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FatFs R0.13c, released October 14, 2018

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Christopher Williams
2019-01-09 17:13:03 -07:00
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<h3>How to Port</h3>
<h4>Basic Considerations</h4>
<p>The FatFs module is assuming following conditions on portability.</p>
<p>The FatFs module assumes following conditions on portability.</p>
<ul>
<li>ANSI C<br>
The FatFs module is a middleware written in ANSI C (C89). There is no platform dependence, so long as the compiler is in compliance with ANSI C.</li>
The FatFs module is a middleware written in ANSI C (C89). There is no platform dependence, so long as the compiler is in compliance with C89 or later. Only exFAT feature requires C99.</li>
<li>Size of integer types<br>
The FatFs module assumes that size of <tt>char</tt>/<tt>short</tt>/<tt>long</tt> are 8/16/32 bit and <tt>int</tt> is 16 or 32 bit. These correspondence are defined in <tt>integer.h</tt>. This will not be a problem on most compilers. When a conflict with existing definitions is occured, you must resolve it with care.</li>
<ul>
<li>When the C standard is in C89, size of <tt>char</tt>/<tt>short</tt>/<tt>long</tt> must be 8/16/32-bit respectively.</li>
<li>When it is in C99 or later, <tt>stdint.h</tt> is used to obtain the correct sizes.</li>
<li>Size of <tt>char</tt> must be 8-bit.</li>
<li>Size of <tt>int</tt>, as well as integer promotion, must be 16-bit or 32-bit.</li>
</ul>
Integer types used in FatFs are defined in <tt>fftypes.h</tt>. This will not be a problem on most platform. When a conflict with existing definitions occured, you must resolve it with care.</li>
</ul>
<h4>System Organizations</h4>
<p>The dependency diagram shown below is a typical but not specific configuration of the embedded system with FatFs module.</p>
<p>The dependency diagram shown below is a typical, but not specific, configuration of the embedded system with FatFs module.</p>
<p><img src="../res/modules.png" width="580" height="280" alt="dependency diagram"></p>
<p>(a) If a working disk module with FatFs disk interface is provided, nothing else will be needed. (b) To attach existing disk drivers with different interface, glue functions are needed to translate the interfaces between FatFs and the drivers.</p>
<p>(a) If a working disk module for FatFs is provided, nothing else will be needed. (b) To attach existing disk drivers with different interface, some glue functions are needed to translate the interfaces between FatFs and the driver.</p>
<p><img src="../res/funcs.png" width="750" height="420" alt="functional diagram"></p>
<h4>Required Functions</h4>
<p>You need to provide only low level disk I/O functions required by FatFs module and nothing else. If a working disk module for the target system is already provided, you need to write only glue functions to attach it to the FatFs module. If not, you need to port another disk I/O module or write it from scratch. Most of defined functions are not that always required. For example, any write function is not required at read-only configuration. Following table shows which function is required depends on the configuration options.</p>
<p>You need to provide only low level disk I/O functions required by FatFs module and nothing else. If a working disk I/O module for the target system is already provided, you need to write only glue functions to attach it to the FatFs module. If not, you need to port another disk I/O module or write it from scratch. Most of defined functions are not that always required. For instance, any write function is not required at read-only configuration. Following table shows which function is required depends on the configuration options.</p>
<table class="lst2">
<tr><th>Function</th><th>Required when</th><th>Note</th></tr>
<tr><td>disk_status<br>disk_initialize<br>disk_read</td><td>Always</td><td rowspan="5">Disk I/O functions.<br>Samples available in ffsample.zip.<br>There are many implementations on the web.</td></tr>
@@ -55,10 +61,11 @@ The FatFs module assumes that size of <tt>char</tt>/<tt>short</tt>/<tt>long</tt>
<tr><td>disk_ioctl (GET_SECTOR_COUNT)<br>disk_ioctl (GET_BLOCK_SIZE)</td><td>FF_USE_MKFS == 1</td></tr>
<tr><td>disk_ioctl (GET_SECTOR_SIZE)</td><td>FF_MAX_SS != FF_MIN_SS</td></tr>
<tr><td>disk_ioctl (CTRL_TRIM)</td><td>FF_USE_TRIM == 1</td></tr>
<tr><td>ff_uni2oem<br>ff_oem2uni<br>ff_wtoupper</td><td>FF_USE_LFN != 0</td><td>Unicode support functions.<br>Just add ffunicode.c to the project.</td></tr>
<tr><td>ff_cre_syncobj<br>ff_del_syncobj<br>ff_req_grant<br>ff_rel_grant</td><td>FF_FS_REENTRANT == 1</td><td rowspan="2">O/S dependent functions.<br>Samples available in ffsystem.c.</td></tr>
<tr><td>ff_uni2oem<br>ff_oem2uni<br>ff_wtoupper</td><td>FF_USE_LFN != 0</td><td>Unicode support functions.<br>Add optional module ffunicode.c to the project.</td></tr>
<tr><td>ff_cre_syncobj<br>ff_del_syncobj<br>ff_req_grant<br>ff_rel_grant</td><td>FF_FS_REENTRANT == 1</td><td rowspan="2">O/S dependent functions.<br>Sample code is available in ffsystem.c.</td></tr>
<tr><td>ff_mem_alloc<br>ff_mem_free</td><td>FF_USE_LFN == 3</td></tr>
</table>
<p>FatFs cares about neither what kind of storage device is used nor how it is implemented. Only a requirement is that it is a block device read/written in fixed-size blocks that accessible via the disk I/O functions defined above.</p>
</div>
<div class="para doc" id="limits">
@@ -89,13 +96,13 @@ The FatFs module assumes that size of <tt>char</tt>/<tt>short</tt>/<tt>long</tt>
<tr class="ral"><td class="cal">Work area<br><small>(FF_FS_TINY == 0)</small></td><td>V*564<br>+ F*552</td><td>V*564<br>+ F*552</td><td>V*564<br>+ F*552</td><td>V*560<br>+ F*546</td><td>V*560<br>+ F*546</td><td>V*560<br>+ F*546</td><td>V*560<br>+ F*546</td><td>V*564<br>+ F*552</td><td>V*564<br>+ F*552</td><td>V*564<br>+ F*552</td><td>V*564<br>+ F*552</td></tr>
<tr class="ral"><td class="cal">Work area<br><small>(FF_FS_TINY == 1)</small></td><td>V*564<br>+ F*40</td><td>V*564<br>+ F*40</td><td>V*564<br>+ F*40</td><td>V*560<br>+ F*34</td><td>V*560<br>+ F*34</td><td>V*560<br>+ F*34</td><td>V*560<br>+ F*34</td><td>V*564<br>+ F*40</td><td>V*564<br>+ F*40</td><td>V*564<br>+ F*40</td><td>V*564<br>+ F*40</td></tr>
</table>
<p>These are the memory usage on some target systems with following condition. The memory sizes are in unit of byte, <em>V</em> denotes number of mounted volumes and <em>F</em> denotes number of open files. All samples here are optimezed in code size.</p>
<p>These are the memory usage of FatFs module without lower layer on some target systems in following condition. <em>V</em> denotes number of mounted volumes and <em>F</em> denotes number of open files. Every samples here are optimezed in code size.</p>
<pre>
FatFs R0.13a options:
FF_FS_READONLY 0 (Read/Write) or 1 (Read only)
FF_FS_MINIMIZE 0 (Full, with all basic functions) or 3 (Min, with fully minimized)
FF_FS_TINY 0 (Default) or 1 (Tiny file object)
And other options are left unchanged from original setting.
And any other options are left unchanged from original setting.
</pre>
</div>
@@ -146,8 +153,8 @@ And other options are left unchanged from original setting.
<h3>Long File Name</h3>
<p>FatFs module supports long file name (LFN). The two different file names, short file name (SFN) and LFN, of a file is transparent on the API except for <tt>f_readdir</tt> function. The support for LFN is disabled by default. To enable the LFN, set <tt><a href="config.html#use_lfn">FF_USE_LFN</a></tt> to 1, 2 or 3, and add <tt>ffunicode.c</tt> to the project. The LFN requiers a certain working buffer in addition. The buffer size can be configured by <tt><a href="config.html#max_lfn">FF_MAX_LFN</a></tt> according to the available memory. The length of an LFN will be up to 255 characters, so that the <tt>FF_MAX_LFN</tt> should be set to 255 for all file names. If the size of working buffer is insufficient for the input file name, the file function fails with <tt>FR_INVALID_NAME</tt>. When use any re-entry to the API with LFN is enabled, <tt>FF_USE_LFN</tt> must be set to 2 or 3. In this case, the file function allocates the working buffer on the stack or heap. The working buffer occupies <tt>(FF_MAX_LFN + 1) * 2</tt> bytes and additional <tt>(FF_MAX_LFN + 44) / 15 * 32</tt> bytes when exFAT is enabled.</p>
<table class="lst2 rset">
<caption>With LFN at CM3+gcc</caption>
<tr><th><tt>FF_CODE_PAGE</tt></th><th>Increment</th></tr>
<caption>With LFN at CM3 + gcc</caption>
<tr><th><tt>FF_CODE_PAGE</tt></th><th>Code size</th></tr>
<tr><td>437-869 (SBCS)</td><td>+3.3k</td></tr>
<tr><td>932 (Japanese)</td><td>+62k</td></tr>
<tr><td>936 (Simplified Chinese)</td><td>+177k</td></tr>
@@ -155,22 +162,22 @@ And other options are left unchanged from original setting.
<tr><td>950 (Traditional Chinese)</td><td>+111k</td></tr>
<tr><td>0 (All code pages)</td><td>+486k</td></tr>
</table>
<p>When the LFN is enabled, the module size will be increased depends on the configured code page. Right table shows increment of code size by LFN function at different code pages. Especially, in the CJK region, tens of thousands of characters are being used. Unfortunately, it requires a huge OEM-Unicode bidirectional conversion table and the module size will be drastically increased as shown in the table. As the result, the FatFs with LFN enebled with those code pages will not able to be ported on the most 8-bit MCU systems.</p>
<p>If you can discard ANSI/OEM code API and backward compatibility with non-ASCII SFN, you will able to configure FatFs for Unicode API with any SBCS.</p>
<p>There ware some restrictions on using LFN for open source project because the support for LFN on the FAT volume was a patent of Microsoft Corporation. The related patents have expired and using the LFN function have got free for any projects.</p>
<p>When the LFN is enabled, the module size will be increased depends on the configured code page. Right table shows increment of code size by LFN function at different code pages. Especially, in the CJK region, tens of thousands of characters are being used. Unfortunately, it requires a huge OEM-Unicode bidirectional conversion table and the module size will be drastically increased as shown in the table.</p>
<p>As the result, the FatFs with LFN enebled with DBCS code pages will not able to be ported on the most 8-bit MCU systems. If the target system is in legacy-free, in only Unicode and any ANSI/OEM code is not used at all, the code page setting gets meaningless. You will able to reduce the code size by configureing FatFs for Unicode with any SBCS code page.</p>
<p>There ware some restrictions on using LFN for open source project because the support for LFN on the FAT volume was a patent of Microsoft Corporation. The related patents have expired and using the LFN function has got free for any projects.</p>
</div>
<div class="para doc" id="unicode">
<h3>Unicode API</h3>
<p>By default, FatFs uses ANSI/OEM code set on the API even at LFN configuration. FatFs can also switch the character encoding on the API to Unicode by configuration option <tt><a href="config.html#lfn_unicode">FF_LFN_UNICODE</a></tt>. This means that FatFs supports the full featured LFN specification. The data type <tt>TCHAR</tt> specifies path name strings on the API is an alias of either <tt>char</tt>(ANSI/OEM or UTF-8), <tt>WCHAR</tt>(UTF-16) or <tt>DWORD</tt>(UTF-32) depends on that option. For more information, refer to the description in the <a href="filename.html#uni">file name</a>.</p>
<p>Note that code page setting, <tt><a href="config.html#code_page">FF_CODE_PAGE</a></tt>, has actually no meaning for the path names at the Unicode API. However it still affects code conversion of string I/O functions at <tt><a href="config.html#strf_encode">FF_STRF_ENCODE</a> != 0</tt> and backward compatibility with legacy systems, so that code page needs to be configured properly when it is considered a problem.</p>
<p>By default, FatFs uses ANSI/OEM code set on the API even at LFN configuration. FatFs can also switch the character encoding on the API to Unicode by configuration option <tt><a href="config.html#lfn_unicode">FF_LFN_UNICODE</a></tt>. This means that FatFs is compliant with the full featured LFN specification. The data type <tt>TCHAR</tt> specifies path name strings on the API is an alias of either <tt>char</tt>(ANSI/OEM or UTF-8), <tt>WCHAR</tt>(UTF-16) or <tt>DWORD</tt>(UTF-32) depends on that option. For more information, refer to the description in the <a href="filename.html#uni">file name</a>.</p>
<p>Note that setting of code page, <tt><a href="config.html#code_page">FF_CODE_PAGE</a></tt>, has actually no meaning when FatFs is configured for the Unicode API. It should be set 437 anyway. However it still affects code conversion of string I/O functions at <tt><a href="config.html#strf_encode">FF_STRF_ENCODE</a> == 0</tt> and backward compatibility with legacy systems, so that code page may need to be configured properly if it is considered a problem.</p>
</div>
<div class="para doc" id="exfat">
<h3>exFAT Filesystem</h3>
<p>The exFAT (Microsoft's Extended File Allocation Table) filesystem is a succession of the FAT/FAT32 filesystem which has been widely used in embedded systems, consumer devices and portable storage media. It is adopted by SDA (SD Association) as a recommended filesystem for high capacity SD cards larger than 32 GB and they are being shipped with this format, so that the exFAT became one of the standard filesystems for removable media as well as FAT. The exFAT filesystem allows the file size beyond the 4 GB limit what FAT filesystem allows upto and some filesystem overhead, especially cluster allocation delay, are reduced as well. This feature improves the write throughput to the file.</p>
<p>Note that the exFAT is a patent of Microsoft Corporation. The exFAT function of FatFs is an implementation based on <cite>US. Pat. App. Pub. No. 2009/0164440 A1</cite>. FatFs module can switch the support for exFAT on/off by configuration option, <tt><a href="config.html#fs_exfat">FF_FS_EXFAT</a></tt>. When enable the exFAT on the commercial products, a license by Microsoft will be needed depends on the final destination of the products.</p>
<p><em>Remark: Enabling exFAT discards ANSI C (C89) compatibility because of need for 64-bit integer type.</em></p>
<p>The exFAT (Microsoft's Extended File Allocation Table) filesystem is a succession of the FAT/FAT32 filesystem which has been widely used in embedded systems, consumer devices and portable storage media. It is adopted by SDA (SD Association) as the regular filesystem for SDXC card, 64 GB and larger, and they are being shipped with this format. Therefore the exFAT is one of the standard filesystems for removable media as well as FAT. The exFAT filesystem allows the file size beyond the 4 GB limit what FAT filesystem allows upto and some filesystem overhead, especially cluster allocation delay, are reduced as well. These features allow to record the large media file without dividing into some files and improve the write throughput to the file.</p>
<p>Note that the exFAT is a patent of Microsoft Corporation. The support for exFAT of FatFs is an implementation based on <cite>US. Pat. App. Pub. No. 2009/0164440 A1</cite>. FatFs module can switch the exFAT on or off by configuration option, <tt><a href="config.html#fs_exfat">FF_FS_EXFAT</a></tt>. When enable the exFAT on the commercial products, a license by Microsoft will be needed depends on the final destination of the products.</p>
<p><em>Remark: Enabling exFAT discards C89 compatibility because of need for 64-bit integer type.</em></p>
</div>
<div class="para doc" id="reentrant">
@@ -245,11 +252,12 @@ Figure 5. Minimized critical section<br>
<h3>Extended Use of FatFs API</h3>
<p>These are examples of extended use of FatFs APIs. New item will be added whenever a useful code is found.</p>
<ol>
<li><a href="http://elm-chan.org/fsw/ff/res/app1.c">Open or create a file for append</a> (for only R0.12 and earlier)</li>
<li><a href="http://elm-chan.org/fsw/ff/res/app2.c">Empty a directory</a></li>
<li><a href="http://elm-chan.org/fsw/ff/res/app3.c">Allocate contiguous area to the file</a> (for only R0.11a and earlier)</li>
<li><a href="http://elm-chan.org/fsw/ff/res/app1.c">Open or create a file for append</a> (for R0.12 and earlier)</li>
<li><a href="http://elm-chan.org/fsw/ff/res/app2.c">Delete a non-empty sub-directory</a> (for R0.12 and later)</li>
<li><a href="http://elm-chan.org/fsw/ff/res/app3.c">Allocate contiguous area to the file</a> (for R0.11a and earlier)</li>
<li><a href="http://elm-chan.org/fsw/ff/res/app4.c">Compatibility checker for low level disk I/O module</a></li>
<li><a href="http://elm-chan.org/fsw/ff/res/mkfatimg.zip">FAT volume image creator</a></li>
<li><a href="http://elm-chan.org/fsw/ff/res/app5.c">Test if the file is contiguous or not</a></li>
</ol>
</div>