FAT − the FAT12, FAT16 and FAT32 filesystems
This page describes these filesystem to the aim of clarify what libllfat does. An overview of the library is in libllfat.txt, a programming guide in fat_lib(3) and a description of all functions in fat_functions(3).
A FAT filesystem is composed of two parts:
+----------+ | sectors | +----------+ | clusters | +----------+
Each cluster is made of a number of consecutive sectors, from 1 to 128. Clusters have all the same size except the first one in a FAT12 and FAT16.
The sectors are further segmented in three parts: the reserved sectors, the first and the second file allocation table (FAT).
+-----------+ | reserved | | sectors | +-----------+ | first | | FAT | +-----------+ | second | | FAT | +===========+ | first | | cluster | +-----------+ | other | | clusters | +-----------+
The FATs could be one or more than two, but are almost always two. The first of the reserved sectors is the boot sector, which contains global data about the filesystem, such as the size of each sector in bytes, of each cluster in sectors, and the first cluster of the root directory. On a FAT32, the second sector may also contain some information.
The content of the directories and the files are all stored in clusters. More precisely, every file and directory has an associated chain of clusters that hold its content. The file or directory only points to the first cluster of the chain, the successor of it and of every following cluster in the chain is stored in the file allocation tables (FATs).
Apart from the first cluster of the root directory in a FAT12 or FAT16, clusters are numbered from 2 up.
Starting from the root, the filesystem content is:
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the first cluster of the root directory is the starting point; its number can be found from the boot sector; the FATs allows determining its successor (if any), the successor of its successor, etc; these clusters hold the content of the root directory | ||
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each cluster of this chain contains 32-bytes blocks called directory entries; each represents a file or subdirectory | ||
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each of these entries contain the shortname of the file/directory, its attributes and and its first cluster | ||
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the first cluster may have a successor, which may have its successor, etc. (the successor of each cluster is again stored in the FATs); they form a chain that hold the file content | ||
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if one of the attributes of the file is 0x10, the file is a subdirectory; the chain contains other directory entries, each representing a file in this subdirectory |
Summarizing, each file or directory has an associated chain of clusters that holds its content. For directories, this content is a sequence of 32-byte directory entries, each representing a file or directory and containing its name and the first cluster of its chain.
A directory entry contains the short name of a file, in the form 8 uppercase characters - dot - 3 uppercase charcters, like EXAMPLES.TXT. The LFN hack allows such entries to be associated to a long name, an UCS2 string.
This is realized by preceding the regular directory entry with a number of fake ones containing the long name. A checksum and a progressive number contained in each allows verifying that the long name is valid and really belonging to the following regular directory entry.
This library has functions for accessing the basic building blocks of a FAT filesystem:
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getting and setting the successor of a cluster | ||
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obtain and change the information in a directory entry | ||
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accessing the global information in the boot sector |
It also provides a number of higher-level functions. An overview is in libllfat.txt, a programming guide in fat_lib(3) and a description of all functions in fat_functions(3).