1# Static Memory 2 3## Working Principles 4 5The static memory is a static array. The block size in the static memory pool is set during initialization and cannot be changed after initialization. 6 7The static memory pool consists of a control block **LOS\_MEMBOX\_INFO** and several memory blocks **LOS\_MEMBOX\_NODE** of the same size. The control block is located at the head of the memory pool and used for memory block management. It contains the memory block size \(**uwBlkSize**\), number of memory blocks \(**uwBlkNum**\), number of allocated memory blocks \(**uwBlkCnt**\), and free list \(**stFreeList**\). Memory is allocated and released by block size. Each memory block contains the pointer **pstNext** that points to the next memory block. 8 9**Figure 1** Static memory 10 11 12## Development Guidelines 13 14### When to Use 15 16Use static memory allocation to obtain memory blocks of the fixed size. When the memory is no longer required, release the static memory. 17 18### Available APIs 19 20The following table describes APIs available for OpenHarmony LiteOS-M static memory management. For more details about the APIs, see the API reference. 21 22**Table 1** APIs of the static memory module 23 24<a name="table1415203765610"></a> 25<table><thead align="left"><tr id="row134151837125611"><th class="cellrowborder" valign="top" width="16.19161916191619%" id="mcps1.2.4.1.1"><p id="p16415637105612"><a name="p16415637105612"></a><a name="p16415637105612"></a>Function</p> 26</th> 27<th class="cellrowborder" valign="top" width="22.472247224722473%" id="mcps1.2.4.1.2"><p id="p11415163718562"><a name="p11415163718562"></a><a name="p11415163718562"></a>API</p> 28</th> 29<th class="cellrowborder" valign="top" width="61.33613361336133%" id="mcps1.2.4.1.3"><p id="p1641533755612"><a name="p1641533755612"></a><a name="p1641533755612"></a>Description</p> 30</th> 31</tr> 32</thead> 33<tbody><tr id="row0415737175610"><td class="cellrowborder" valign="top" width="16.19161916191619%" headers="mcps1.2.4.1.1 "><p id="p2990613114416"><a name="p2990613114416"></a><a name="p2990613114416"></a>Initializing the static memory pool</p> 34</td> 35<td class="cellrowborder" valign="top" width="22.472247224722473%" headers="mcps1.2.4.1.2 "><p id="p169901113194416"><a name="p169901113194416"></a><a name="p169901113194416"></a>LOS_MemboxInit</p> 36</td> 37<td class="cellrowborder" valign="top" width="61.33613361336133%" headers="mcps1.2.4.1.3 "><p id="p5990113174414"><a name="p5990113174414"></a><a name="p5990113174414"></a>Initialize a static memory pool and sets the start address, total size, and size of each memory block based on the input parameters.</p> 38</td> 39</tr> 40<tr id="row1841519376561"><td class="cellrowborder" valign="top" width="16.19161916191619%" headers="mcps1.2.4.1.1 "><p id="p9367164904413"><a name="p9367164904413"></a><a name="p9367164904413"></a>Clearing static memory blocks</p> 41</td> 42<td class="cellrowborder" valign="top" width="22.472247224722473%" headers="mcps1.2.4.1.2 "><p id="p103671549144411"><a name="p103671549144411"></a><a name="p103671549144411"></a>LOS_MemboxClr</p> 43</td> 44<td class="cellrowborder" valign="top" width="61.33613361336133%" headers="mcps1.2.4.1.3 "><p id="p18367184916448"><a name="p18367184916448"></a><a name="p18367184916448"></a>Clears the memory blocks allocated from the static memory pool.</p> 45</td> 46</tr> 47<tr id="row1187514443616"><td class="cellrowborder" rowspan="2" valign="top" width="16.19161916191619%" headers="mcps1.2.4.1.1 "><p id="p64541711458"><a name="p64541711458"></a><a name="p64541711458"></a>Allocating or releasing static memory</p> 48</td> 49<td class="cellrowborder" valign="top" width="22.472247224722473%" headers="mcps1.2.4.1.2 "><p id="p84547710452"><a name="p84547710452"></a><a name="p84547710452"></a>LOS_MemboxAlloc</p> 50</td> 51<td class="cellrowborder" valign="top" width="61.33613361336133%" headers="mcps1.2.4.1.3 "><p id="p5454177164520"><a name="p5454177164520"></a><a name="p5454177164520"></a>Allocates a memory block from a specified static memory pool.</p> 52</td> 53</tr> 54<tr id="row1745415527441"><td class="cellrowborder" valign="top" headers="mcps1.2.4.1.1 "><p id="p645457204512"><a name="p645457204512"></a><a name="p645457204512"></a>LOS_MemboxFree</p> 55</td> 56<td class="cellrowborder" valign="top" headers="mcps1.2.4.1.2 "><p id="p545415704514"><a name="p545415704514"></a><a name="p545415704514"></a>Releases a memory block allocated from the static memory pool.</p> 57</td> 58</tr> 59<tr id="row19101718144518"><td class="cellrowborder" rowspan="2" valign="top" width="16.19161916191619%" headers="mcps1.2.4.1.1 "><p id="p15927427144615"><a name="p15927427144615"></a><a name="p15927427144615"></a>Obtaining or printing static memory pool information</p> 60</td> 61<td class="cellrowborder" valign="top" width="22.472247224722473%" headers="mcps1.2.4.1.2 "><p id="p792715277464"><a name="p792715277464"></a><a name="p792715277464"></a>LOS_MemboxStatisticsGet</p> 62</td> 63<td class="cellrowborder" valign="top" width="61.33613361336133%" headers="mcps1.2.4.1.3 "><p id="p139271327114620"><a name="p139271327114620"></a><a name="p139271327114620"></a>Obtains information about a specified static memory pool, including the total number of memory blocks in the memory pool, number of allocated memory blocks, and size of each memory block.</p> 64</td> 65</tr> 66<tr id="row1346314166464"><td class="cellrowborder" valign="top" headers="mcps1.2.4.1.1 "><p id="p8927102754619"><a name="p8927102754619"></a><a name="p8927102754619"></a>LOS_ShowBox</p> 67</td> 68<td class="cellrowborder" valign="top" headers="mcps1.2.4.1.2 "><p id="p49274279460"><a name="p49274279460"></a><a name="p49274279460"></a>Prints information about all nodes in a specified static memory pool (the print level is <strong id="b1937798153019"><a name="b1937798153019"></a><a name="b1937798153019"></a>LOS_INFO_LEVEL</strong>). The information includes the start address of the memory pool, memory block size, total number of memory blocks, start address of each idle memory block, and start addresses of all memory blocks.</p> 69</td> 70</tr> 71</tbody> 72</table> 73 74> **NOTE**<br/> 75>The number of memory blocks in the memory pool after initialization is not equal to the total memory size divided by the memory block size. The reason is the control block of the memory pool and the control header of each memory block have memory overheads. When setting the total memory size, you need to consider these factors. 76 77### How to Develop 78 79The typical development process of static memory is as follows: 80 811. Plan a memory space as the static memory pool. 822. Call the **LOS\_MemboxInit** API to initialize the static memory pool. 83 84 During initialization, the memory space specified by the input parameter is divided into multiple blocks \(the number of blocks depends on the total static memory size and the block size\). Insert all memory blocks to the free list, and place the control header at the beginning of the memory. 85 863. Call the **LOS\_MemboxAlloc** API to allocate static memory. 87 88 The system allocates the first free memory block from the free list and returns the start address of this memory block. 89 904. Call the **LOS\_MemboxClr** API. 91 92 Clear the memory block corresponding to the address contained in the input parameter. 93 945. Call the **LOS\_MemboxFree** API. 95 96 Add the memory block to the free list. 97 98 99### Development Example 100 101This example implements the following: 102 1031. Initialize a static memory pool. 1042. Allocate a memory block from the static memory pool. 1053. Store a piece of data in a memory block. 1064. Print the data in the memory block. 1075. Clear the data in the memory block. 1086. Release the memory block. 109 110 The sample code is as follows: 111 112 113``` 114#include "los_membox.h" 115 116VOID Example_StaticMem(VOID) 117{ 118 UINT32 *mem = NULL; 119 UINT32 blkSize = 10; 120 UINT32 boxSize = 100; 121 UINT32 boxMem[1000]; 122 UINT32 ret; 123 124 /* Initialize the memory pool.*/ 125 ret = LOS_MemboxInit(&boxMem[0], boxSize, blkSize); 126 if(ret != LOS_OK) { 127 printf("Membox init failed!\n"); 128 return; 129 } else { 130 printf("Membox init success!\n"); 131 } 132 133 /* Request a memory block.*/ 134 mem = (UINT32 *)LOS_MemboxAlloc(boxMem); 135 if (NULL == mem) { 136 printf("Mem alloc failed!\n"); 137 return; 138 } 139 printf("Mem alloc success!\n"); 140 141 /* Assign a value.*/ 142 *mem = 828; 143 printf("*mem = %d\n", *mem); 144 145 /* Clear data in the memory block. */ 146 LOS_MemboxClr(boxMem, mem); 147 printf("Mem clear success \n *mem = %d\n", *mem); 148 149 /* Release the memory.*/ 150 ret = LOS_MemboxFree(boxMem, mem); 151 if (LOS_OK == ret) { 152 printf("Mem free success!\n"); 153 } else { 154 printf("Mem free failed!\n"); 155 } 156 157 return; 158} 159``` 160 161### Verification 162 163The output is as follows: 164 165``` 166Membox init success! 167Mem alloc success! 168*mem = 828 169Mem clear success 170*mem = 0 171Mem free success! 172``` 173 174
