90 lines
3.7 KiB
C++
90 lines
3.7 KiB
C++
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/* ipc.cc: Single unix specification IPC interface for Cygwin
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Originally written by Robert Collins <robert.collins@hotmail.com>
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Updated to 64 bit key_t by Charles Wilson <cygwin@cwilson.fastmail.fm>
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This file is part of Cygwin.
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This software is a copyrighted work licensed under the terms of the
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Cygwin license. Please consult the file "CYGWIN_LICENSE" for
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details. */
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#include "winsup.h"
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#include <sys/stat.h>
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/* Notes: we return a valid key even if id's low order 8 bits are 0. */
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extern "C" key_t
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ftok (const char *path, int id)
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{
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struct stat statbuf;
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key_t tmp;
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if (stat64 (path, &statbuf))
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{
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/* stat set the appropriate errno for us */
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return (key_t) -1;
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}
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/* Since Cygwin 1.5
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dev_t is 32bits for cygwin
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ino_t is 64bits for cygwin
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and we need 8 bits for the id.
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thus key_t needs 104 bits total -- but we only have 64 (long long)
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We will have to alias; leaving open the possibility that the same
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key will be returned for multiple files. This possibility exists
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also on Linux; the question is, how to minimize this possibility.
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How to solve? Well, based on C. Vinschen's research, the nFileIndex*
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words vary as follows, on a partition with > 110,000 files
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nFileIndexHigh: 564 values between 0x00010000 -- 0xffff0000
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nFileIndexLow : 103812 values between 0x00000000 -- 0x0003ffff
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R. Collins suggests that these may represent a tree path,
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and that it would require ~2.9M files to force the tree depth
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to increase and reveal more bit usage.
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Implementation details: dev_t is 32bits, but is formed by
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device(32bits) << 16 | unit(32bits)
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But device is ACTUALLY == status & FH_DEVMASK, where FH_DEVMASK
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is 0x00000fff --> 12 bits
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As it happens, the maximum number of devices is actually
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FH_NDEV, not FH_DEVMASK, where FH_NDEV is currently 0x0000001d.
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However, FH_NDEV grows as new device types are added. So
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currently the device number needs 5 bits, but later? Let's
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take a cue from Linux, and use the lower 8 bits (instead of the
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lower 12 or 16) for the device (major?) number.
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Similarly, while 'units' is an int (32bits), it is unclear
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how many of these are significant. For most devices, it seems that
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'units' is equivalent to 'minor'. For FH_TAPE, it's obvious that
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only 8 bits are important. However, for FH_SOCKET...it might be
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as high as 16 significant bits.
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Let's assume that we only need 8 bits from device (major) and
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only 8 bits from unit (minor). (On linux, only 8 bits of minor
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are used, and none from major).
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---> so, we only need 0x00ff00ff (16 bits) of dev_t
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---> we MUST have all 8 bits of id.
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---> So, we only have 64 - 8 - 16 = 40 bits for ino_t. But, we
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need 0xffff0000 for nFileIndexHigh and 0x0003ffff for nFileIndexLow
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minimum, or 16 + 18 = 34 bits. Lucky us - we have 6 more bits
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to distribute.
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For lack of a better idea, we'll allocate 2 of the extra bits to
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nFileIndexHigh and 4 to nFileIndexLow. */
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/* get 8 bits from dev_t (major), put into 0xff00000000000000L */
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tmp = (((key_t) statbuf.st_dev) & 0x0000000000ff0000LL) << 40;
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/* get 8 bits from dev_t (minor), put into 0x00ff000000000000L */
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tmp |= (((key_t) statbuf.st_dev) & 0x00000000000000ffLL) << 48;
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/* get upper 16+2 bits from nFileInfoHigh, put into 0x0000ffffc0000000L
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shift down first, then mask, to avoid sign extension on rightshift */
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tmp |= (((key_t) statbuf.st_ino) & 0xffffc00000000000LL) >> 16;
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/* get lower 18+4 bits from nFileInfoLow, put into 0x000000003fffff00L */
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tmp |= (((key_t) statbuf.st_ino) & 0x00000000003fffffLL) << 8;
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/* use all 8 bits of id, and put into 0x00000000000000ffL */
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tmp |= (id & 0x00ff);
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return tmp;
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}
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