REFERENCE / ZLIB × LEAN

zlib API manual

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Start here to find a zlib function, understand its caller obligations, or follow a stream's lifetime. Each entry gives the C signature, purpose, Pre (requirements), Post (result), Effects (permitted changes), and edge cases that affect use.

Choose a starting point

If you want to…	Read
Understand the contract vocabulary and build scope	How to read a contract
Manage buffers, ownership, resets and stream call order	Shared stream rules
Compress incrementally through `z_stream`	Stream compression
Decompress incrementally through `z_stream`	Stream decompression
Supply decompression input and receive output through callbacks	Callback decompression
Compress or decompress a whole buffer, or query the library	Buffer utilities and library information
Compute or combine Adler-32 and CRC-32 checksums	Checksums
Read, write, position or close a gzip file handle	Gzip files

The manual follows the pinned source plus the listed patches, first for the default LP64 Linux static build and default allocator. Entries state their additional limits. The Windows gzopen_w entry is separate from that build. These are focused documentary contracts; some preconditions are narrower than the public header. Outside a stated Pre is outside that contract, not automatically undefined C. See scope deviations.

The behavior read from C is unfinished by implementation proof. This manual does not report completed verification or change the broader registered public goals. Progress is the single record of proof status and remaining work.

Find a function

The following index links all 96 entries, grouped by how they are used. The function pages retain C names, including trailing underscores supplied by initialization macros.

Stream compression

Function	One line
deflateInit_	initialise with default window, memory level and strategy
deflateInit2_	initialise with all parameters
deflateSetDictionary	preload a dictionary
deflateGetDictionary	copy out the current dictionary window
deflateResetKeep	restart a stream but keep the window and hash (undocumented)
deflateReset	restart a stream
deflateSetHeader	give the gzip header to write
deflatePending	report pending output
deflateUsed	report the bits used in the last byte of the last flush
deflatePrime	insert bits into the output
deflateParams	change level and strategy
deflateTune	set the internal match-search tuning values
deflateBound_z	upper bound on the compressed size (size type)
deflateBound	upper bound on the compressed size (`uLong`)
deflate	compress
deflateEnd	free the stream
deflateCopy	duplicate a stream

Stream decompression

Function	One line
inflateResetKeep	restart a stream and keep the window contents (undocumented)
inflateReset	restart a stream, drop the window contents
inflateReset2	restart a stream with a new window size
inflateInit2_	initialise with a window size
inflateInit_	initialise with the default window size
inflatePrime	insert bits into the input
inflate	decompress
inflateEnd	free the stream
inflateGetDictionary	copy out the window
inflateSetDictionary	give the dictionary
inflateGetHeader	ask for the gzip header
inflateSync	search for a full flush point
inflateSyncPoint	test whether stopped at a sync flush point
inflateCopy	duplicate a stream
inflateUndermine	request distance-checking option; default build returns `Z_DATA_ERROR`
inflateValidate	control wrapper integrity checks
inflateMark	report where `inflate` is inside a block or code
inflateCodesUsed	report the code table entries used by the current block (undocumented)

Callback decompression

Function	One line
inflateBackInit_	initialise for callback decompression
inflateBack	decompress with input and output callbacks
inflateBackEnd	free the stream

Whole-buffer operations and library information

Function	One line
compress2_z	compress a buffer at a level (size type)
compress2	compress a buffer at a level
compress_z	compress a buffer at the default level (size type)
compress	compress a buffer at the default level
compressBound_z	bound on the compressed size (size type)
compressBound	bound on the compressed size
uncompress2_z	decompress a buffer, report input used (size type)
uncompress2	decompress a buffer, report input used
uncompress_z	decompress a buffer (size type)
uncompress	decompress a buffer
zlibVersion	the release string of the library
zlibCompileFlags	the type sizes and options the library was built with
zError	text for a return code

Checksums

Function	One line
adler32_z	Adler-32 of a buffer (size type)
adler32	Adler-32 of a buffer
adler32_combine	combine two Adler-32 values
adler32_combine64	combine two Adler-32 values (64-bit length)
get_crc_table	the CRC-32 table
crc32_z	CRC-32 of a buffer (size type)
crc32	CRC-32 of a buffer
crc32_combine_gen64	operator for combining (64-bit length)
crc32_combine_gen	operator for combining
crc32_combine_op	combine with an operator
crc32_combine64	combine two CRC-32 values (64-bit length)
crc32_combine	combine two CRC-32 values

Gzip files

Function	One line
gzopen, gzopen64, gzdopen, gzopen_w	open a file
gzbuffer, gzsetparams	buffer size, compression parameters
gzread, gzfread, gzgetc, gzgetc_, gzungetc, gzgets	read
gzwrite, gzfwrite, gzputc, gzputs, gzprintf, gzvprintf, gzflush	write
gzrewind, gzseek, gzseek64, gztell, gztell64, gzoffset, gzoffset64	position
gzeof, gzdirect, gzerror, gzclearerr	state
gzclose, gzclose_r, gzclose_w	close

Build variants and macros

Compared with the functions that zlib.h declares, these are not compiled into the default Linux library:

Name	Why
`gzopen_w`	declared only for Windows (`_WIN32`); the definition in `gzlib.c` is under the wide-character option
`deflateInit`, `deflateInit2`, `inflateInit`, `inflateInit2`, `inflateBackInit`	these are macros in `zlib.h`; they call `deflateInit_`, `deflateInit2_`, `inflateInit_`, `inflateInit2_`, `inflateBackInit_` with the header's own release string and `sizeof(z_stream)`
`gzgetc` as a macro	`zlib.h` also defines `gzgetc` as a macro that reads the buffer directly and calls `gzgetc_` when empty; the function `gzgetc` exists as well

The index contains all 95 default-build entries and the separate Windows entry gzopen_w. Internal helper contracts describe routines used inside the library.