xz_embedded_sys::xz_dec_init

pub unsafe extern fn xz_dec_init(mode: xz_mode, dict_max: u32) -> *mut xz_dec

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 2ⁿ bytes or 2ⁿ + 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.