257 lines
13 KiB
Plaintext
257 lines
13 KiB
Plaintext
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ziplimit.txt
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A1) Hard limits of the Zip archive format (without Zip64 extensions):
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Number of entries in Zip archive: 64 Ki (2^16 - 1 entries)
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Compressed size of archive entry: 4 GiByte (2^32 - 1 Bytes)
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Uncompressed size of entry: 4 GiByte (2^32 - 1 Bytes)
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Size of single-volume Zip archive: 4 GiByte (2^32 - 1 Bytes)
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Per-volume size of multi-volume archives: 4 GiByte (2^32 - 1 Bytes)
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Number of parts for multi-volume archives: 64 Ki (2^16 - 1 parts)
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Total size of multi-volume archive: 256 TiByte (4G * 64k)
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The number of archive entries and of multivolume parts are limited by
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the structure of the "end-of-central-directory" record, where the these
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numbers are stored in 2-Byte fields.
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Some Zip and/or UnZip implementations (for example Info-ZIP's) allow
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handling of archives with more than 64k entries. (The information
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from "number of entries" field in the "end-of-central-directory" record
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is not really neccessary to retrieve the contents of a Zip archive;
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it should rather be used for consistency checks.)
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Length of an archive entry name: 64 KiByte (2^16 - 1)
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Length of archive member comment: 64 KiByte (2^16 - 1)
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Total length of "extra field": 64 KiByte (2^16 - 1)
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Length of a single e.f. block: 64 KiByte (2^16 - 1)
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Length of archive comment: 64 KiByte (2^16 - 1)
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Additional limitation claimed by PKWARE:
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Size of local-header structure (fixed fields of 30 Bytes + filename
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local extra field): < 64 KiByte
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Size of central-directory structure (46 Bytes + filename +
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central extra field + member comment): < 64 KiByte
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A2) Hard limits of the Zip archive format with Zip64 extensions:
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In 2001, PKWARE has published version 4.5 of the Zip format specification
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(together with the release of PKZIP for Windows 4.5). This specification
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defines new extra field blocks that allow to break the size limits of the
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standard zipfile structures. This extended "Zip64" format enlarges the
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theoretical limits to the following values:
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Number of entries in Zip archive: 16 Ei (2^64 - 1 entries)
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Compressed size of archive entry: 16 EiByte (2^64 - 1 Bytes)
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Uncompressed size of entry: 16 EiByte (2^64 - 1 Bytes)
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Size of single-volume Zip archive: 16 EiByte (2^64 - 1 Bytes)
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Per-volume size of multi-volume archives: 16 EiByte (2^64 - 1 Bytes)
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Number of parts for multi-volume archives: 4 Gi (2^32 - 1 parts)
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Total size of multi-volume archive: 2^96 Byte (16 Ei * 4Gi)
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The Info-ZIP software releases (beginning with Zip 3.0 and UnZip 6.0)
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support Zip64 archives on selected environments (where the underlying
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operating system capabilities are sufficient, e.g. Unix, VMS and Win32).
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B) Implementation limits of UnZip:
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1. Size limits caused by file I/O and decompression handling:
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a) Without "Zip64" and "LargeFile" extensions:
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Size of Zip archive: 2 GiByte (2^31 - 1 Bytes)
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Compressed size of archive entry: 2 GiByte (2^31 - 1 Bytes)
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b) With "Zip64" enabled and "LargeFile" supported:
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Size of Zip archive: 8 EiByte (2^63 - 1 Bytes)
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Compressed size of archive entry: 8 EiByte (2^63 - 1 Bytes)
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Uncompressed size of entry: 8 EiByte (2^63 - 1 Bytes)
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Note: On some systems, even UnZip without "LargeFile" extensions enabled
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may support archive sizes up to 4 GiByte. To get this support, the
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target environment has to meet the following requirements:
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a) The compiler's intrinsic "long" data types must be able to hold
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integer numbers of 2^32. In other words - the standard intrinsic
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integer types "long" and "unsigned long" have to be wider than
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32 bit.
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b) The system has to supply a C runtime library that is compatible
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with the more-than-32-bit-wide "long int" type of condition a)
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c) The standard file positioning functions fseek(), ftell() (and/or
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the Unix style lseek() and tell() functions) have to be capable
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to move to absolute file offsets of up to 4 GiByte from the file
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start.
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On 32-bit CPU hardware, you generally cannot expect that a C compiler
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provides a "long int" type that is wider than 32-bit. So, many of the
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most popular systems (i386, PowerPC, 680x0, et. al) are out of luck.
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You may find environment that provide all requirements on systems
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with 64-bit CPU hardware. Examples might be Cray number crunchers,
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Compaq (former DEC) Alpha AXP machines, or Intel/AMD x64 computers.
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The number of Zip archive entries is unlimited. The "number-of-entries"
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field of the "end-of-central-dir" record is checked against the "number
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of entries found in the central directory" modulus 64k (2^16) (without
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Zip64 extension) or modulus 2^64 (with Zip64 extensions enabled for
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Zip64 archives).
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Multi-volume archive extraction is not (yet) supported.
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Memory requirements are mostly independent of the archive size
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and archive contents.
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In general, UnZip needs a fixed amount of internal buffer space
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plus the size to hold the complete information of the currently
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processed entry's local header. Here, a large extra field
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(could be up to 64 kByte) may exceed the available memory
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for MSDOS 16-bit executables (when they were compiled in small
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or medium memory model, with a fixed 64 KiByte limit on data space).
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The other exception where memory requirements scale with "larger"
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archives is the "restore directory attributes" feature. Here, the
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directory attributes info for each restored directory has to be held
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in memory until the whole archive has been processed. So, the amount
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of memory needed to keep this info scales with the number of restored
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directories and may cause memory problems when a lot of directories
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are restored in a single run.
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C) Implementation limits of the Zip executables:
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1. Size limits caused by file I/O and compression handling:
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a) Without "Zip64" and "LargeFile" extensions:
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Size of Zip archive: 2 GiByte (2^31 - 1 Bytes)
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Compressed size of archive entry: 2 GiByte (2^31 - 1 Bytes)
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Uncompressed size of entry: 2 GiByte (2^31 - 1 Bytes),
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(could/should be 4 GiBytes...)
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b) With "Zip64" enabled and "LargeFile" supported:
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Size of Zip archive: 8 EiByte (2^63 - 1 Bytes)
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Compressed size of archive entry: 8 EiByte (2^63 - 1 Bytes)
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Uncompressed size of entry: 8 EiByte (2^63 - 1 Bytes)
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Multi-volume archive creation now supported in the form of split
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archives. Currently up to 99,999 splits are supported.
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2. Limits caused by handling of archive contents lists
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2.1. Number of archive entries (freshen, update, delete)
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a) 16-bit executable: 64k (2^16 -1) or 32k (2^15 - 1),
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(unsigned vs. signed type of size_t)
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a1) 16-bit executable: <16k ((2^16)/4)
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(The smaller limit a1) results from the array size limit of
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the "qsort()" function.)
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32-bit executable: <1G ((2^32)/4)
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(usual system limit of the "qsort()" function on 32-bit systems)
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64-bit executable: <2Ei ((2^64)/8)
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(theoretical limit of 64-bit flat memory model, the actual limit of
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currently available OS implementations is several orders of magnitude
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lower)
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b) stack space needed by qsort to sort list of archive entries
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NOTE: In the current executables, overflows of limits a) and b) are NOT
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checked!
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c) amount of free memory to hold "central directory information" of
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all archive entries; one entry needs:
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128 bytes (Zip64), 96 bytes (32-bit) resp. 80 bytes (16-bit)
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+ 3 * length of entry name
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+ length of zip entry comment (when present)
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+ length of extra field(s) (when present, e.g.: UT needs 9 bytes)
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+ some bytes for book-keeping of memory allocation
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Conclusion:
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For systems with limited memory space (MSDOS, small AMIGAs, other
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environments without virtual memory), the number of archive entries
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is most often limited by condition c).
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For example, with approx. 100 kBytes of free memory after loading and
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initializing the program, a 16-bit DOS Zip cannot process more than 600
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to 1000 (+) archive entries. (For the 16-bit Windows DLL or the 16-bit
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OS/2 port, limit c) is less important because Windows or OS/2 executables
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are not restricted to the 1024k area of real mode memory. These 16-bit
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ports are limited by conditions a1) and b), say: at maximum approx.
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16000 entries!)
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2.2. Number of "new" entries (add operation)
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In addition to the restrictions above (2.1.), the following limits
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caused by the handling of the "new files" list apply:
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a) 16-bit executable: <16k ((2^64)/4)
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b) stack size required for "qsort" operation on "new entries" list.
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NOTE: In the current executables, the overflow checks for these limits
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are missing!
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c) amount of free memory to hold the directory info list for new entries;
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one entry needs:
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32 bytes (Zip64), 24 bytes (32-bit) resp. 22 bytes (16-bit)
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+ 3 * length of filename
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NOTE: For larger systems, the actual usability limits may be more
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performance issues (how long you want to wait) rather than available
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memory and other resources.
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D) Some technical remarks:
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1. For executables without support for "Zip64" archives and "LargeFile"
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I/O extensions, the 2GiByte size limit on archive files is a consequence
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of the portable C implementation used for the Info-ZIP programs.
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Zip archive processing requires random access to the archive file for
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jumping between different parts of the archive's structure.
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In standard C, this is done via stdio functions fseek()/ftell() resp.
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unix-io functions lseek()/tell(). In many (most?) C implementations,
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these functions use "signed long" variables to hold offset pointers
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into sequential files. In most cases, this is a signed 32-bit number,
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which is limited to ca. 2E+09. There may be specific C runtime library
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implementations that interpret the offset numbers as unsigned, but for
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us, this is not reliable in the context of portable programming.
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2. Similarly, for executables without "Zip64" and "LargeFile" support,
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the 2GiByte limit on the size of a single compressed archive member
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is again a consequence of the implementation in C.
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The variables used internally to count the size of the compressed
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data stream are of type "long", which is guaranted to be at least
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32-bit wide on all supported environments.
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But, why do we use "signed" long and not "unsigned long"?
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Throughout the I/O handling of the compressed data stream, the sign bit
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of the "long" numbers is (mis-)used as a kind of overflow detection.
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In the end, this is caused by the fact that standard C lacks any
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overflow checking on integer arithmetics and does not support access
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to the underlying hardware's overflow detection (the status bits,
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especially "carry" and "overflow" of the CPU's flags-register) in a
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system-independent manner.
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So, we "misuse" the most-significant bit of the compressed data size
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counters as carry bit for efficient overflow/underflow detection. We
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could change the code to a different method of overflow detection, by
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using a bunch of "sanity" comparisons (kind of "is the calculated result
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plausible when compared with the operands"). But, this would "blow up"
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the code of the "inner loop", with remarkable loss of processing speed.
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Or, we could reduce the amount of consistency checks of the compressed
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data (e.g. detection of premature end of stream) to an absolute minimum,
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at the cost of the programs' stability when processing corrupted data.
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3. The argumentation above is somewhat out-dated. Beginning with the
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releases of Zip 3 and UnZip 6, Info-ZIP programs support archive
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sizes larger than 4GiB on systems where the required underlying
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support for 64-bit file offsets and file sizes is available from
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the OS (and the C runtime environment).
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For executables with support for "Zip64" archive format and "LargeFile"
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extension, the I/O limits are lifted by applying extended 64-bit off_t
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file offsets. All limits discussed above are then based on integer
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sizes of 64 bits instead of 32, this should allow to handle file and
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archive sizes up to the limits of manufacturable hardware for the
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foreseeable future. The reduction of the theoretical limits from
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(2^64 - 1) to (2^63 - 1) because of the throughout use of signed
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numbers can be neglected with the currently imaginable hardware.
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However, this new support partially breaks compatibility with older
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"legacy" systems. And it should be noted that the portability and
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readability of the UnZip and Zip code has suffered somehow caused
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by the extensive use of non-standard language extension needed for
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64-bit support on the major target systems.
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Please report any problems to: Zip-Bugs at www.info-zip.org
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Last updated: 25 May 2008, Ed Gordon
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02 January 2009, Christian Spieler
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