zlib.h File Reference

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:

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.

Normally the parameter flush is set to Z_NO_FLUSH, which allows deflate to decide how much data to accumualte before producing output, in order to maximize compression.

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 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). In the case of a Z_FULL_FLUSH or Z_SYNC_FLUSH, make sure that avail_out is greater than six to avoid repeated flush markers due to avail_out == 0 on return.

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 the value returned by deflateBound (see below). 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 strm->data_type if it can make a good guess about the input data type (Z_BINARY or Z_TEXT). 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). Note that Z_BUF_ERROR is not fatal, and deflate() can be called again with more input and more output space to continue compressing.

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).

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:

Before the call of 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 of 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.

The flush parameter of inflate() can be Z_NO_FLUSH, Z_SYNC_FLUSH, Z_FINISH, or Z_BLOCK. Z_SYNC_FLUSH requests that inflate() flush as much output as possible to the output buffer. Z_BLOCK requests that inflate() stop if and when it gets to the next deflate block boundary. When decoding the zlib or gzip format, this will cause inflate() to return immediately after the header and before the first block. When doing a raw inflate, inflate() will go ahead and process the first block, and will return when it gets to the end of that block, or when it runs out of data.

The Z_BLOCK option assists in appending to or combining deflate streams. Also to assist in this, on return inflate() will set strm->data_type to the number of unused bits in the last byte taken from strm->next_in, plus 64 if inflate() is currently decoding the last block in the deflate stream, plus 128 if inflate() returned immediately after decoding an end-of-block code or decoding the complete header up to just before the first byte of the deflate stream. The end-of-block will not be indicated until all of the uncompressed data from that block has been written to strm->next_out. The number of unused bits may in general be greater than seven, except when bit 7 of data_type is set, in which case the number of unused bits will be less than eight.

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 of 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 approach may be used for the single inflate() call.

In this implementation, inflate() always flushes as much output as possible to the output buffer, and always uses the faster approach on the first call. So the only effect of the flush parameter in this implementation is on the return value of inflate(), as noted below, or when it returns early because Z_BLOCK is used.

If a preset dictionary is needed after this call (see inflateSetDictionary below), inflate sets strm->adler to the adler32 checksum of the dictionary chosen by the compressor and returns Z_NEED_DICT; otherwise it sets strm->adler to the adler32 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 adler32 checksum is equal to that saved by the compressor and returns Z_STREAM_END only if the checksum is correct.

inflate() will decompress and check either zlib-wrapped or gzip-wrapped deflate data. The header type is detected automatically. Any information contained in the gzip header is not retained, so applications that need that information should instead use raw inflate, see inflateInit2() below, or inflateBack() and perform their own processing of the gzip header and trailer.

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 check value), 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. Note that Z_BUF_ERROR is not fatal, and inflate() can be called again with more input and more output space to continue decompressing. If Z_DATA_ERROR is returned, the application may then call inflateSync() to look for a good compression block if a partial recovery of the data is desired.

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

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 of 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. In addition, the current implementation of deflate will use at most the window size minus 262 bytes of the provided dictionary.

Upon return of this function, strm->adler is set to the adler32 value of the dictionary; the decompressor may later use this value to determine which dictionary has been used by the compressor. (The adler32 value applies to the whole dictionary even if only a subset of the dictionary is actually used by the compressor.) If a raw deflate was requested, then the adler32 value is not computed and strm->adler is not set.

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.

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.

Dynamically update 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 of deflate().

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

Fine tune deflate's internal compression parameters. This should only be used by someone who understands the algorithm used by zlib's deflate for searching for the best matching string, and even then only by the most fanatic optimizer trying to squeeze out the last compressed bit for their specific input data. Read the deflate.c source code for the meaning of the max_lazy, good_length, nice_length, and max_chain parameters.

deflateTune() can be called after deflateInit() or deflateInit2()

deflateBound() returns an upper bound on the compressed size after deflation of sourceLen bytes. It must be called after deflateInit() or deflateInit2(). This would be used to allocate an output buffer for deflation in a single pass, and so would be called before deflate().

deflatePrime() inserts bits in the deflate output stream. The intent is that this function is used to start off the deflate output with the bits leftover from a previous deflate stream when appending to it. As such, this function can only be used for raw deflate, and must be used before the first deflate() call after a deflateInit2() or deflateReset(). bits must be less than or equal to 16, and that many of the least significant bits of value will be inserted in the output.

deflateSetHeader() provides gzip header information for when a gzip stream is requested by deflateInit2(). deflateSetHeader() may be called after deflateInit2() or deflateReset() and before the first call of deflate(). The text, time, os, extra field, name, and comment information in the provided gz_header structure are written to the gzip header (xflag is ignored -- the extra flags are set according to the compression level). The caller must assure that, if not Z_NULL, name and comment are terminated with a zero byte, and that if extra is not Z_NULL, that extra_len bytes are available there. If hcrc is true, a gzip header crc is included. Note that the current versions of the command-line version of gzip (up through version 1.3.x) do not support header crc's, and will report that it is a "multi-part gzip file" and give up.

If deflateSetHeader is not used, the default gzip header has text false, the time set to zero, and os set to 3, with no extra, name, or comment fields. The gzip header is returned to the default state by deflateReset().

Initializes the decompression dictionary from the given uncompressed byte sequence. This function must be called immediately after a call of inflate, if that call returned Z_NEED_DICT. The dictionary chosen by the compressor can be determined from the adler32 value returned by that call of inflate. The compressor and decompressor must use exactly the same dictionary (see deflateSetDictionary). For raw inflate, this function can be called immediately after inflateInit2() or inflateReset() and before any call of inflate() to set the dictionary. The application must insure that the dictionary that was used for compression is provided.

inflateSetDictionary does not perform any decompression: this will be done by subsequent calls of inflate().

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.

Sets the destination stream as a complete copy of the source stream.

This function can be useful when randomly accessing a large stream. The first pass through the stream can periodically record the inflate state, allowing restarting inflate at those points when randomly accessing the stream.

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.

This function inserts bits in the inflate input stream. The intent is that this function is used to start inflating at a bit position in the middle of a byte. The provided bits will be used before any bytes are used from next_in. This function should only be used with raw inflate, and should be used before the first inflate() call after inflateInit2() or inflateReset(). bits must be less than or equal to 16, and that many of the least significant bits of value will be inserted in the input.

inflateGetHeader() requests that gzip header information be stored in the provided gz_header structure. inflateGetHeader() may be called after inflateInit2() or inflateReset(), and before the first call of inflate(). As inflate() processes the gzip stream, head->done is zero until the header is completed, at which time head->done is set to one. If a zlib stream is being decoded, then head->done is set to -1 to indicate that there will be no gzip header information forthcoming. Note that Z_BLOCK can be used to force inflate() to return immediately after header processing is complete and before any actual data is decompressed.

The text, time, xflags, and os fields are filled in with the gzip header contents. hcrc is set to true if there is a header CRC. (The header CRC was valid if done is set to one.) If extra is not Z_NULL, then extra_max contains the maximum number of bytes to write to extra. Once done is true, extra_len contains the actual extra field length, and extra contains the extra field, or that field truncated if extra_max is less than extra_len. If name is not Z_NULL, then up to name_max characters are written there, terminated with a zero unless the length is greater than name_max. If comment is not Z_NULL, then up to comm_max characters are written there, terminated with a zero unless the length is greater than comm_max. When any of extra, name, or comment are not Z_NULL and the respective field is not present in the header, then that field is set to Z_NULL to signal its absence. This allows the use of deflateSetHeader() with the returned structure to duplicate the header. However if those fields are set to allocated memory, then the application will need to save those pointers elsewhere so that they can be eventually freed.

If inflateGetHeader is not used, then the header information is simply discarded. The header is always checked for validity, including the header CRC if present. inflateReset() will reset the process to discard the header information. The application would need to call inflateGetHeader() again to retrieve the header from the next gzip stream.

inflateBack() does a raw inflate with a single call using a call-back interface for input and output. This is more efficient than inflate() for file i/o applications in that it avoids copying between the output and the sliding window by simply making the window itself the output buffer. This function trusts the application to not change the output buffer passed by the output function, at least until inflateBack() returns.

inflateBackInit() must be called first to allocate the internal state and to initialize the state with the user-provided window buffer. inflateBack() may then be used multiple times to inflate a complete, raw deflate stream with each call. inflateBackEnd() is then called to free the allocated state.

A raw deflate stream is one with no zlib or gzip header or trailer. This routine would normally be used in a utility that reads zip or gzip files and writes out uncompressed files. The utility would decode the header and process the trailer on its own, hence this routine expects only the raw deflate stream to decompress. This is different from the normal behavior of inflate(), which expects either a zlib or gzip header and trailer around the deflate stream.

inflateBack() uses two subroutines supplied by the caller that are then called by inflateBack() for input and output. inflateBack() calls those routines until it reads a complete deflate stream and writes out all of the uncompressed data, or until it encounters an error. The function's parameters and return types are defined above in the in_func and out_func typedefs. inflateBack() will call in(in_desc, &buf) which should return the number of bytes of provided input, and a pointer to that input in buf. If there is no input available, in() must return zero--buf is ignored in that case--and inflateBack() will return a buffer error. inflateBack() will call out(out_desc, buf, len) to write the uncompressed data buf[0..len-1]. out() should return zero on success, or non-zero on failure. If out() returns non-zero, inflateBack() will return with an error. Neither in() nor out() are permitted to change the contents of the window provided to inflateBackInit(), which is also the buffer that out() uses to write from. The length written by out() will be at most the window size. Any non-zero amount of input may be provided by in().

For convenience, inflateBack() can be provided input on the first call by setting strm->next_in and strm->avail_in. If that input is exhausted, then in() will be called. Therefore strm->next_in must be initialized before calling inflateBack(). If strm->next_in is Z_NULL, then in() will be called immediately for input. If strm->next_in is not Z_NULL, then strm->avail_in must also be initialized, and then if strm->avail_in is not zero, input will initially be taken from strm->next_in[0 .. strm->avail_in - 1].

The in_desc and out_desc parameters of inflateBack() is passed as the first parameter of in() and out() respectively when they are called. These descriptors can be optionally used to pass any information that the caller- supplied in() and out() functions need to do their job.

On return, inflateBack() will set strm->next_in and strm->avail_in to pass back any unused input that was provided by the last in() call. The return values of inflateBack() can be Z_STREAM_END on success, Z_BUF_ERROR if in() or out() returned an error, Z_DATA_ERROR if there was a format error in the deflate stream (in which case strm->msg is set to indicate the nature of the error), or Z_STREAM_ERROR if the stream was not properly initialized. In the case of Z_BUF_ERROR, an input or output error can be distinguished using strm->next_in which will be Z_NULL only if in() returned an error. If strm->next is not Z_NULL, then the Z_BUF_ERROR was due to out() returning non-zero. (in() will always be called before out(), so strm->next_in is assured to be defined if out() returns non-zero.) Note that inflateBack() cannot return Z_OK.

All memory allocated by inflateBackInit() is freed.

Return flags indicating compile-time options.

Type sizes, two bits each, 00 = 16 bits, 01 = 32, 10 = 64, 11 = other: 1.0: size of uInt 3.2: size of uLong 5.4: size of voidpf (pointer) 7.6: size of z_off_t

Compiler, assembler, and debug options: 8: DEBUG 9: ASMV or ASMINF -- use ASM code 10: ZLIB_WINAPI -- exported functions use the WINAPI calling convention 11: 0 (reserved)

One-time table building (smaller code, but not thread-safe if true): 12: BUILDFIXED -- build static block decoding tables when needed 13: DYNAMIC_CRC_TABLE -- build CRC calculation tables when needed 14,15: 0 (reserved)

Library content (indicates missing functionality): 16: NO_GZCOMPRESS -- gz* functions cannot compress (to avoid linking deflate code when not needed) 17: NO_GZIP -- deflate can't write gzip streams, and inflate can't detect and decode gzip streams (to avoid linking crc code) 18-19: 0 (reserved)

Operation variations (changes in library functionality): 20: PKZIP_BUG_WORKAROUND -- slightly more permissive inflate 21: FASTEST -- deflate algorithm with only one, lowest compression level 22,23: 0 (reserved)

The sprintf variant used by gzprintf (zero is best): 24: 0 = vs*, 1 = s* -- 1 means limited to 20 arguments after the format 25: 0 = *nprintf, 1 = *printf -- 1 means gzprintf() not secure! 26: 0 = returns value, 1 = void -- 1 means inferred string length returned

Remainder: 27-31: 0 (reserved)