savelij13
ead60b6ce1
- Added support for 64-bit LBA and GUID partition table (FF_LBA64 = 1) - Changed some API functions, f_mkfs() and f_fdisk(). - Fixed f_open() function cannot find the file with file name in length of FF_MAX_LFN characters. - Fixed f_readdir() function cannot retrieve long file names in length of FF_MAX_LFN - 1 characters. - Fixed f_readdir() function returns file names with wrong case conversion. (appeared at R0.12) - Fixed f_mkfs() function can fail to create exFAT volume in the second partition. (appeared at R0.12)
132 lines
7.0 KiB
HTML
132 lines
7.0 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
|
|
<html lang="en">
|
|
<head>
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<meta http-equiv="Content-Style-Type" content="text/css">
|
|
<link rel="up" title="FatFs" href="../00index_e.html">
|
|
<link rel="alternate" hreflang="ja" title="Japanese" href="../ja/lseek.html">
|
|
<link rel="stylesheet" href="../css_e.css" type="text/css" media="screen" title="ELM Default">
|
|
<title>FatFs - f_lseek</title>
|
|
</head>
|
|
|
|
<body>
|
|
|
|
<div class="para func">
|
|
<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 expand the file size (cluster pre-allocation). </p>
|
|
|
|
<pre>
|
|
FRESULT f_lseek (
|
|
FIL* <span class="arg">fp</span>, <span class="c">/* [IN] File object */</span>
|
|
FSIZE_t <span class="arg">ofs</span> <span class="c">/* [IN] File read/write pointer */</span>
|
|
);
|
|
</pre>
|
|
</div>
|
|
|
|
<div class="para arg">
|
|
<h4>Parameters</h4>
|
|
<dl class="par">
|
|
<dt>fp</dt>
|
|
<dd>Pointer to the open file object.</dd>
|
|
<dt>ofs</dt>
|
|
<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>
|
|
</dl>
|
|
</div>
|
|
|
|
|
|
<div class="para ret">
|
|
<h4>Return Values</h4>
|
|
<p>
|
|
<a href="rc.html#ok">FR_OK</a>,
|
|
<a href="rc.html#de">FR_DISK_ERR</a>,
|
|
<a href="rc.html#ie">FR_INT_ERR</a>,
|
|
<a href="rc.html#io">FR_INVALID_OBJECT</a>,
|
|
<a href="rc.html#tm">FR_TIMEOUT</a>
|
|
</p>
|
|
</div>
|
|
|
|
|
|
<div class="para desc">
|
|
<h4>Description</h4>
|
|
<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.</p>
|
|
<p>When 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 the expected value, either of followings has been occured.</p>
|
|
<ul>
|
|
<li>End of file. The specified <tt class="arg">ofs</tt> was clipped at end of the file because the file has been opened in read-only mode.</li>
|
|
<li>Disk full. There is no free space on the volume to expand the file.</li>
|
|
</ul>
|
|
<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>
|
|
<p>The fast seek mode is enabled 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 first item and call <tt>f_lseek</tt> function with <tt class="arg">ofs</tt><tt> = CREATE_LINKMAP</tt>. After the function succeeded and CLMT is created, 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 first item of the array. The number of items to be used 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>
|
|
</div>
|
|
|
|
|
|
<div class="para comp">
|
|
<h4>QuickInfo</h4>
|
|
<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>
|
|
</div>
|
|
|
|
|
|
<div class="para use">
|
|
<h4>Example</h4>
|
|
<pre>
|
|
<span class="c">/* Open file */</span>
|
|
fp = malloc(sizeof (FIL));
|
|
res = f_open(fp, "file.dat", FA_READ|FA_WRITE);
|
|
if (res) ...
|
|
|
|
<span class="c">/* Move to offset of 5000 from top of the file */</span>
|
|
res = <em>f_lseek</em>(fp, 5000);
|
|
|
|
<span class="c">/* Move to end of the file to append data */</span>
|
|
res = <em>f_lseek</em>(fp, f_size(fp));
|
|
|
|
<span class="c">/* Forward 3000 bytes */</span>
|
|
res = <em>f_lseek</em>(fp, f_tell(fp) + 3000);
|
|
|
|
<span class="c">/* Rewind 2000 bytes (take care on wraparound) */</span>
|
|
res = <em>f_lseek</em>(fp, f_tell(fp) - 2000);
|
|
</pre>
|
|
<pre>
|
|
<span class="c">/* Cluster pre-allocation (to prevent buffer overrun on streaming write) */</span>
|
|
|
|
res = f_open(fp, recfile, FA_CREATE_NEW | FA_WRITE); <span class="c">/* Create a file */</span>
|
|
|
|
res = <em>f_lseek</em>(fp, PRE_SIZE); <span class="c">/* Expand file size (cluster pre-allocation) */</span>
|
|
if (res || f_tell(fp) != PRE_SIZE) ... <span class="c">/* Check if the file has been expanded successfly */</span>
|
|
|
|
res = <em>f_lseek</em>(fp, DATA_START); <span class="c">/* Record data stream WITHOUT cluster allocation delay */</span>
|
|
... <span class="c">/* Write operation should be aligned to sector boundary to optimize the write throughput */</span>
|
|
|
|
res = f_truncate(fp); <span class="c">/* Truncate unused area */</span>
|
|
res = <em>f_lseek</em>(fp, 0); <span class="c">/* Set file header */</span>
|
|
...
|
|
|
|
res = f_close(fp);
|
|
</pre>
|
|
<pre>
|
|
<span class="c">/* Using fast seek mode */</span>
|
|
|
|
DWORD clmt[SZ_TBL]; <span class="c">/* Cluster link map table buffer */</span>
|
|
|
|
res = f_open(fp, fname, FA_READ | FA_WRITE); <span class="c">/* Open a file */</span>
|
|
|
|
res = <em>f_lseek</em>(fp, ofs1); <span class="c">/* This is normal seek (cltbl is nulled on file open) */</span>
|
|
|
|
fp->cltbl = clmt; <span class="c">/* Enable fast seek mode (cltbl != NULL) */</span>
|
|
clmt[0] = SZ_TBL; <span class="c">/* Set table size */</span>
|
|
res = <em>f_lseek</em>(fp, CREATE_LINKMAP); <span class="c">/* Create CLMT */</span>
|
|
...
|
|
|
|
res = <em>f_lseek</em>(fp, ofs2); <span class="c">/* This is fast seek */</span>
|
|
</pre>
|
|
</div>
|
|
|
|
|
|
<div class="para ref">
|
|
<h4>See Also</h4>
|
|
<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>
|
|
</div>
|
|
|
|
<p class="foot"><a href="../00index_e.html">Return</a></p>
|
|
</body>
|
|
</html>
|