1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> 2<html lang="en"> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 5<meta http-equiv="Content-Style-Type" content="text/css"> 6<link rel="up" title="FatFs" href="../00index_e.html"> 7<link rel="alternate" hreflang="ja" title="Japanese" href="../ja/lseek.html"> 8<link rel="stylesheet" href="../css_e.css" type="text/css" media="screen" title="ELM Default"> 9<title>FatFs - f_lseek</title> 10</head> 11 12<body> 13 14<div class="para func"> 15<h2>f_lseek</h2> 16<p>The f_lseek function moves the file read/write pointer of an open file object. It can also be used to expand the file size (cluster pre-allocation). </p> 17 18<pre> 19FRESULT f_lseek ( 20 FIL* <span class="arg">fp</span>, <span class="c">/* [IN] File object */</span> 21 FSIZE_t <span class="arg">ofs</span> <span class="c">/* [IN] Offset of file read/write pointer to be set */</span> 22); 23</pre> 24<pre> 25FRESULT f_rewind ( 26 FIL* <span class="arg">fp</span> <span class="c">/* [IN] File object */</span> 27); 28</pre> 29</div> 30 31<div class="para arg"> 32<h4>Parameters</h4> 33<dl class="par"> 34<dt>fp</dt> 35<dd>Pointer to the open file object.</dd> 36<dt>ofs</dt> 37<dd>Byte offset from top of the file to set read/write pointer. The data type <tt>FSIZE_t</tt> is an alias of either <tt>DWORD</tt>(32-bit) or <tt>QWORD</tt>(64-bit) depends on the configuration option <tt><a href="config.html#fs_exfat">FF_FS_EXFAT</a></tt>.</dd> 38</dl> 39</div> 40 41 42<div class="para ret"> 43<h4>Return Values</h4> 44<p> 45<a href="rc.html#ok">FR_OK</a>, 46<a href="rc.html#de">FR_DISK_ERR</a>, 47<a href="rc.html#ie">FR_INT_ERR</a>, 48<a href="rc.html#io">FR_INVALID_OBJECT</a>, 49<a href="rc.html#tm">FR_TIMEOUT</a> 50</p> 51</div> 52 53 54<div class="para desc"> 55<h4>Description</h4> 56<p>File read/write ponter in the open file object points the data byte to be read/written at next read/write operation. It advances as the number of bytes read/written. The <tt>f_lseek</tt> function moves the file read/write pointer without any read/write operation to the file. The <tt>f_rewind</tt> function is impremented as a macro.</p> 57<pre> 58#define <em>f_rewind</em>(fp) f_lseek((fp), 0) 59</pre> 60<p>If an offset beyond the file size is specified in write mode, the file size is expanded to the specified offset. The file data in the expanded part is <em>undefined</em>, because no data is written to the file in this process. This is suitable to pre-allocate a data area to the file quickly for fast write operation. When a contiguous data area needs to be allocated to the file, use <tt>f_expand</tt> function instead. After the <tt>f_lseek</tt> function succeeded, the current read/write pointer should be checked in order to make sure the read/write pointer has been moved correctry. In case of the read/write pointer is not pointing expected offset, either of followings has been occured.</p> 61<ul> 62<li>End of file. The specified <tt class="arg">ofs</tt> was clipped at end of the file in read-only mode.</li> 63<li>Disk full. There is no free space on the volume to expand the file.</li> 64</ul> 65<p>The fast seek feature enables fast backward/long seek operations without FAT access by using an on-memory CLMT (cluster link map table). It is applied to <tt>f_read</tt> and <tt>f_write</tt> function as well, however, the file size cannot be expanded by <tt>f_write</tt>, <tt>f_lseek</tt> function while the file is at fast seek mode.</p> 66<p>The fast seek mode is available when <tt>FF_USE_FASTSEEK = 1</tt>. The CLMT must be created into the <tt>DWORD</tt> array prior to use the fast seek mode. To create the CLMT, set address of the <tt>DWORD</tt> array to the member <tt>cltbl</tt> in the open file object, set the size of array in unit of items to the <tt>cltbl[0]</tt> and then call <tt>f_lseek</tt> function with <tt class="arg">ofs</tt><tt> = CREATE_LINKMAP</tt>. After the function succeeded, no FAT access is occured in subsequent <tt>f_read</tt>, <tt>f_write</tt>, <tt>f_lseek</tt> function to the file. The number of items used or required is returned into the <tt>cltbl[0]</tt>. The number of items needed is (number of the file fragments + 1) * 2. For example, 12 items in the array will be used for the file fragmented in 5 portions. If the function failed with <tt>FR_NOT_ENOUGH_CORE</tt>, the size of given array is insufficient for the file.</p> 67</div> 68 69 70<div class="para comp"> 71<h4>QuickInfo</h4> 72<p>Available when <tt><a href="config.html#fs_minimize">FF_FS_MINIMIZE</a> <= 2</tt>. To use fast seek function, <tt><a href="config.html#use_fastseek">FF_USE_FASTSEEK</a></tt> needs to be set 1 to enable this feature.</p> 73</div> 74 75 76<div class="para use"> 77<h4>Example</h4> 78<pre> 79 <span class="c">/* Open file */</span> 80 fp = malloc(sizeof (FIL)); 81 res = f_open(fp, "file.dat", FA_READ|FA_WRITE); 82 if (res) ... 83 84 <span class="c">/* Set read/write pointer to 5000 */</span> 85 res = <em>f_lseek</em>(fp, 5000); 86 87 <span class="c">/* Set read/write pointer to end of the file to append data */</span> 88 res = <em>f_lseek</em>(fp, f_size(fp)); 89 90 <span class="c">/* Advance read/write pointer 3000 bytes */</span> 91 res = <em>f_lseek</em>(fp, f_tell(fp) + 3000); 92 93 <span class="c">/* Rewind read/write pointer 2000 bytes (take care on wraparound) */</span> 94 res = <em>f_lseek</em>(fp, f_tell(fp) - 2000); 95</pre> 96<pre> 97<span class="c">/* Cluster pre-allocation (to prevent buffer overrun on streaming write) */</span> 98 99 res = f_open(fp, recfile, FA_CREATE_NEW | FA_WRITE); <span class="c">/* Create a file */</span> 100 101 res = <em>f_lseek</em>(fp, PRE_SIZE); <span class="c">/* Expand file size (cluster pre-allocation) */</span> 102 if (res || f_tell(fp) != PRE_SIZE) ... <span class="c">/* Check if the file has been expanded successfly */</span> 103 104 res = <em>f_lseek</em>(fp, OFS_DATA); <span class="c">/* Record data stream with free from cluster allocation delay */</span> 105 ... <span class="c">/* Write operation should be aligned to sector boundary to optimize the write throughput */</span> 106 107 res = f_truncate(fp); <span class="c">/* Truncate unused area */</span> 108 res = <em>f_lseek</em>(fp, OFS_HEADER); <span class="c">/* Set file header */</span> 109 ... 110 111 res = f_close(fp); 112</pre> 113<pre> 114<span class="c">/* Using fast seek mode */</span> 115 116 DWORD clmt[SZ_TBL]; <span class="c">/* Cluster link map table buffer */</span> 117 118 res = f_open(fp, fname, FA_READ | FA_WRITE); <span class="c">/* Open a file */</span> 119 120 res = <em>f_lseek</em>(fp, ofs1); <span class="c">/* This is normal seek (cltbl is nulled on file open) */</span> 121 122 fp->cltbl = clmt; <span class="c">/* Enable fast seek mode (cltbl != NULL) */</span> 123 clmt[0] = SZ_TBL; <span class="c">/* Set table size */</span> 124 res = <em>f_lseek</em>(fp, CREATE_LINKMAP); <span class="c">/* Create CLMT */</span> 125 ... 126 127 res = <em>f_lseek</em>(fp, ofs2); <span class="c">/* This is fast seek */</span> 128</pre> 129</div> 130 131 132<div class="para ref"> 133<h4>See Also</h4> 134<p><tt><a href="open.html">f_open</a>, <a href="truncate.html">f_truncate</a>, <a href="expand.html">f_expand</a>, <a href="sfile.html">FIL</a></tt></p> 135</div> 136 137<p class="foot"><a href="../00index_e.html">Return</a></p> 138</body> 139</html> 140