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More documentation may still by worthwhile (design documentation?), but for now this provides a reasonable baseline. - readme - license - header documentation
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Copyright (c) 2016 Christopher Haster | ||
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Permission is hereby granted, free of charge, to any person obtaining a | ||
copy of this software and associated documentation files (the "Software"), | ||
to deal in the Software without restriction, including without limitation | ||
the rights to use, copy, modify, merge, publish, distribute, sublicense, | ||
and/or sell copies of the Software, and to permit persons to whom the | ||
Software is furnished to do so, subject to the following conditions: | ||
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The above copyright notice and this permission notice shall be included in | ||
all copies or substantial portions of the Software. | ||
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THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | ||
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | ||
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL | ||
THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | ||
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING | ||
FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER | ||
DEALINGS IN THE SOFTWARE. |
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## The little filesystem | ||
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A little fail-safe filesystem designed for low ram/rom footprint. | ||
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**Fail-safe** - The littlefs is designed to work consistently with random power | ||
failures. During filesystem operations the storage on disk is always kept | ||
in a valid state. The filesystem also has strong copy-on-write garuntees. | ||
When updating a file, the original file will remain unmodified until the | ||
file is closed, or sync is called. | ||
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**Handles bad blocks** - While the littlefs does not implement static wear | ||
leveling, if the underlying block device reports write errors, the littlefs | ||
uses a form of dynamic wear leveling to manage blocks that go bad during | ||
the lifetime of the filesystem. | ||
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**Constrained memory** - The littlefs is designed to work in bounded memory, | ||
recursion is avoided, and dynamic memory is kept to a minimum. The littlefs | ||
allocates two fixed-size buffers for general operations, and one fixed-size | ||
buffer per file. If there is only ever one file in use, these buffers can be | ||
provided statically. | ||
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## Example | ||
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Here's a simple example that updates a file named `boot_count` every time | ||
main runs. The program can be interrupted at any time without losing track | ||
of how many times it has been booted and without corrupting the filesystem: | ||
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``` c | ||
#include "lfs.h" | ||
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// variables used by the filesystem | ||
lfs_t lfs; | ||
lfs_file_t file; | ||
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// configuration of the filesystem is provided by this struct | ||
const struct lfs_config cfg = { | ||
// block device operations | ||
.read = user_provided_block_device_read, | ||
.prog = user_provided_block_device_prog, | ||
.erase = user_provided_block_device_erase, | ||
.sync = user_provided_block_device_sync, | ||
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// block device configuration | ||
.read_size = 16, | ||
.prog_size = 16, | ||
.block_size = 4096, | ||
.block_count = 128, | ||
.lookahead = 128, | ||
}; | ||
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// entry point | ||
int main(void) { | ||
// mount the filesystem | ||
int err = lfs_mount(&lfs, &cfg); | ||
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// reformat if we can't mount the filesystem | ||
// this should only happen on the first boot | ||
if (err) { | ||
lfs_format(&lfs, &cfg); | ||
lfs_mount(&lfs, &cfg); | ||
} | ||
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// read current count | ||
uint32_t boot_count = 0; | ||
lfs_file_open(&lfs, &file, "boot_count", LFS_O_RDWR | LFS_O_CREAT); | ||
lfs_file_read(&lfs, &file, &boot_count, sizeof(boot_count)); | ||
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// update boot count | ||
boot_count += 1; | ||
printf("boot_count: %ld\n", boot_count); | ||
lfs_file_rewind(&lfs, &file); | ||
lfs_file_write(&lfs, &file, &boot_count, sizeof(boot_count)); | ||
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// remember the storage is not updated until the file is closed successfully | ||
lfs_file_close(&lfs, &file); | ||
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// release and resources we were using | ||
lfs_unmount(&lfs); | ||
} | ||
``` | ||
## Usage | ||
Detailed documentation (or at least as much detail as is currently available) | ||
can be cound in the comments in [lfs.h](lfs.h). | ||
As you may have noticed, the littlefs takes in a configuration structure that | ||
defines how the filesystem operates. The configuration struct provides the | ||
filesystem with the block device operations and dimensions, tweakable | ||
parameters that tradeoff memory usage for performance, and optional | ||
static buffers if the user wants to avoid dynamic memory. | ||
The state of the littlefs is stored in the `lfs_t` type which is left up | ||
to the user to allocate, allowing multiple filesystems to be in use | ||
simultaneously. With the `lfs_t` and configuration struct, a user can either | ||
format a block device or mount the filesystem. | ||
Once mounted, the littlefs provides a full set of posix-like file and | ||
directory functions, with the deviation that the allocation of filesystem | ||
structures must be provided by the user. An important addition is that | ||
no file updates will actually be written to disk until a sync or close | ||
is called. | ||
## Other notes | ||
All littlefs have the potential to return a negative error code. The errors | ||
can be either one of those found in the `enum lfs_error` in [lfs.h](lfs.h), | ||
or an error returned by the user's block device operations. | ||
It should also be noted that the littlefs does not do anything to insure | ||
that the data written to disk is machine portable. It should be fine as | ||
long as the machines involved share endianness and don't have really | ||
strange padding requirements. If the question does come up, the littlefs | ||
metadata should be stored on disk in little-endian format. | ||
## Testing | ||
The littlefs comes with a test suite designed to run on a pc using the | ||
[emulated block device](emubd/lfs_emubd.h) found in the emubd directory. | ||
The tests assume a linux environment and can be started with make: | ||
``` bash | ||
make test | ||
``` |
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