ZLIB(3)

ZLIB(3) | Library Functions Manual | ZLIB(3)

# NAME

zlib — general purpose compression library

# SYNOPSIS

#include <zlib.h>

## Basic functions

const char *
zlibVersion(void);

int
deflateInit(z_streamp strm, int level);

int
deflate(z_streamp strm, int flush);

int
deflateEnd(z_streamp strm);

int
inflateInit(z_streamp strm);

int
inflate(z_streamp strm, int flush);

int
inflateEnd(z_streamp strm);

## Advanced functions

int
deflateInit2(z_streamp strm, int level, int method, int windowBits, int memLevel, int strategy);

int
deflateSetDictionary(z_streamp strm, const Bytef *dictionary, uInt dictLength);

int
deflateCopy(z_streamp dest, z_streamp source);

int
deflateReset(z_streamp strm);

int
deflateParams(z_streamp strm, int level, int strategy);

int
inflateInit2(z_streamp strm, int windowBits);

int
inflateSetDictionary(z_streamp strm, const Bytef *dictionary, uInt dictLength);

int
inflateSync(z_streamp strm);

int
inflateReset(z_streamp strm);

## Utility functions

typedef voidp gzFile ;

int
compress(Bytef *dest, uLongf *destLen, const Bytef *source, uLong sourceLen);

int
compress2(Bytef *dest, uLongf *destLen, const Bytef *source, uLong sourceLen, int level);

int
uncompress(Bytef *dest, uLongf *destLen, const Bytef *source, uLong sourceLen);

gzFile
gzopen(const char *path, const char *mode);

gzFile
gzdopen(int fd, const char *mode);

int
gzsetparams(gzFile file, int level, int strategy);

int
gzread(gzFile file, voidp buf, unsigned len);

int
gzwrite(gzFile file, const voidp buf, unsigned len);

int
gzprintf(gzFile file, const char *format, ...);

int
gzputs(gzFile file, const char *s);

char *
gzgets(gzFile file, char *buf, int len);

int
gzputc(gzFile file, int c);

int
gzgetc(gzFile file);

int
gzflush(gzFile file, int flush);

z_off_t
gzseek(gzFile file, z_off_t offset, int whence);

int
gzrewind(gzFile file);

z_off_t
gztell(gzFile file);

int
gzeof(gzFile file);

int
gzclose(gzFile file);

const char *
gzerror(gzFile file, int *errnum);

## Checksum functions

uLong
adler32(uLong adler, const Bytef *buf, uInt len);

uLong
crc32(uLong crc, const Bytef *buf, uInt len);

# DESCRIPTION

This manual page describes the zlib general purpose compression library, version 1.1.4.

The zlib compression library provides in-memory compression and decompression functions, including integrity checks of the uncompressed data. This version of the library supports only one compression method (deflation) but other algorithms will be added later and will have the same stream interface.

Compression can be done in a single step if the buffers are large enough (for example if an input file is mmap'ed), or can be done by repeated calls of the compression function. In the latter case, the application must provide more input and/or consume the output (providing more output space) before each call.

The library also supports reading and writing files in gzip(1) (.gz) format with an interface similar to that of stdio(3).

The library does not install any signal handler. The decoder checks the consistency of the compressed data, so the library should never crash even in case of corrupted input.

The functions within the library are divided into the following sections:

- Basic functions
- Advanced functions
- Utility functions
- Checksum functions

# BASIC FUNCTIONS

const char * zlibVersion(void);

  The application can compare zlibVersion() and ZLIB_VERSION for consistency. If the first character differs, the library code actually used is not compatible with the <zlib.h> header file used by the application. This check is automatically made by deflateInit() and inflateInit().

int deflateInit(z_streamp strm, int level);

  The deflateInit() function initializes the internal stream state for compression. The fields zalloc, zfree, and opaque must be initialized before by the caller. If zalloc and zfree are set to Z_NULL, deflateInit() updates them to use default allocation functions.

  The compression level must be Z_DEFAULT_COMPRESSION, or between 0 and 9: 1 gives best speed, 9 gives best compression, 0 gives no compression at all (the input data is simply copied a block at a time).

  Z_DEFAULT_COMPRESSION requests a default compromise between speed and compression (currently equivalent to level 6).

  deflateInit() returns Z_OK if successful, Z_MEM_ERROR if there was not enough memory, Z_STREAM_ERROR if level is not a valid compression level, Z_VERSION_ERROR if the zlib library version (zlib_version) is incompatible with the version assumed by the caller (ZLIB_VERSION). msg is set to null if there is no error message. deflateInit() does not perform any compression: this will be done by deflate().

int deflate(z_streamp strm, int flush);

  deflate() compresses as much data as possible, and stops when the input buffer becomes empty or the output buffer becomes full. It may introduce some output latency (reading input without producing any output) except when forced to flush.

  The detailed semantics are as follows. deflate() performs one or both of the following actions:

  Compress more input starting at next_in and update next_in and avail_in accordingly. If not all input can be processed (because there is not enough room in the output buffer), next_in and avail_in are updated and processing will resume at this point for the next call to deflate().

  Provide more output starting at next_out and update next_out and avail_out accordingly. This action is forced if the parameter flush is non-zero. Forcing flush frequently degrades the compression ratio, so this parameter should be set only when necessary (in interactive applications). Some output may be provided even if flush is not set.

  Before the call to deflate(), the application should ensure that at least one of the actions is possible, by providing more input and/or consuming more output, and updating avail_in or avail_out accordingly; avail_out should never be zero before the call. The application can consume the compressed output when it wants, for example when the output buffer is full (avail_out == 0), or after each call to deflate(). If deflate() returns Z_OK and with zero avail_out, it must be called again after making room in the output buffer because there might be more output pending.

  If the parameter flush is set to Z_SYNC_FLUSH, all pending output is flushed to the output buffer and the output is aligned on a byte boundary, so that the decompressor can get all input data available so far. (In particular, avail_in is zero after the call if enough output space has been provided before the call.) Flushing may degrade compression for some compression algorithms and so it should be used only when necessary.

  If flush is set to Z_FULL_FLUSH, all output is flushed as with Z_SYNC_FLUSH, and the compression state is reset so that decompression can restart from this point if previous compressed data has been damaged or if random access is desired. Using Z_FULL_FLUSH too often can seriously degrade the compression.

  If deflate() returns with avail_out == 0, this function must be called again with the same value of the flush parameter and more output space (updated avail_out), until the flush is complete (deflate() returns with non-zero avail_out).

  If the parameter flush is set to Z_FINISH, pending input is processed, pending output is flushed and deflate() returns with Z_STREAM_END if there was enough output space; if deflate() returns with Z_OK, this function must be called again with Z_FINISH and more output space (updated avail_out but no more input data, until it returns with Z_STREAM_END or an error. After deflate() has returned Z_STREAM_END, the only possible operations on the stream are deflateReset() or deflateEnd().

  Z_FINISH can be used immediately after deflateInit() if all the compression is to be done in a single step. In this case, avail_out must be at least 0.1% larger than avail_in plus 12 bytes. If deflate() does not return Z_STREAM_END, then it must be called again as described above.

  deflate() sets strm->adler to the Adler-32 checksum of all input read so far (that is, total_in bytes).

  deflate() may update data_type if it can make a good guess about the input data type (Z_ASCII or Z_BINARY). If in doubt, the data is considered binary. This field is only for information purposes and does not affect the compression algorithm in any manner.

  deflate() returns Z_OK if some progress has been made (more input processed or more output produced), Z_STREAM_END if all input has been consumed and all output has been produced (only when flush is set to Z_FINISH), Z_STREAM_ERROR if the stream state was inconsistent (for example, if next_in or next_out was NULL), Z_BUF_ERROR if no progress is possible (for example, avail_in or avail_out was zero).

int deflateEnd(z_streamp strm);

  All dynamically allocated data structures for this stream are freed. This function discards any unprocessed input and does not flush any pending output.

  deflateEnd() returns Z_OK if successful, Z_STREAM_ERROR if the stream state was inconsistent, Z_DATA_ERROR if the stream was freed prematurely (some input or output was discarded). In the error case, msg may be set but then points to a static string (which must not be deallocated).

int inflateInit(z_streamp strm);
  The inflateInit() function initializes the internal stream state for decompression. The fields next_in, avail_in, zalloc, zfree, and opaque must be initialized before by the caller. If next_in is not Z_NULL and avail_in is large enough (the exact value depends on the compression method), inflateInit() determines the compression method from the zlib header and allocates all data structures accordingly; otherwise the allocation will be deferred to the first call to inflate(). If zalloc and zfree are set to Z_NULL, inflateInit() updates them to use default allocation functions.

  inflateInit() returns Z_OK if successful, Z_MEM_ERROR if there was not enough memory, Z_VERSION_ERROR if the zlib library version is incompatible with the version assumed by the caller. msg is set to null if there is no error message. inflateInit() does not perform any decompression apart from reading the zlib header if present: this will be done by inflate(). (So next_in and avail_in may be modified, but next_out and avail_out are unchanged.)

int inflate(z_streamp strm, int flush);
  inflate() decompresses as much data as possible, and stops when the input buffer becomes empty or the output buffer becomes full. It may introduce some output latency (reading input without producing any output) except when forced to flush.

  The detailed semantics are as follows. inflate() performs one or both of the following actions:

  Decompress more input starting at next_in and update next_in and avail_in accordingly. If not all input can be processed (because there is not enough room in the output buffer), next_in is updated and processing will resume at this point for the next call to inflate().

  Provide more output starting at next_out and update next_out and avail_out accordingly. inflate() provides as much output as possible, until there is no more input data or no more space in the output buffer (see below about the flush parameter).

  Before the call to inflate(), the application should ensure that at least one of the actions is possible, by providing more input and/or consuming more output, and updating the next_* and avail_* values accordingly. The application can consume the uncompressed output when it wants, for example when the output buffer is full (avail_out == 0), or after each call to inflate(). If inflate() returns Z_OK and with zero avail_out, it must be called again after making room in the output buffer because there might be more output pending.

  If the parameter flush is set to Z_SYNC_FLUSH, inflate() flushes as much output as possible to the output buffer. The flushing behavior of inflate() is not specified for values of the flush parameter other than Z_SYNC_FLUSH and Z_FINISH, but the current implementation actually flushes as much output as possible anyway.

  inflate() should normally be called until it returns Z_STREAM_END or an error. However if all decompression is to be performed in a single step (a single call to inflate), the parameter flush should be set to Z_FINISH. In this case all pending input is processed and all pending output is flushed; avail_out must be large enough to hold all the uncompressed data. (The size of the uncompressed data may have been saved by the compressor for this purpose.) The next operation on this stream must be inflateEnd() to deallocate the decompression state. The use of Z_FINISH is never required, but can be used to inform inflate() that a faster routine may be used for the single inflate() call.

  If a preset dictionary is needed at this point (see inflateSetDictionary() below), inflate() sets strm->adler to the Adler-32 checksum of the dictionary chosen by the compressor and returns Z_NEED_DICT; otherwise it sets strm->adler to the Adler-32 checksum of all output produced so far (that is, total_out bytes) and returns Z_OK, Z_STREAM_END, or an error code as described below. At the end of the stream, inflate() checks that its computed Adler-32 checksum is equal to that saved by the compressor and returns Z_STREAM_END only if the checksum is correct.

  inflate() returns Z_OK if some progress has been made (more input processed or more output produced), Z_STREAM_END if the end of the compressed data has been reached and all uncompressed output has been produced, Z_NEED_DICT if a preset dictionary is needed at this point, Z_DATA_ERROR if the input data was corrupted (input stream not conforming to the zlib format or incorrect Adler-32 checksum), Z_STREAM_ERROR if the stream structure was inconsistent (for example, if next_in or next_out was NULL), Z_MEM_ERROR if there was not enough memory, Z_BUF_ERROR if no progress is possible or if there was not enough room in the output buffer when Z_FINISH is used. In the Z_DATA_ERROR case, the application may then call inflateSync() to look for a good compression block.

int inflateEnd(z_streamp strm);
  All dynamically allocated data structures for this stream are freed. This function discards any unprocessed input and does not flush any pending output.

  inflateEnd() returns Z_OK if successful, or Z_STREAM_ERROR if the stream state was inconsistent. In the error case, msg may be set but then points to a static string (which must not be deallocated).

# ADVANCED FUNCTIONS

The following functions are needed only in some special applications.

int deflateInit2(z_streamp strm, int level, int method, int windowBits, int memLevel, int strategy);

  This is another version of deflateInit() with more compression options. The fields next_in, zalloc, zfree, and opaque must be initialized before by the caller.

  The method parameter is the compression method. It must be Z_DEFLATED in this version of the library.

  The windowBits parameter is the base two logarithm of the window size (the size of the history buffer). It should be in the range 8..15 for this version of the library. Larger values of this parameter result in better compression at the expense of memory usage. The default value is 15 if deflateInit() is used instead.

  The memLevel parameter specifies how much memory should be allocated for the internal compression state. memLevel=1 uses minimum memory but is slow and reduces compression ratio; memLevel=9 uses maximum memory for optimal speed. The default value is 8. See <zconf.h> for total memory usage as a function of windowBits and memLevel.

  The strategy parameter is used to tune the compression algorithm. Use the value Z_DEFAULT_STRATEGY for normal data; Z_FILTERED for data produced by a filter (or predictor); or Z_HUFFMAN_ONLY to force Huffman encoding only (no string match). Filtered data consists mostly of small values with a somewhat random distribution. In this case, the compression algorithm is tuned to compress them better. The effect of Z_FILTERED is to force more Huffman coding and less string matching; it is somewhat intermediate between Z_DEFAULT and Z_HUFFMAN_ONLY. The strategy parameter only affects the compression ratio but not the correctness of the compressed output, even if it is not set appropriately.

  deflateInit2() returns Z_OK if successful, Z_MEM_ERROR if there was not enough memory, Z_STREAM_ERROR if a parameter is invalid (such as an invalid method). msg is set to null if there is no error message. deflateInit2() does not perform any compression: this will be done by deflate().

int deflateSetDictionary(z_streamp strm, const Bytef *dictionary, uInt dictLength);

  Initializes the compression dictionary from the given byte sequence without producing any compressed output. This function must be called immediately after deflateInit(), deflateInit2(), or deflateReset(), before any call to deflate(). The compressor and decompressor must use exactly the same dictionary (see inflateSetDictionary()).

  The dictionary should consist of strings (byte sequences) that are likely to be encountered later in the data to be compressed, with the most commonly used strings preferably put towards the end of the dictionary. Using a dictionary is most useful when the data to be compressed is short and can be predicted with good accuracy; the data can then be compressed better than with the default empty dictionary.

  Depending on the size of the compression data structures selected by deflateInit() or deflateInit2(), a part of the dictionary may in effect be discarded, for example if the dictionary is larger than the window size in deflate() or deflate2(). Thus the strings most likely to be useful should be put at the end of the dictionary, not at the front.

  Upon return of this function, strm->adler is set to the Adler-32 value of the dictionary; the decompressor may later use this value to determine which dictionary has been used by the compressor. (The Adler-32 value applies to the whole dictionary even if only a subset of the dictionary is actually used by the compressor.)

  deflateSetDictionary() returns Z_OK if successful, or Z_STREAM_ERROR if a parameter is invalid (such as NULL dictionary) or the stream state is inconsistent (for example if deflate() has already been called for this stream or if the compression method is bsort). deflateSetDictionary() does not perform any compression: this will be done by deflate().

int deflateCopy(z_streamp dest, z_streamp source);

  The deflateCopy() function sets the destination stream as a complete copy of the source stream.

  This function can be useful when several compression strategies will be tried, for example when there are several ways of pre-processing the input data with a filter. The streams that will be discarded should then be freed by calling deflateEnd(). Note that deflateCopy() duplicates the internal compression state which can be quite large, so this strategy is slow and can consume lots of memory.

  deflateCopy() returns Z_OK if successful, Z_MEM_ERROR if there was not enough memory, Z_STREAM_ERROR if the source stream state was inconsistent (such as zalloc being NULL). msg is left unchanged in both source and destination.

int deflateReset(z_streamp strm);

  This function is equivalent to deflateEnd() followed by deflateInit(), but does not free and reallocate all the internal compression state. The stream will keep the same compression level and any other attributes that may have been set by deflateInit2().

  deflateReset() returns Z_OK if successful, or Z_STREAM_ERROR if the source stream state was inconsistent (such as zalloc or state being NULL).

int deflateParams(z_streamp strm, int level, int strategy);

  The deflateParams() function dynamically updates the compression level and compression strategy. The interpretation of level and strategy is as in deflateInit2(). This can be used to switch between compression and straight copy of the input data, or to switch to a different kind of input data requiring a different strategy. If the compression level is changed, the input available so far is compressed with the old level (and may be flushed); the new level will take effect only at the next call to deflate().

  Before the call to deflateParams(), the stream state must be set as for a call to deflate(), since the currently available input may have to be compressed and flushed. In particular, strm->avail_out must be non-zero.

  deflateParams() returns Z_OK if successful, Z_STREAM_ERROR if the source stream state was inconsistent or if a parameter was invalid, or Z_BUF_ERROR if strm->avail_out was zero.

int inflateInit2(z_streamp strm, int windowBits);

  This is another version of inflateInit() with an extra parameter. The fields next_in, avail_in, zalloc, zfree, and opaque must be initialized before by the caller.

  The windowBits parameter is the base two logarithm of the maximum window size (the size of the history buffer). It should be in the range 8..15 for this version of the library. The default value is 15 if inflateInit() is used instead. If a compressed stream with a larger window size is given as input, inflate() will return with the error code Z_DATA_ERROR instead of trying to allocate a larger window.

  inflateInit2() returns Z_OK if successful, Z_MEM_ERROR if there was not enough memory, Z_STREAM_ERROR if a parameter is invalid (such as a negative memLevel). msg is set to null if there is no error message. inflateInit2() does not perform any decompression apart from reading the zlib header if present: this will be done by inflate(). (So next_in and avail_in may be modified, but next_out and avail_out are unchanged.)

int inflateSetDictionary(z_streamp strm, const Bytef *dictionary, uInt dictLength);

  Initializes the decompression dictionary from the given uncompressed byte sequence. This function must be called immediately after a call to inflate() if this call returned Z_NEED_DICT. The dictionary chosen by the compressor can be determined from the Adler-32 value returned by this call to inflate(). The compressor and decompressor must use exactly the same dictionary (see deflateSetDictionary()).

  inflateSetDictionary() returns Z_OK if successful, Z_STREAM_ERROR if a parameter is invalid (such as NULL dictionary) or the stream state is inconsistent, Z_DATA_ERROR if the given dictionary doesn't match the expected one (incorrect Adler-32 value). inflateSetDictionary() does not perform any decompression: this will be done by subsequent calls of inflate().

int inflateSync(z_streamp strm);

  Skips invalid compressed data until a full flush point (see above the description of deflate() with Z_FULL_FLUSH) can be found, or until all available input is skipped. No output is provided.

  inflateSync() returns Z_OK if a full flush point has been found, Z_BUF_ERROR if no more input was provided, Z_DATA_ERROR if no flush point has been found, or Z_STREAM_ERROR if the stream structure was inconsistent. In the success case, the application may save the current value of total_in which indicates where valid compressed data was found. In the error case, the application may repeatedly call inflateSync(), providing more input each time, until success or end of the input data.

int inflateReset(z_streamp strm);

  This function is equivalent to inflateEnd() followed by inflateInit(), but does not free and reallocate all the internal decompression state. The stream will keep attributes that may have been set by inflateInit2().

  inflateReset() returns Z_OK if successful, or Z_STREAM_ERROR if the source stream state was inconsistent (such as zalloc or state being NULL).

# UTILITY FUNCTIONS

The following utility functions are implemented on top of the basic stream-oriented functions. To simplify the interface, some default options are assumed (compression level and memory usage, standard memory allocation functions). The source code of these utility functions can easily be modified if you need special options.

int compress(Bytef *dest, uLongf *destLen, const Bytef *source, uLong sourceLen);

  The compress() function compresses the source buffer into the destination buffer. sourceLen is the byte length of the source buffer. Upon entry, destLen is the total size of the destination buffer, which must be at least 0.1% larger than sourceLen plus 12 bytes. Upon exit, destLen is the actual size of the compressed buffer. This function can be used to compress a whole file at once if the input file is mmap'ed.

  compress() returns Z_OK if successful, Z_MEM_ERROR if there was not enough memory, or Z_BUF_ERROR if there was not enough room in the output buffer.

int compress2(Bytef *dest, uLongf *destLen, const Bytef *source, uLong sourceLen, int level);

  The compress2() function compresses the source buffer into the destination buffer. The level parameter has the same meaning as in deflateInit(). sourceLen is the byte length of the source buffer. Upon entry, destLen is the total size of the destination buffer, which must be at least 0.1% larger than sourceLen plus 12 bytes. Upon exit, destLen is the actual size of the compressed buffer.

  compress2() returns Z_OK if successful, Z_MEM_ERROR if there was not enough memory, Z_BUF_ERROR if there was not enough room in the output buffer, or Z_STREAM_ERROR if the level parameter is invalid.

int uncompress(Bytef *dest, uLongf *destLen, const Bytef *source, uLong sourceLen);

  The uncompress() function decompresses the source buffer into the destination buffer. sourceLen is the byte length of the source buffer. Upon entry, destLen is the total size of the destination buffer, which must be large enough to hold the entire uncompressed data. (The size of the uncompressed data must have been saved previously by the compressor and transmitted to the decompressor by some mechanism outside the scope of this compression library.) Upon exit, destLen is the actual size of the compressed buffer. This function can be used to decompress a whole file at once if the input file is mmap'ed.

  uncompress() returns Z_OK if successful, Z_MEM_ERROR if there was not enough memory, Z_BUF_ERROR if there was not enough room in the output buffer, or Z_DATA_ERROR if the input data was corrupted.

gzFile gzopen(const char *path, const char *mode);

  The gzopen() function opens a gzip (.gz) file for reading or writing. The mode parameter is as in fopen(3) (“rb” or “wb”) but can also include a compression level (“wb9”) or a strategy: ‘f’ for filtered data, as in “wb6f”; ‘h’ for Huffman only compression, as in “wb1h”. (See the description of deflateInit2() for more information about the strategy parameter.)

  gzopen() can be used to read a file which is not in gzip format; in this case gzread() will directly read from the file without decompression.

  gzopen() returns NULL if the file could not be opened or if there was insufficient memory to allocate the (de)compression state; errno can be checked to distinguish the two cases (if errno is zero, the zlib error is Z_MEM_ERROR).

gzFile gzdopen(int fd, const char *mode);

  The gzdopen() function associates a gzFile with the file descriptor fd. File descriptors are obtained from calls like open(2), dup(2), creat(3), pipe(2), or fileno(3) (if the file has been previously opened with fopen(3)). The mode parameter is as in gzopen().

  The next call to gzclose() on the returned gzFile will also close the file descriptor fd, just like fclose(fdopen(fd), mode) closes the file descriptor fd. If you want to keep fd open, use gzdopen(dup(fd), mode).

  gzdopen() returns NULL if there was insufficient memory to allocate the (de)compression state.

int gzsetparams(gzFile file, int level, int strategy);

  The gzsetparams() function dynamically updates the compression level or strategy. See the description of deflateInit2() for the meaning of these parameters.

  gzsetparams() returns Z_OK if successful, or Z_STREAM_ERROR if the file was not opened for writing.

int gzread(gzFile file, voidp buf, unsigned len);

  The gzread() function reads the given number of uncompressed bytes from the compressed file. If the input file was not in gzip format, gzread() copies the given number of bytes into the buffer.

  gzread() returns the number of uncompressed bytes actually read (0 for end of file, -1 for error).

int gzwrite(gzFile file, const voidp buf, unsigned len);

  The gzwrite() function writes the given number of uncompressed bytes into the compressed file. gzwrite() returns the number of uncompressed bytes actually written (0 in case of error).

int gzprintf(gzFile file, const char *format, ...);

  The gzprintf() function converts, formats, and writes the args to the compressed file under control of the format string, as in fprintf(3). gzprintf() returns the number of uncompressed bytes actually written (0 in case of error).

int gzputs(gzFile file, const char *s);

  The gzputs() function writes the given null-terminated string to the compressed file, excluding the terminating null character.

  gzputs() returns the number of characters written, or -1 in case of error.

char * gzgets(gzFile file, char *buf, int len);

  The gzgets() function reads bytes from the compressed file until len-1 characters are read, or a newline character is read and transferred to buf, or an end-of-file condition is encountered. The string is then terminated with a null character.

  gzgets() returns buf, or Z_NULL in case of error.

int gzputc(gzFile file, int c);

  The gzputc() function writes c, converted to an unsigned char, into the compressed file. gzputc() returns the value that was written, or -1 in case of error.

int gzgetc(gzFile file);

  The gzgetc() function reads one byte from the compressed file. gzgetc() returns this byte or -1 in case of end of file or error.

int gzflush(gzFile file, int flush);

  The gzflush() function flushes all pending output into the compressed file. The parameter flush is as in the deflate() function. The return value is the zlib error number (see function gzerror() below). gzflush() returns Z_OK if the flush parameter is Z_FINISH and all output could be flushed.

  gzflush() should be called only when strictly necessary because it can degrade compression.

z_off_t gzseek(gzFile file, z_off_t offset, int whence);

  Sets the starting position for the next gzread() or gzwrite() on the given compressed file. The offset represents a number of bytes in the uncompressed data stream. The whence parameter is defined as in lseek(2); the value SEEK_END is not supported.

  If the file is opened for reading, this function is emulated but can be extremely slow. If the file is opened for writing, only forward seeks are supported; gzseek() then compresses a sequence of zeroes up to the new starting position.

  gzseek() returns the resulting offset location as measured in bytes from the beginning of the uncompressed stream, or -1 in case of error, in particular if the file is opened for writing and the new starting position would be before the current position.

int gzrewind(gzFile file);

  The gzrewind() function rewinds the given file. This function is supported only for reading.

  gzrewind(file) is equivalent to (int)gzseek(file, 0L, SEEK_SET).

z_off_t gztell(gzFile file);

  The gztell() function returns the starting position for the next gzread() or gzwrite() on the given compressed file. This position represents a number of bytes in the uncompressed data stream.

  gztell(file) is equivalent to gzseek(file, 0L, SEEK_CUR).

int gzeof(gzFile file);

  The gzeof() function returns 1 when EOF has previously been detected reading the given input stream, otherwise zero.

int gzclose(gzFile file);

  The gzclose() function flushes all pending output if necessary, closes the compressed file and deallocates all the (de)compression state. The return value is the zlib error number (see function gzerror() below).

const char * gzerror(gzFile file, int *errnum);

  The gzerror() function returns the error message for the last error which occurred on the given compressed file. errnum is set to the zlib error number. If an error occurred in the file system and not in the compression library, errnum is set to Z_ERRNO and the application may consult errno to get the exact error code.

# CHECKSUM FUNCTIONS

These functions are not related to compression but are exported anyway because they might be useful in applications using the compression library.

uLong adler32(uLong adler, const Bytef *buf, uInt len);
  The adler32() function updates a running Adler-32 checksum with the bytes buf[0..len-1] and returns the updated checksum. If buf is NULL, this function returns the required initial value for the checksum.

  An Adler-32 checksum is almost as reliable as a CRC32 but can be computed much faster. Usage example:

      uLong adler = adler32(0L, Z_NULL, 0);

      while (read_buffer(buffer, length) != EOF) {
      adler = adler32(adler, buffer, length);
      }
      if (adler != original_adler) error();

uLong crc32(uLong crc, const Bytef *buf, uInt len);
  The crc32() function updates a running CRC with the bytes buf[0..len-1] and returns the updated CRC. If buf is NULL, this function returns the required initial value for the CRC. Pre- and post-conditioning (one's complement) is performed within this function so it shouldn't be done by the application. Usage example:

      uLong crc = crc32(0L, Z_NULL, 0);

      while (read_buffer(buffer, length) != EOF) {
      crc = crc32(crc, buffer, length);
      }
      if (crc != original_crc) error();

# STRUCTURES

    struct internal_state;

    typedef struct z_stream_s {
        Bytef    *next_in;  /* next input byte */
        uInt     avail_in;  /* number of bytes available at next_in */
        uLong    total_in;  /* total nb of input bytes read so far */

        Bytef    *next_out; /* next output byte should be put there */
        uInt     avail_out; /* remaining free space at next_out */
        uLong    total_out; /* total nb of bytes output so far */

        char     *msg;      /* last error message, NULL if no error */
        struct internal_state FAR *state; /* not visible by applications */

        alloc_func zalloc;  /* used to allocate the internal state */
        free_func  zfree;   /* used to free the internal state */
        voidpf     opaque;  /* private data object passed to zalloc and zfree*/

        int     data_type;  /*best guess about the data type: ascii or binary*/
        uLong   adler;      /* Adler-32 value of the uncompressed data */
        uLong   reserved;   /* reserved for future use */
    } z_stream;

    typedef z_stream FAR * z_streamp;

The application must update next_in and avail_in when avail_in has dropped to zero. It must update next_out and avail_out when avail_out has dropped to zero. The application must initialize zalloc, zfree, and opaque before calling the init function. All other fields are set by the compression library and must not be updated by the application.

The opaque value provided by the application will be passed as the first parameter for calls to zalloc() and zfree(). This can be useful for custom memory management. The compression library attaches no meaning to the opaque value.

zalloc must return Z_NULL if there is not enough memory for the object. If zlib is used in a multi-threaded application, zalloc and zfree must be thread safe.

On 16-bit systems, the functions zalloc and zfree must be able to allocate exactly 65536 bytes, but will not be required to allocate more than this if the symbol MAXSEG_64K is defined (see <zconf.h>).

WARNING: On MSDOS, pointers returned by zalloc for objects of exactly 65536 bytes *must* have their offset normalized to zero. The default allocation function provided by this library ensures this (see zutil.c). To reduce memory requirements and avoid any allocation of 64K objects, at the expense of compression ratio, compile the library with -DMAX_WBITS=14 (see <zconf.h>).

The fields total_in and total_out can be used for statistics or progress reports. After compression, total_in holds the total size of the uncompressed data and may be saved for use in the decompressor (particularly if the decompressor wants to decompress everything in a single step).

# CONSTANTS

    #define Z_NO_FLUSH      0
    #define Z_PARTIAL_FLUSH 1 /* will be removed, use Z_SYNC_FLUSH instead */
    #define Z_SYNC_FLUSH    2
    #define Z_FULL_FLUSH    3
    #define Z_FINISH        4
    /* Allowed flush values; see deflate() below for details */

    #define Z_OK            0
    #define Z_STREAM_END    1
    #define Z_NEED_DICT     2
    #define Z_ERRNO        (-1)
    #define Z_STREAM_ERROR (-2)
    #define Z_DATA_ERROR   (-3)
    #define Z_MEM_ERROR    (-4)
    #define Z_BUF_ERROR    (-5)
    #define Z_VERSION_ERROR (-6)
    /* Return codes for the compression/decompression functions.
     * Negative values are errors,
     * positive values are used for special but normal events.
     */

    #define Z_NO_COMPRESSION         0
    #define Z_BEST_SPEED             1
    #define Z_BEST_COMPRESSION       9
    #define Z_DEFAULT_COMPRESSION  (-1)
    /* compression levels */

    #define Z_FILTERED            1
    #define Z_HUFFMAN_ONLY        2
    #define Z_DEFAULT_STRATEGY    0
    /* compression strategy; see deflateInit2() below for details */

    #define Z_BINARY   0
    #define Z_ASCII    1
    #define Z_UNKNOWN  2
    /* Possible values of the data_type field */

    #define Z_DEFLATED   8
    /* The deflate compression method
     * (the only one supported in this version)
    */

    #define Z_NULL  0  /* for initializing zalloc, zfree, opaque */

    #define zlib_version zlibVersion()
    /* for compatibility with versions < 1.0.2 */

# VARIOUS HACKS

deflateInit and inflateInit are macros to allow checking the zlib version and the compiler's view of z_stream.

int deflateInit_(z_stream strm, int level, const char *version, int stream_size);
int inflateInit_(z_stream strm, const char *version, int stream_size);
int deflateInit2_(z_stream strm, int level, int method, int windowBits, int memLevel, int strategy, const char *version, int stream_size);
int inflateInit2_(z_stream strm, int windowBits, const char *version, int stream_size);
const char * zError(int err);
int inflateSyncPoint(z_streamp z);
const uLongf * get_crc_table(void);

# SEE ALSO

RFC 1950
  ZLIB Compressed Data Format Specification.
RFC 1951
  DEFLATE Compressed Data Format Specification.
RFC 1952
  GZIP File Format Specification.

zlib

# HISTORY

This manual page is based on an HTML version of <zlib.h> converted by piaip <piaip@csie.ntu.edu.tw> and was converted to mdoc format by the OpenBSD project.

# AUTHORS

Jean-loup Gailly <jloup@gzip.org> Mark Adler <madler@alumni.caltech.edu>

May 1, 2004 | NetBSD 7.1