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The text of the specification assumes a basic background in programming at the level of bits and other primitive data representations.
+---+ | | <-- the vertical bars might be missing +---+represents one byte; a box like this:
+==============+ | | +==============+represents a variable number of bytes.
Bytes stored within a computer do not have a "bit order", since they are always treated as a unit. However, a byte considered as an integer between 0 and 255 does have a most- and least-significant bit, and since we write numbers with the most-significant digit on the left, we also write bytes with the most-significant bit on the left. In the diagrams below, we number the bits of a byte so that bit 0 is the least-significant bit, i.e., the bits are numbered:
+--------+ |76543210| +--------+Within a computer, a number may occupy multiple bytes. All multi-byte numbers in the format described here are stored with the MOST-significant byte first (at the lower memory address). For example, the decimal number 520 is stored as:
0 1
+--------+--------+
|00000010|00001000|
+--------+--------+
^ ^
| |
| + less significant byte = 8
+ more significant byte = 2 x 256
0 1 +---+---+ |CMF|FLG| (more-->) +---+---+(if FLG.FDICT set)
0 1 2 3 +---+---+---+---+ | DICTID | (more-->) +---+---+---+---+
+=====================+---+---+---+---+ |...compressed data...| ADLER32 | +=====================+---+---+---+---+Any data which may appear after ADLER32 are not part of the zlib stream.
bits 0 to 3 CM Compression method bits 4 to 7 CINFO Compression info
bits 0 to 4 FCHECK (check bits for CMF and FLG) bit 5 FDICT (preset dictionary) bits 6 to 7 FLEVEL (compression level)The FCHECK value must be such that CMF and FLG, when viewed as a 16-bit unsigned integer stored in MSB order (CMF*256 + FLG), is a multiple of 31.
0 - compressor used fastest algorithm 1 - compressor used fast algorithm 2 - compressor used default algorithm 3 - compressor used maximum compression, slowest algorithmThe information in FLEVEL is not needed for decompression; it is there to indicate if recompression might be worthwhile.
Other compressed data formats are not specified in this version of the zlib specification.
Adler-32 is composed of two sums accumulated per byte: s1 is the sum of all bytes, s2 is the sum of all s1 values. Both sums are done modulo 65521. s1 is initialized to 1, s2 to zero. The Adler-32 checksum is stored as s2*65536 + s1 in most-significant-byte first (network) order.
A compliant decompressor must check CMF, FLG, and ADLER32, and provide an error indication if any of these have incorrect values. A compliant decompressor must give an error indication if CM is not one of the values defined in this specification (only the value 8 is permitted in this version), since another value could indicate the presence of new features that would cause subsequent data to be interpreted incorrectly. A compliant decompressor must give an error indication if FDICT is set and DICTID is not the identifier of a known preset dictionary. A decompressor may ignore FLEVEL and still be compliant. When the zlib data format is being used as a part of another standard format, a compliant decompressor must support all the preset dictionaries specified by the other format. When the other format does not use the preset dictionary feature, a compliant decompressor must reject any stream in which the FDICT flag is set.
[1] Deutsch, L.P.,"GZIP Compressed Data Format Specification", available in ftp://ftp.uu.net/pub/archiving/zip/doc/
[2] Thomas Boutell, "PNG (Portable Network Graphics) specification", available in ftp://ftp.uu.net/graphics/png/documents/
[3] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification", available in ftp://ftp.uu.net/pub/archiving/zip/doc/
[4] Fletcher, J. G., "An Arithmetic Checksum for Serial Transmissions," IEEE Transactions on Communications, Vol. COM-30, No. 1, January 1982, pp. 247-252.
[5] ITU-T Recommendation X.224, Annex D, "Checksum Algorithms," November, 1993, pp. 144, 145. (Available from gopher://info.itu.ch). ITU-T X.244 is also the same as ISO 8073.
Jean-Loup Gailly and Mark Adler designed the zlib format and wrote the related software described in this specification. Glenn Randers-Pehrson converted this document to RFC and HTML format.
Aladdin Enterprises 203 Santa Margarita Ave. Menlo Park, CA 94025 Phone: (415) 322-0103 (AM only) FAX: (415) 322-1734 EMail: <ghost@aladdin.com>Questions about the technical content of this specification can be sent by email to:
Jean-Loup Gailly <jloup@gzip.org> and Mark Adler <madler@alumni.caltech.edu>Editorial comments on this specification can be sent by email to:
L. Peter Deutsch <ghost@aladdin.com> and Glenn Randers-Pehrson <randeg@alumni.rpi.edu>
The modulo on unsigned long accumulators can be delayed for 5552 bytes, so the modulo operation time is negligible. If the bytes are a, b, c, the second sum is 3a + 2b + c + 3, and so is position and order sensitive, unlike the first sum, which is just a checksum. That 65521 is prime is important to avoid a possible large class of two-byte errors that leave the check unchanged. (The Fletcher checksum uses 255, which is not prime and which also makes the Fletcher check insensitive to single byte changes 0 <-> 255.)
The sum s1 is initialized to 1 instead of zero to make the length of the sequence part of s2, so that the length does not have to be checked separately. (Any sequence of zeroes has a Fletcher checksum of zero.)
& Bitwise AND operator.
>> Bitwise right shift operator. When applied to an
unsigned quantity, as here, right shift inserts zero bit(s)
at the left.
<< Bitwise left shift operator. Left shift inserts zero
bit(s) at the right.
++ "n++" increments the variable n.
% modulo operator: a % b is the remainder of a divided by b.
#define BASE 65521 /* largest prime smaller than 65536 */
/*
Update a running Adler-32 checksum with the bytes buf[0..len-1]
and return the updated checksum. The Adler-32 checksum should be
initialized to 1.
Usage example:
unsigned long adler = 1L;
while (read_buffer(buffer, length) != EOF) {
adler = update_adler32(adler, buffer, length);
}
if (adler != original_adler) error();
*/
unsigned long update_adler32(unsigned long adler,
unsigned char *buf, int len)
{
unsigned long s1 = adler & 0xffff;
unsigned long s2 = (adler >> 16) & 0xffff;
int n;
for (n = 0; n < len; n++) {
s1 = (s1 + buf[n]) % BASE;
s2 = (s2 + s1) % BASE;
}
return (s2 << 16) + s1;
}
/* Return the adler32 of the bytes buf[0..len-1] */
unsigned long adler32(unsigned char *buf, int len)
{
return update_adler32(1L, buf, len);
}