof(mem_size_t))
  #else
  #define MEM_LIBC_STATSHELPER_SIZE 0
  #endif
  
  /**
   * Allocate a block of memory with a minimum of 'size' bytes.
   *
   * @param size is the minimum size of the requested block in bytes.
   * @return pointer to allocated memory or NULL if no free memory was found.
   *
   * Note that the returned value must always be aligned (as defined by MEM_ALIGNMENT).
   */
  void *
  mem_malloc(mem_size_t size)
  {
    void* ret = mem_clib_malloc(size + MEM_LIBC_STATSHELPER_SIZE);
    if (ret == NULL) {
      MEM_STATS_INC(err);
    } else {
      LWIP_ASSERT("malloc() must return aligned memory", LWIP_MEM_ALIGN(ret) == ret);
  #if LWIP_STATS && MEM_STATS
      *(mem_size_t*)ret = size;
      ret = (u8_t*)ret + MEM_LIBC_STATSHELPER_SIZE;
      MEM_STATS_INC_USED(used, size);
  #endif
    }
    return ret;
  }
  
  /** Put memory back on the heap
   *
   * @param rmem is the pointer as returned by a previous call to mem_malloc()
   */
  void
  mem_free(void *rmem)
  {
    LWIP_ASSERT("rmem != NULL", (rmem != NULL));
    LWIP_ASSERT("rmem == MEM_ALIGN(rmem)", (rmem == LWIP_MEM_ALIGN(rmem)));
  #if LWIP_STATS && MEM_STATS
    rmem = (u8_t*)rmem - MEM_LIBC_STATSHELPER_SIZE;
    MEM_STATS_DEC_USED(used, *(mem_size_t*)rmem);
  #endif
    mem_clib_free(rmem);
  }
  
  #elif MEM_USE_POOLS
  
  /* lwIP heap implemented with different sized pools */
  
  /**
   * Allocate memory: determine the smallest pool that is big enough
   * to contain an element of 'size' and get an element from that pool.
   *
   * @param size the size in bytes of the memory needed
   * @return a pointer to the allocated memory or NULL if the pool is empty
   */
  void *
  mem_malloc(mem_size_t size)
  {
    void *ret;
    struct memp_malloc_helper *element = NULL;
    memp_t poolnr;
    mem_size_t required_size = size + LWIP_MEM_ALIGN_SIZE(sizeof(struct memp_malloc_helper));
  
    for (poolnr = MEMP_POOL_FIRST; poolnr <= MEMP_POOL_LAST; poolnr = (memp_t)(poolnr + 1)) {
      /* is this pool big enough to hold an element of the required size
         plus a struct memp_malloc_helper that saves the pool this element came from? */
      if (required_size <= memp_pools[poolnr]->size) {
        element = (struct memp_malloc_helper*)memp_malloc(poolnr);
        if (element == NULL) {
          /* No need to DEBUGF or ASSERT: This error is already taken care of in memp.c */
  #if MEM_USE_POOLS_TRY_BIGGER_POOL
          /** Try a bigger pool if this one is empty! */
          if (poolnr < MEMP_POOL_LAST) {
            continue;
          }
  #endif /* MEM_USE_POOLS_TRY_BIGGER_POOL */
          MEM_STATS_INC(err);
          return NULL;
        }
        break;
      }
    }
    if (poolnr > MEMP_POOL_LAST) {
      LWIP_ASSERT("mem_malloc(): no pool is that big!", 0);
      MEM_STATS_INC(err);
      return NULL;
    }
  
    /* save the pool number this element came from */
    element->poolnr = poolnr;
    /* and return a pointer to the memory directly after the struct memp_malloc_helper */
    ret = (u8_t*)element + LWIP_MEM_ALIGN_SIZE(sizeof(struct memp_malloc_helper));
  
  #if MEMP_OVERFLOW_CHECK || (LWIP_STATS && MEM_STATS)
    /* truncating to u16_t is safe because struct memp_desc::size is u16_t */
    element->size = (u16_t)size;
    MEM_STATS_INC_USED(used, element->size);
  #endif /* MEMP_OVERFLOW_CHECK || (LWIP_STATS && MEM_STATS) */
  #if MEMP_OVERFLOW_CHECK
    /* initialize unused memory (diff between requested size and selected pool's size) */
    memset((u8_t*)ret + size, 0xcd, memp_pools[poolnr]->size - size);
  #endif /* MEMP_OVERFLOW_CHECK */
    return ret;
  }
  
  /**
   * Free memory previously allocated by mem_malloc. Loads the pool number
   * and calls memp_free with that pool number to put the element back into
   * its pool
   *
   * @param rmem the memory element to free
   */
  void
  mem_free(void *rmem)
  {
    struct memp_malloc_helper *hmem;
  
    LWIP_ASSERT("rmem != NULL", (rmem != NULL));
    LWIP_ASSERT("rmem == MEM_ALIGN(rmem)", (rmem == LWIP_MEM_ALIGN(rmem)));
  
    /* get the original struct memp_malloc_helper */
    /* cast through void* to get rid of alignment warnings */
    hmem = (struct memp_malloc_helper*)(void*)((u8_t*)rmem - LWIP_MEM_ALIGN_SIZE(sizeof(struct memp_malloc_helper)));
  
    LWIP_ASSERT("hmem != NULL", (hmem != NULL));
    LWIP_ASSERT("hmem == MEM_ALIGN(hmem)", (hmem == LWIP_MEM_ALIGN(hmem)));
    LWIP_ASSERT("hmem->poolnr < MEMP_MAX", (hmem->poolnr < MEMP_MAX));
  
    MEM_STATS_DEC_USED(used, hmem->size);
  #if MEMP_OVERFLOW_CHECK
    {
       u16_t i;
       LWIP_ASSERT("MEM_USE_POOLS: invalid chunk size",
          hmem->size <= memp_pools[hmem->poolnr]->size);
       /* check that unused memory remained untouched (diff between requested size and selected pool's size) */
       for (i = hmem->size; i < memp_pools[hmem->poolnr]->size; i++) {
          u8_t data = *((u8_t*)rmem + i);
          LWIP_ASSERT("MEM_USE_POOLS: mem overflow detected", data == 0xcd);
       }
    }
  #endif /* MEMP_OVERFLOW_CHECK */
  
    /* and put it in the pool we saved earlier */
    memp_free(hmem->poolnr, hmem);
  }
  
  #else /* MEM_USE_POOLS */
  /* lwIP replacement for your libc malloc() */
  
  /**
   * The heap is made up as a list of structs of this type.
   * This does not have to be aligned since for getting its size,
   * we only use the macro SIZEOF_STRUCT_MEM, which automatically aligns.
   */
  struct mem {
    /** index (-> ram[next]) of the next struct */
    mem_size_t next;
    /** index (-> ram[prev]) of the previous struct */
    mem_size_t prev;
    /** 1: this area is used; 0: this area is unused */
    u8_t used;
  };
  
  /** All allocated blocks will be MIN_SIZE bytes big, at least!
   * MIN_SIZE can be overridden to suit your needs. Smaller values save space,
   * larger values could prevent too small blocks to fragment the RAM too much. */
  #ifndef MIN_SIZE
  #define MIN_SIZE             12
  #endif /* MIN_SIZE */
  /* some alignment macros: we define them here for better source code layout */
  #define MIN_SIZE_ALIGNED     LWIP_MEM_ALIGN_SIZE(MIN_SIZE)
  #define SIZEOF_STRUCT_MEM    LWIP_MEM_ALIGN_SIZE(sizeof(struct mem))
  #define MEM_SIZE_ALIGNED     LWIP_MEM_ALIGN_SIZE(MEM_SIZE)
  
  /** If you want to relocate the heap to external memory, simply define
   * LWIP_RAM_HEAP_POINTER as a void-pointer to that location.
   * If so, make sure the memory at that location is big enough (see below on
   * how that space is calculated). */
  #ifndef LWIP_RAM_HEAP_POINTER
  /** the heap. we need one struct mem at the end and some room for alignment */
  LWIP_DECLARE_MEMORY_ALIGNED(ram_heap, MEM_SIZE_ALIGNED + (2U*SIZEOF_STRUCT_MEM));
  #define LWIP_RAM_HEAP_POINTER ram_heap
  #endif /* LWIP_RAM_HEAP_POINTER */
  
  /** pointer to the heap (ram_heap): for alignment, ram is now a pointer instead of an array */
  static u8_t *ram;
  /** the last entry, always unused! */
  static struct mem *ram_end;
  /** pointer to the lowest free block, this is used for faster search */
  static struct mem *lfree;
  
  /** concurrent access protection */
  #if !NO_SYS
  static sys_mutex_t mem_mutex;
  #endif
  
  #if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT
  
  static volatile u8_t mem_free_count;
  
  /* Allow mem_free from other (e.g. interrupt) context */
  #define LWIP_MEM_FREE_DECL_PROTECT()  SYS_ARCH_DECL_PROTECT(lev_free)
  #define LWIP_MEM_FREE_PROTECT()       SYS_ARCH_PROTECT(lev_free)
  #define LWIP_MEM_FREE_UNPROTECT()     SYS_ARCH_UNPROTECT(lev_free)
  #define LWIP_MEM_ALLOC_DECL_PROTECT() SYS_ARCH_DECL_PROTECT(lev_alloc)
  #define LWIP_MEM_ALLOC_PROTECT()      SYS_ARCH_PROTECT(lev_alloc)
  #define LWIP_MEM_ALLOC_UNPROTECT()    SYS_ARCH_UNPROTECT(lev_alloc)
  
  #else /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */
  
  /* Protect the heap only by using a semaphore */
  #define LWIP_MEM_FREE_DECL_PROTECT()
  #define LWIP_MEM_FREE_PROTECT()    sys_mutex_lock(&mem_mutex)
  #define LWIP_MEM_FREE_UNPROTECT()  sys_mutex_unlock(&mem_mutex)
  /* mem_malloc is protected using semaphore AND LWIP_MEM_ALLOC_PROTECT */
  #define LWIP_MEM_ALLOC_DECL_PROTECT()
  #define LWIP_MEM_ALLOC_PROTECT()
  #define LWIP_MEM_ALLOC_UNPROTECT()
  
  #endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */
  
  
  /**
   * "Plug holes" by combining adjacent empty struct mems.
   * After this function is through, there should not exist
   * one empty struct mem pointing to another empty struct mem.
   *
   * @param mem this points to a struct mem which just has been freed
   * @internal this function is only called by mem_free() and mem_trim()
   *
   * This assumes access to the heap is protected by the calling function
   * already.
   */
  static void
  plug_holes(struct mem *mem)
  {
    struct mem *nmem;
    struct mem *pmem;
  
    LWIP_ASSERT("plug_holes: mem >= ram", (u8_t *)mem >= ram);
    LWIP_ASSERT("plug_holes: mem < ram_end", (u8_t *)mem < (u8_t *)ram_end);
    LWIP_ASSERT("plug_holes: mem->used == 0", mem->used == 0);
  
    /* plug hole forward */
    LWIP_ASSERT("plug_holes: mem->next <= MEM_SIZE_ALIGNED", mem->next <= MEM_SIZE_ALIGNED);
  
    nmem = (struct mem *)(void *)&ram[mem->next];
    if (mem != nmem && nmem->used == 0 && (u8_t *)nmem != (u8_t *)ram_end) {
      /* if mem->next is unused and not end of ram, combine mem and mem->next */
      if (lfree == nmem) {
        lfree = mem;
      }
      mem->next = nmem->next;
      ((struct mem *)(void *)&ram[nmem->next])->prev = (mem_size_t)((u8_t *)mem - ram);
    }
  
    /* plug hole backward */
    pmem = (struct mem *)(void *)&ram[mem->prev];
    if (pmem != mem && pmem->used == 0) {
      /* if mem->prev is unused, combine mem and mem->prev */
      if (lfree == mem) {
        lfree = pmem;
      }
      pmem->next = mem->next;
      ((struct mem *)(void *)&ram[mem->next])->prev = (mem_size_t)((u8_t *)pmem - ram);
    }
  }
  
  /**
   * Zero the heap and initialize start, end and lowest-free
   */
  void
  mem_init(void)
  {
    struct mem *mem;
  
    LWIP_ASSERT("Sanity check alignment",
      (SIZEOF_STRUCT_MEM & (MEM_ALIGNMENT-1)) == 0);
  
    /* align the heap */
    ram = (u8_t *)LWIP_MEM_ALIGN(LWIP_RAM_HEAP_POINTER);
    /* initialize the start of the heap */
    mem = (struct mem *)(void *)ram;
    mem->next = MEM_SIZE_ALIGNED;
    mem->prev = 0;
    mem->used = 0;
    /* initialize the end of the heap */
    ram_end = (struct mem *)(void *)&ram[MEM_SIZE_ALIGNED];
    ram_end->used = 1;
    ram_end->next = MEM_SIZE_ALIGNED;
    ram_end->prev = MEM_SIZE_ALIGNED;
  
    /* initialize the lowest-free pointer to the start of the heap */
    lfree = (struct mem *)(void *)ram;
  
    MEM_STATS_AVAIL(avail, MEM_SIZE_ALIGNED);
  
    if (sys_mutex_new(&mem_mutex) != ERR_OK) {
      LWIP_ASSERT("failed to create mem_mutex", 0);
    }
  }
  
  /**
   * Put a struct mem back on the heap
   *
   * @param rmem is the data portion of a struct mem as returned by a previous
   *             call to mem_malloc()
   */
  void
  mem_free(void *rmem)
  {
    struct mem *mem;
    LWIP_MEM_FREE_DECL_PROTECT();
  
    if (rmem == NULL) {
      LWIP_DEBUGF(MEM_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_SERIOUS, ("mem_free(p == NULL) was called.\n"));
      return;
    }
    LWIP_ASSERT("mem_free: sanity check alignment", (((mem_ptr_t)rmem) & (MEM_ALIGNMENT-1)) == 0);
  
    LWIP_ASSERT("mem_free: legal memory", (u8_t *)rmem >= (u8_t *)ram &&
      (u8_t *)rmem < (u8_t *)ram_end);
  
    if ((u8_t *)rmem < (u8_t *)ram || (u8_t *)rmem >= (u8_t *)ram_end) {
      SYS_ARCH_DECL_PROTECT(lev);
      LWIP_DEBUGF(MEM_DEBUG | LWIP_DBG_LEVEL_SEVERE, ("mem_free: illegal memory\n"));
      /* protect mem stats from concurrent access */
      SYS_ARCH_PROTECT(lev);
      MEM_STATS_INC(illegal);
      SYS_ARCH_UNPROTECT(lev);
      return;
    }
    /* protect the heap from concurrent access */
    LWIP_MEM_FREE_PROTECT();
    /* Get the corresponding struct mem ... */
    /* cast through void* to get rid of alignment warnings */
    mem = (struct mem *)(void *)((u8_t *)rmem - SIZEOF_STRUCT_MEM);
    /* ... which has to be in a used state ... */
    LWIP_ASSERT("mem_free: mem->used", mem->used);
    /* ... and is now unused. */
    mem->used = 0;
  
    if (mem < lfree) {
      /* the newly freed struct is now the lowest */
      lfree = mem;
    }
  
    MEM_STATS_DEC_USED(used, mem->next - (mem_size_t)(((u8_t *)mem - ram)));
  
    /* finally, see if prev or next are free also */
    plug_holes(mem);
  #if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT
    mem_free_count = 1;
  #endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */
    LWIP_MEM_FREE_UNPROTECT();
  }
  
  /**
   * Shrink memory returned by mem_malloc().
   *
   * @param rmem pointer to memory allocated by mem_malloc the is to be shrinked
   * @param newsize required size after shrinking (needs to be smaller than or
   *                equal to the previous size)
   * @return for compatibility reasons: is always == rmem, at the moment
   *         or NULL if newsize is > old size, in which case rmem is NOT touched
   *         or freed!
   */
  void *
  mem_trim(void *rmem, mem_size_t newsize)
  {
    mem_size_t size;
    mem_size_t ptr, ptr2;
    struct mem *mem, *mem2;
    /* use the FREE_PROTECT here: it protects with sem OR SYS_ARCH_PROTECT */
    LWIP_MEM_FREE_DECL_PROTECT();
  
    /* Expand the size of the allocated memory region so that we can
       adjust for alignment. */
    newsize = LWIP_MEM_ALIGN_SIZE(newsize);
  
    if (newsize < MIN_SIZE_ALIGNED) {
      /* every data block must be at least MIN_SIZE_ALIGNED long */
      newsize = MIN_SIZE_ALIGNED;
    }
  
    if (newsize > MEM_SIZE_ALIGNED) {
      return NULL;
    }
  
    LWIP_ASSERT("mem_trim: legal memory", (u8_t *)rmem >= (u8_t *)ram &&
     (u8_t *)rmem < (u8_t *)ram_end);
  
    if ((u8_t *)rmem < (u8_t *)ram || (u8_t *)rmem >= (u8_t *)ram_end) {
      SYS_ARCH_DECL_PROTECT(lev);
      LWIP_DEBUGF(MEM_DEBUG | LWIP_DBG_LEVEL_SEVERE, ("mem_trim: illegal memory\n"));
      /* protect mem stats from concurrent access */
      SYS_ARCH_PROTECT(lev);
      MEM_STATS_INC(illegal);
      SYS_ARCH_UNPROTECT(lev);
      return rmem;
    }
    /* Get the corresponding struct mem ... */
    /* cast through void* to get rid of alignment warnings */
    mem = (struct mem *)(void *)((u8_t *)rmem - SIZEOF_STRUCT_MEM);
    /* ... and its offset pointer */
    ptr = (mem_size_t)((u8_t *)mem - ram);
  
    size = mem->next - ptr - SIZEOF_STRUCT_MEM;
    LWIP_ASSERT("mem_trim can only shrink memory", newsize <= size);
    if (newsize > size) {
      /* not supported */
      return NULL;
    }
    if (newsize == size) {
      /* No change in size, simply return */
      return rmem;
    }
  
    /* protect the heap from concurrent access */
    LWIP_MEM_FREE_PROTECT();
  
    mem2 = (struct mem *)(void *)&ram[mem->next];
    if (mem2->used == 0) {
      /* The next struct is unused, we can simply move it at little */
      mem_size_t next;
      /* remember the old next pointer */
      next = mem2->next;
      /* create new struct mem which is moved directly after the shrinked mem */
      ptr2 = ptr + SIZEOF_STRUCT_MEM + newsize;
      if (lfree == mem2) {
        lfree = (struct mem *)(void *)&ram[ptr2];
      }
      mem2 = (struct mem *)(void *)&ram[ptr2];
      mem2->used = 0;
      /* restore the next pointer */
      mem2->next = next;
      /* link it back to mem */
      mem2->prev = ptr;
      /* link mem to it */
      mem->next = ptr2;
      /* last thing to restore linked list: as we have moved mem2,
       * let 'mem2->next->prev' point to mem2 again. but only if mem2->next is not
       * the end of the heap */
      if (mem2->next != MEM_SIZE_ALIGNED) {
        ((struct mem *)(void *)&ram[mem2->next])->prev = ptr2;
      }
      MEM_STATS_DEC_USED(used, (size - newsize));
      /* no need to plug holes, we've already done that */
    } else if (newsize + SIZEOF_STRUCT_MEM + MIN_SIZE_ALIGNED <= size) {
      /* Next struct is used but there's room for another struct mem with
       * at least MIN_SIZE_ALIGNED of data.
       * Old size ('size') must be big enough to contain at least 'newsize' plus a struct mem
       * ('SIZEOF_STRUCT_MEM') with some data ('MIN_SIZE_ALIGNED').
       * @todo we could leave out MIN_SIZE_ALIGNED. We would create an empty
       *       region that couldn't hold data, but when mem->next gets freed,
       *       the 2 regions would be combined, resulting in more free memory */
      ptr2 = ptr + SIZEOF_STRUCT_MEM + newsize;
      mem2 = (struct mem *)(void *)&ram[ptr2];
      if (mem2 < lfree) {
        lfree = mem2;
      }
      mem2->used = 0;
      mem2->next = mem->next;
      mem2->prev = ptr;
      mem->next = ptr2;
      if (mem2->next != MEM_SIZE_ALIGNED) {
        ((struct mem *)(void *)&ram[mem2->next])->prev = ptr2;
      }
      MEM_STATS_DEC_USED(used, (size - newsize));
      /* the original mem->next is used, so no need to plug holes! */
    }
    /* else {
      next struct mem is used but size between mem and mem2 is not big enough
      to create another struct mem
      -> don't do anyhting.
      -> the remaining space stays unused since it is too small
    } */
  #if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT
    mem_free_count = 1;
  #endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */
    LWIP_MEM_FREE_UNPROTECT();
    return rmem;
  }
  
  /**
   * Allocate a block of memory with a minimum of 'size' bytes.
   *
   * @param size is the minimum size of the requested block in bytes.
   * @return pointer to allocated memory or NULL if no free memory was found.
   *
   * Note that the returned value will always be aligned (as defined by MEM_ALIGNMENT).
   */
  void *
  mem_malloc(mem_size_t size)
  {
    mem_size_t ptr, ptr2;
    struct mem *mem, *mem2;
  #if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT
    u8_t local_mem_free_count = 0;
  #endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */
    LWIP_MEM_ALLOC_DECL_PROTECT();
  
    if (size == 0) {
      return NULL;
    }
  
    /* Expand the size of the allocated memory region so that we can
       adjust for alignment. */
    size = LWIP_MEM_ALIGN_SIZE(size);
  
    if (size < MIN_SIZE_ALIGNED) {
      /* every data block must be at least MIN_SIZE_ALIGNED long */
      size = MIN_SIZE_ALIGNED;
    }
  
    if (size > MEM_SIZE_ALIGNED) {
      return NULL;
    }
  
    /* protect the heap from concurrent access */
    sys_mutex_lock(&mem_mutex);
    LWIP_MEM_ALLOC_PROTECT();
  #if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT
    /* run as long as a mem_free disturbed mem_malloc or mem_trim */
    do {
      local_mem_free_count = 0;
  #endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */
  
      /* Scan through the heap searching for a free block that is big enough,
       * beginning with the lowest free block.
       */
      for (ptr = (mem_size_t)((u8_t *)lfree - ram); ptr < MEM_SIZE_ALIGNED - size;
           ptr = ((struct mem *)(void *)&ram[ptr])->next) {
        mem = (struct mem *)(void *)&ram[ptr];
  #if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT
        mem_free_count = 0;
        LWIP_MEM_ALLOC_UNPROTECT();
        /* allow mem_free or mem_trim to run */
        LWIP_MEM_ALLOC_PROTECT();
        if (mem_free_count != 0) {
          /* If mem_free or mem_trim have run, we have to restart since they
             could have altered our current struct mem. */
          local_mem_free_count = 1;
          break;
        }
  #endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */
  
        if ((!mem->used) &&
            (mem->next - (ptr + SIZEOF_STRUCT_MEM)) >= size) {
          /* mem is not used and at least perfect fit is possible:
           * mem->next - (ptr + SIZEOF_STRUCT_MEM) gives us the 'user data size' of mem */
  
          if (mem->next - (ptr + SIZEOF_STRUCT_MEM) >= (size + SIZEOF_STRUCT_MEM + MIN_SIZE_ALIGNED)) {
            /* (in addition to the above, we test if another struct mem (SIZEOF_STRUCT_MEM) containing
             * at least MIN_SIZE_ALIGNED of data also fits in the 'user data space' of 'mem')
             * -> split large block, create empty remainder,
             * remainder must be large enough to contain MIN_SIZE_ALIGNED data: if
             * mem->next - (ptr + (2*SIZEOF_STRUCT_MEM)) == size,
             * struct mem would fit in but no data between mem2 and mem2->next
             * @todo we could leave out MIN_SIZE_ALIGNED. We would create an empty
             *       region that couldn't hold data, but when mem->next gets freed,
             *       the 2 regions would be combined, resulting in more free memory
             */
            ptr2 = ptr + SIZEOF_STRUCT_MEM + size;
            /* create mem2 struct */
            mem2 = (struct mem *)(void *)&ram[ptr2];
            mem2->used = 0;
            mem2->next = mem->next;
            mem2->prev = ptr;
            /* and insert it between mem and mem->next */
            mem->next = ptr2;
            mem->used = 1;
  
            if (mem2->next != MEM_SIZE_ALIGNED) {
              ((struct mem *)(void *)&ram[mem2->next])->prev = ptr2;
            }
            MEM_STATS_INC_USED(used, (size + SIZEOF_STRUCT_MEM));
          } else {
            /* (a mem2 struct does no fit into the user data space of mem and mem->next will always
             * be used at this point: if not we have 2 unused structs in a row, plug_holes should have
             * take care of this).
             * -> near fit or exact fit: do not split, no mem2 creation
             * also can't move mem->next directly behind mem, since mem->next
             * will always be used at this point!
             */
            mem->used = 1;
            MEM_STATS_INC_USED(used, mem->next - (mem_size_t)((u8_t *)mem - ram));
          }
  #if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT
  mem_malloc_adjust_lfree:
  #endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */
          if (mem == lfree) {
            struct mem *cur = lfree;
            /* Find next free block after mem and update lowest free pointer */
            while (cur->used && cur != ram_end) {
  #if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT
              mem_free_count = 0;
              LWIP_MEM_ALLOC_UNPROTECT();
              /* prevent high interrupt latency... */
              LWIP_MEM_ALLOC_PROTECT();
              if (mem_free_count != 0) {
                /* If mem_free or mem_trim have run, we have to restart since they
                   could have altered our current struct mem or lfree. */
                goto mem_malloc_adjust_lfree;
              }
  #endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */
              cur = (struct mem *)(void *)&ram[cur->next];
            }
            lfree = cur;
            LWIP_ASSERT("mem_malloc: !lfree->used", ((lfree == ram_end) || (!lfree->used)));
          }
          LWIP_MEM_ALLOC_UNPROTECT();
          sys_mutex_unlock(&mem_mutex);
          LWIP_ASSERT("mem_malloc: allocated memory not above ram_end.",
           (mem_ptr_t)mem + SIZEOF_STRUCT_MEM + size <= (mem_ptr_t)ram_end);
          LWIP_ASSERT("mem_malloc: allocated memory properly aligned.",
           ((mem_ptr_t)mem + SIZEOF_STRUCT_MEM) % MEM_ALIGNMENT == 0);
          LWIP_ASSERT("mem_malloc: sanity check alignment",
            (((mem_ptr_t)mem) & (MEM_ALIGNMENT-1)) == 0);
  
          return (u8_t *)mem + SIZEOF_STRUCT_MEM;
        }
      }
  #if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT
      /* if we got interrupted by a mem_free, try again */
    } while (local_mem_free_count != 0);
  #endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */
    LWIP_DEBUGF(MEM_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("mem_malloc: could not allocate %"S16_F" bytes\n", (s16_t)size));
    MEM_STATS_INC(err);
    LWIP_MEM_ALLOC_UNPROTECT();
    sys_mutex_unlock(&mem_mutex);
    return NULL;
  }
  
  #endif /* MEM_USE_POOLS */
  
  #if MEM_LIBC_MALLOC && (!LWIP_STATS || !MEM_STATS)
  void *
  mem_calloc(mem_size_t count, mem_size_t size)
  {
    return mem_clib_calloc(count, size);
  }
  
  #else /* MEM_LIBC_MALLOC && (!LWIP_STATS || !MEM_STATS) */
  /**
   * Contiguously allocates enough space for count objects that are size bytes
   * of memory each and returns a pointer to the allocated memory.
   *
   * The allocated memory is filled with bytes of value zero.
   *
   * @param count number of objects to allocate
   * @param size size of the objects to allocate
   * @return pointer to allocated memory / NULL pointer if there is an error
   */
  void *
  mem_calloc(mem_size_t count, mem_size_t size)
  {
    void *p;
  
    /* allocate 'count' objects of size 'size' */
    p = mem_malloc(count * size);
    if (p) {
      /* zero the memory */
      memset(p, 0, (size_t)count * (size_t)size);
    }
    return p;
  }
  #endif /* MEM_LIBC_MALLOC && (!LWIP_STATS || !MEM_STATS) */
  