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| <section id="file-mapping-emulation-mmap"> |
| <h1>File mapping emulation (mmap)<a class="headerlink" href="#file-mapping-emulation-mmap" title="Permalink to this heading"></a></h1> |
| <p>NuttX operates in a flat open address space and is focused on MCUs that do |
| support Memory Management Units (MMUs). Therefore, NuttX generally does not |
| require mmap() functionality and the MCUs generally cannot support true |
| memory-mapped files.</p> |
| <p>However, memory mapping of files is the mechanism used by NXFLAT, the NuttX |
| tiny binary format, to get files into memory in order to execute them. |
| mmap() support is therefore required to support NXFLAT. There are two |
| conditions where mmap() can be supported:</p> |
| <ol class="arabic"> |
| <li><p>mmap can be used to support eXecute In Place (XIP) on random access media |
| under the following very restrictive conditions:</p> |
| <ol class="loweralpha simple"> |
| <li><p>The filesystem implements the mmap file operation. Any file |
| system that maps files contiguously on the media should support |
| this ioctl. (vs. file system that scatter files over the media |
| in non-contiguous sectors). As of this writing, ROMFS is the |
| only file system that meets this requirement.</p></li> |
| <li><p>The underlying block driver supports the BIOC_XIPBASE ioctl |
| command that maps the underlying media to a randomly accessible |
| address. At present, only the RAM/ROM disk driver does this.</p></li> |
| </ol> |
| <p>Some limitations of this approach are as follows:</p> |
| <ol class="loweralpha simple"> |
| <li><p>Since no real mapping occurs, all of the file contents are “mapped” |
| into memory.</p></li> |
| <li><p>All mapped files are read-only.</p></li> |
| <li><p>There are no access privileges.</p></li> |
| </ol> |
| </li> |
| <li><p>If CONFIG_FS_RAMMAP is defined in the configuration, then mmap() will |
| support simulation of memory mapped files by copying files whole |
| into RAM. These copied files have some of the properties of |
| standard memory mapped files. There are many, many exceptions, |
| however. Some of these include:</p> |
| <ol class="loweralpha"> |
| <li><p>The goal is to have a single region of memory that represents a single |
| file and can be shared by many threads. That is, given a filename a |
| thread should be able to open the file, get a file descriptor, and |
| call mmap() to get a memory region. Different file descriptors opened |
| with the same file path should get the same memory region when mapped.</p> |
| <p>The limitation in the current design is that there is insufficient |
| knowledge to know that these different file descriptors correspond to |
| the same file. So, for the time being, a new memory region is created |
| each time that rammap() is called. Not very useful!</p> |
| </li> |
| <li><p>The entire mapped portion of the file must be present in memory. |
| Since it is assumed that the MCU does not have an MMU, on-demanding |
| paging in of file blocks cannot be supported. Since the while mapped |
| portion of the file must be present in memory, there are limitations |
| in the size of files that may be memory mapped (especially on MCUs |
| with no significant RAM resources).</p></li> |
| <li><p>All mapped files are read-only. You can write to the in-memory image, |
| but the file contents will not change.</p></li> |
| <li><p>There are no access privileges.</p></li> |
| <li><p>Since there are no processes in NuttX, all mmap() and munmap() |
| operations have immediate, global effects. Under Linux, for example, |
| munmap() would eliminate only the mapping with a process; the mappings |
| to the same file in other processes would not be effected.</p></li> |
| <li><p>Like true mapped file, the region will persist after closing the file |
| descriptor. However, at present, these ram copied file regions are |
| <strong>not</strong> automatically “unmapped” (i.e., freed) when a thread is terminated. |
| This is primarily because it is not possible to know how many users |
| of the mapped region there are and, therefore, when would be the |
| appropriate time to free the region (other than when munmap is called).</p> |
| <p>NOTE: Note, if the design limitation of a) were solved, then it would be |
| easy to solve exception d) as well.</p> |
| </li> |
| </ol> |
| </li> |
| </ol> |
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