include/linux/xz.h

Source file repositories/reference/linux-study-clean/include/linux/xz.h

File Facts

System
Linux kernel
Corpus path
include/linux/xz.h
Extension
.h
Size
16032 bytes
Lines
360
Domain
Core OS
Bucket
Core Kernel Interface
Inferred role
Core OS: implementation source
Status
source implementation candidate

Why This File Exists

Core operating-system implementation surface: boot, tasks, memory, VFS, syscall-facing interfaces, synchronization, credentials, and isolation.

Dependency Surface

Detected Declarations

Annotated Snippet

struct xz_buf {
	const uint8_t *in;
	size_t in_pos;
	size_t in_size;

	uint8_t *out;
	size_t out_pos;
	size_t out_size;
};

/*
 * struct xz_dec - Opaque type to hold the XZ decoder state
 */
struct xz_dec;

/**
 * xz_dec_init() - Allocate and initialize a XZ decoder state
 * @mode:       Operation mode
 * @dict_max:   Maximum size of the LZMA2 dictionary (history buffer) for
 *              multi-call decoding. This is ignored in single-call mode
 *              (mode == XZ_SINGLE). LZMA2 dictionary is always 2^n bytes
 *              or 2^n + 2^(n-1) bytes (the latter sizes are less common
 *              in practice), so other values for dict_max don't make sense.
 *              In the kernel, dictionary sizes of 64 KiB, 128 KiB, 256 KiB,
 *              512 KiB, and 1 MiB are probably the only reasonable values,
 *              except for kernel and initramfs images where a bigger
 *              dictionary can be fine and useful.
 *
 * Single-call mode (XZ_SINGLE): xz_dec_run() decodes the whole stream at
 * once. The caller must provide enough output space or the decoding will
 * fail. The output space is used as the dictionary buffer, which is why
 * there is no need to allocate the dictionary as part of the decoder's
 * internal state.
 *
 * Because the output buffer is used as the workspace, streams encoded using
 * a big dictionary are not a problem in single-call mode. It is enough that
 * the output buffer is big enough to hold the actual uncompressed data; it
 * can be smaller than the dictionary size stored in the stream headers.
 *
 * Multi-call mode with preallocated dictionary (XZ_PREALLOC): dict_max bytes
 * of memory is preallocated for the LZMA2 dictionary. This way there is no
 * risk that xz_dec_run() could run out of memory, since xz_dec_run() will
 * never allocate any memory. Instead, if the preallocated dictionary is too
 * small for decoding the given input stream, xz_dec_run() will return
 * XZ_MEMLIMIT_ERROR. Thus, it is important to know what kind of data will be
 * decoded to avoid allocating excessive amount of memory for the dictionary.
 *
 * Multi-call mode with dynamically allocated dictionary (XZ_DYNALLOC):
 * dict_max specifies the maximum allowed dictionary size that xz_dec_run()
 * may allocate once it has parsed the dictionary size from the stream
 * headers. This way excessive allocations can be avoided while still
 * limiting the maximum memory usage to a sane value to prevent running the
 * system out of memory when decompressing streams from untrusted sources.
 *
 * On success, xz_dec_init() returns a pointer to struct xz_dec, which is
 * ready to be used with xz_dec_run(). If memory allocation fails,
 * xz_dec_init() returns NULL.
 */
struct xz_dec *xz_dec_init(enum xz_mode mode, uint32_t dict_max);

/**
 * xz_dec_run() - Run the XZ decoder
 * @s:          Decoder state allocated using xz_dec_init()
 * @b:          Input and output buffers
 *
 * The possible return values depend on build options and operation mode.
 * See enum xz_ret for details.
 *
 * Note that if an error occurs in single-call mode (return value is not
 * XZ_STREAM_END), b->in_pos and b->out_pos are not modified and the
 * contents of the output buffer from b->out[b->out_pos] onward are
 * undefined. This is true even after XZ_BUF_ERROR, because with some filter
 * chains, there may be a second pass over the output buffer, and this pass
 * cannot be properly done if the output buffer is truncated. Thus, you
 * cannot give the single-call decoder a too small buffer and then expect to
 * get that amount valid data from the beginning of the stream. You must use
 * the multi-call decoder if you don't want to uncompress the whole stream.
 */
enum xz_ret xz_dec_run(struct xz_dec *s, struct xz_buf *b);

/**
 * xz_dec_reset() - Reset an already allocated decoder state
 * @s:          Decoder state allocated using xz_dec_init()
 *
 * This function can be used to reset the multi-call decoder state without
 * freeing and reallocating memory with xz_dec_end() and xz_dec_init().
 *
 * In single-call mode, xz_dec_reset() is always called in the beginning of
 * xz_dec_run(). Thus, explicit call to xz_dec_reset() is useful only in
 * multi-call mode.

Annotation

Implementation Notes