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Introduced the LFS_O_SNAPSHOT flag

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LFS_O_SNAPSHOT brings back some of littlefs's idiosyncratic behavior
removed in the changes to open file syncing in a form that may be more
useful for users.

LFS_O_SNAPSHOT allows you to open a "snapshot" of a file. This is a cheap,
local copy of a file who's changes are not reflected on disk.

Internally, snapshot files use the same mechanism as pending writes. A
separate, copy-on-write CTZ skip-list is created, with read-only
references to the existing data blocks until a write occurs. The
difference is that snapshot files are not enrolled in the mlist, meaning
they won't get updates from open file syncs, and during close their
contents are simply discarded.

As an extra benefit, LFS_O_CREAT | LFS_O_SNAPSHOT is equivalent to
Linux's O_TMPFILE, making it easy to create temporary, unnamed files.

This may be useful for embedded development, where unnamed flash-backed
buffers may provide a slower, but larger, alternative to RAM-backed
buffers.
This commit is contained in:
Christopher Haster
2020-12-24 18:44:14 -06:00
parent deeaa17317
commit c7820e653e
4 changed files with 549 additions and 97 deletions
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28
lfs.h
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@@ -123,26 +123,30 @@ enum lfs_type {
// File open flags
enum lfs_open_flags {
// open flags
LFS_O_RDONLY = 1, // Open a file as read only
LFS_O_RDONLY = 1, // Open a file as read only
#ifndef LFS_READONLY
LFS_O_WRONLY = 2, // Open a file as write only
LFS_O_RDWR = 3, // Open a file as read and write
LFS_O_CREAT = 0x0100, // Create a file if it does not exist
LFS_O_EXCL = 0x0200, // Fail if a file already exists
LFS_O_TRUNC = 0x0400, // Truncate the existing file to zero size
LFS_O_APPEND = 0x0800, // Move to end of file on every write
LFS_O_WRONLY = 2, // Open a file as write only
LFS_O_RDWR = 3, // Open a file as read and write
#endif
#ifndef LFS_READONLY
LFS_O_CREAT = 0x0100, // Create a file if it does not exist
LFS_O_EXCL = 0x0200, // Fail if a file already exists
LFS_O_TRUNC = 0x0400, // Truncate the existing file to zero size
LFS_O_APPEND = 0x0800, // Move to end of file on every write
LFS_O_SNAPSHOT = 0x1000, // Open a temporary snapshot, ignore changes
#endif
// internally used flags
#ifndef LFS_READONLY
LFS_F_DIRTY = 0x010000, // File does not match storage
LFS_F_WRITING = 0x020000, // File has been written since last flush
LFS_F_DIRTY = 0x010000, // File does not match storage
#endif
LFS_F_READING = 0x040000, // File has been read since last flush
LFS_F_READING = 0x020000, // File has been read since last flush
#ifndef LFS_READONLY
LFS_F_ERRED = 0x080000, // An error occurred during write
LFS_F_WRITING = 0x040000, // File has been written since last flush
LFS_F_ZOMBIE = 0x080000, // An error occurred during write
#endif
LFS_F_INLINE = 0x100000, // Currently inlined in directory entry
LFS_F_INLINE = 0x100000, // Currently inlined in directory entry
};
// File seek flags