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| <title>FatFs - f_lseek</title> | |
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| <div class="para"> | |
| <h2>f_lseek</h2> | |
| <p>The f_lseek function moves the file read/write pointer of an open file object. It can also be used to increase the file size (cluster pre-allocation). </p> | |
| <pre> | |
| FRESULT f_lseek ( | |
| FIL* <em>FileObject</em>, <span class="c">/* Pointer to the file object structure */</span> | |
| DWORD <em>Offset</em> <span class="c">/* File offset in unit of byte */</span> | |
| ); | |
| </pre> | |
| </div> | |
| <div class="para"> | |
| <h4>Parameters</h4> | |
| <dl class="par"> | |
| <dt>FileObject</dt> | |
| <dd>Pointer to the open file object.</dd> | |
| <dt>Offset</dt> | |
| <dd>Number of bytes where from start of the file</dd> | |
| </dl> | |
| </div> | |
| <div class="para"> | |
| <h4>Return Values</h4> | |
| <dl class="ret"> | |
| <dt>FR_OK (0)</dt> | |
| <dd>The function succeeded.</dd> | |
| <dt>FR_DISK_ERR</dt> | |
| <dd>The function failed due to an error in the disk function.</dd> | |
| <dt>FR_INT_ERR</dt> | |
| <dd>The function failed due to a wrong FAT structure or an internal error.</dd> | |
| <dt>FR_NOT_READY</dt> | |
| <dd>The disk drive cannot work due to no medium in the drive or any other reason.</dd> | |
| <dt>FR_INVALID_OBJECT</dt> | |
| <dd>The file object is invalid.</dd> | |
| <dt>FR_NOT_ENOUGH_CORE</dt> | |
| <dd>Insufficient size of link map table for the file.</dd> | |
| </dl> | |
| </div> | |
| <div class="para"> | |
| <h4>Description</h4> | |
| <p>The f_lseek function moves the file read/write pointer of an open file. The offset can be specified in only origin from top of the file. When an offset above the file size is specified in write mode, the file size is increased to the offset and the data in the expanded area is undefined. This is suitable to create a large file quickly, for fast write operation. After the f_lseek function succeeded, member fptr in the file object should be checked in order to make sure the read/write pointer has been moved correctry. In case of fptr is not the expected value, either of followings has been occured.</p> | |
| <ul> | |
| <li>End of file. The specified Offset was clipped at the file size because the file has been opened in read-only mode.</li> | |
| <li>Disk full. There is insufficient free space on the volume to expand the file size.</li> | |
| </ul> | |
| <p>Fast seek mode is enabled when <tt>_USE_FASTSEEK</tt> is set to 1 and <tt>cltbl</tt> member in the file object is not NULL. This feature enables fast backward/long seek operations without FAT access by cluster link map table (CLMT) stored in the user defined table. It is also applied to f_read/f_write functions. In this mode, the file size cannot be increased by f_write/f_lseek functions.</p> | |
| <p>The CLMT must be created in the user defined DWORD array prior to use fast seek feature. To create the CLMT, set pointer to the DWORD array to <tt>cltbl</tt> member in the file object, set the array size in unit of items into the first item and call the f_lseek function with <tt>Offset = CREATE_LINKMAP</tt>. After the function succeeded and CLMT is created, no FAT access is occured in subsequent f_read/f_write/f_lseek functions to the file. If the function failed with <tt>FR_NOT_ENOUGH_CORE</tt>, the given array size is insufficient for the file and the required items is returned into the first item of the array. The required array size is (number of fragments + 1) * 2 items. For example, when the file is fragmented in 5, 12 items will be required for the CLMT. </p> | |
| </div> | |
| <div class="para"> | |
| <h4>QuickInfo</h4> | |
| <p>Available when <tt>_FS_MINIMIZE <= 2</tt>.</p> | |
| </div> | |
| <div class="para"> | |
| <h4>Example</h4> | |
| <pre> | |
| <span class="c">/* Open file */</span> | |
| file = malloc(sizeof(FIL)); | |
| if (!file) ... | |
| res = f_open(file, "file.dat", FA_READ|FA_WRITE); | |
| if (res) ... | |
| <span class="c">/* Move to offset of 5000 from top of the file */</span> | |
| res = f_lseek(file, 5000); | |
| <span class="c">/* Move to end of the file to append data */</span> | |
| res = f_lseek(file, f_size(file)); | |
| <span class="c">/* Forward 3000 bytes */</span> | |
| res = f_lseek(file, f_tell(file) + 3000); | |
| <span class="c">/* Rewind 2000 bytes (take care on overflow) */</span> | |
| res = f_lseek(file, f_tell(file) - 2000); | |
| </pre> | |
| <pre> <span class="c">/* Cluster pre-allocation (to prevent buffer overrun on streaming write) */</span> | |
| res = f_open(file, recfile, FA_CREATE_NEW | FA_WRITE); <span class="c">/* Create a file */</span> | |
| res = f_lseek(file, PRE_SIZE); <span class="c">/* Pre-allocate clusters */</span> | |
| if (res || file->fptr != PRE_SIZE) ... <span class="c">/* Check if the file size has been increased correctly */</span> | |
| res = f_lseek(file, DATA_START); <span class="c">/* Record data stream without cluster allocation delay */</span> | |
| ... | |
| res = f_truncate(file); <span class="c">/* Truncate unused area */</span> | |
| res = f_lseek(file, 0); <span class="c">/* Put file header */</span> | |
| ... | |
| res = f_close(file); | |
| </pre> | |
| <pre> <span class="c">/* Using fast seek feature */</span> | |
| DWORD lktbl[SZ_TBL]; <span class="c">/* Link map table buffer */</span> | |
| res = f_lseek(file, ofs1); <span class="c">/* This is normal seek (file.cltbl is nulled on file open) */</span> | |
| file.cltbl = lktbl; <span class="c">/* Enable fast seek feature */</span> | |
| lktbl[0] = SZ_TBL; <span class="c">/* Set table size to the first item */</span> | |
| res = f_lseek(file, CREATE_LINKMAP); <span class="c">/* Create CLMT */</span> | |
| ... | |
| res = f_lseek(file, ofs2); <span class="c">/* This is fast seek (file.cltbl != NULL) */</span> | |
| </pre> | |
| </div> | |
| <div class="para"> | |
| <h4>See Also</h4> | |
| <p><tt><a href="open.html">f_open</a>, <a href="truncate.html">f_truncate</a>, <a href="sfile.html">FIL</a></tt></p> | |
| </div> | |
| <p class="foot"><a href="../00index_e.html">Return</a></p> | |
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