zlib 1.1.4 Manual

1. Prologue
2. Introduction
3. Utility functions
4. Basic functions
5. Advanced functions
6. Constants
7. struct z_stream_s
8. Checksum functions
9. Misc

Jean-loup Gailly

jloup@gzip.org

Mark Adler

madler@alumni.caltech.edu

This manual is converted from zlib.h by piaip

Visit http://ftp.cdrom.com/pub/infozip/zlib/ for the official zlib web page.

Function list

• 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);
• typedef voidp gzFile;
• 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 VA 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);
• z_off_t gztell (gzFile file);
• int gzrewind (gzFile file);
• int gzeof (gzFile file);
• int gzclose (gzFile file);
• const char * gzerror (gzFile file, int *errnum);

Function description

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

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 success, Z_MEM_ERROR if there was not enough memory, 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);

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 success, Z_MEM_ERROR if there was not enough memory, Z_BUF_ERROR if there was not enough room in the output buffer, Z_STREAM_ERROR if the level parameter is invalid.

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

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 success, 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.

typedef voidp gzFile;

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

Opens a gzip (.gz) file for reading or writing. The mode parameter is as in fopen ("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);

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

The next call of 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);

Dynamically update the compression level or strategy. See the description of deflateInit2 for the meaning of these parameters.

gzsetparams returns Z_OK if success, or Z_STREAM_ERROR if the file was not opened for writing.

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

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);

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

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

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

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);

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);

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);

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);

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);

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);

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);

Returns 1 when EOF has previously been detected reading the given input stream, otherwise zero.

int gzclose (gzFile file);

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);

Returns the error message for the last error which occurred on the given compressed file. errnum is set to 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.

• 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);

Function description

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);

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 success, 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 of 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 of 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 of 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 adler32 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). 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 success, 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);

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 of inflate. If zalloc and zfree are set to Z_NULL, inflateInit updates them to use default allocation functions.

inflateInit returns Z_OK if success, 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 some 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),