/* ipc.cc: Single unix specification IPC interface for Cygwin
- Copyright 2001, 2002 Red Hat, Inc.
+ Copyright 2001, 2002, 2003 Red Hat, Inc.
Originally written by Robert Collins <robert.collins@hotmail.com>
+ Updated to 64 bit key_t by Charles Wilson <cygwin@cwilson.fastmail.fm>
This file is part of Cygwin.
#include <cygwin/ipc.h>
#include <sys/stat.h>
-extern "C"
-{
-
/* Notes: we return a valid key even if id's low order 8 bits are 0. */
-key_t
+extern "C" key_t
ftok (const char *path, int id)
{
struct __stat64 statbuf;
+ key_t tmp;
if (stat64 (path, &statbuf))
{
/* stat set the appropriate errno for us */
return (key_t) -1;
}
- /* dev_t is short for cygwin
- * ino_t is long for cygwin
- * and we need 8 bits for the id.
- * thus key_t is long long.
- */
- return ((long long) statbuf.st_dev << (5*8)) | (statbuf.st_ino << (8) ) | (id & 0x00ff);
-}
+ /* Since __CYGWIN_USE_BIG_TYPES__,
+ dev_t is 32bits for cygwin
+ ino_t is 64bits for cygwin
+ and we need 8 bits for the id.
+ thus key_t needs 104 bits total -- but we only have 64 (long long)
+ We will have to alias; leaving open the possibility that the same
+ key will be returned for multiple files. This possibility exists
+ also on Linux; the question is, how to minimize this possibility.
+
+ How to solve? Well, based on C. Vinschen's research, the nFileIndex*
+ words vary as follows, on a partition with > 110,000 files
+ nFileIndexHigh: 564 values between 0x00010000 -- 0xffff0000
+ nFileIndexLow : 103812 values between 0x00000000 -- 0x0003ffff
+ R. Collins suggests that these may represent a tree path,
+ and that it would require ~2.9M files to force the tree depth
+ to increase and reveal more bit usage.
+
+ Implementation details: dev_t is 32bits, but is formed by
+ device(32bits) << 16 | unit(32bits)
+ But device is ACTUALLY == status & FH_DEVMASK, where FH_DEVMASK
+ is 0x00000fff --> 12 bits
+
+ As it happens, the maximum number of devices is actually
+ FH_NDEV, not FH_DEVMASK, where FH_NDEV is currently 0x0000001d.
+ However, FH_NDEV grows as new device types are added. So
+ currently the device number needs 5 bits, but later? Let's
+ take a cue from Linux, and use the lower 8 bits (instead of the
+ lower 12 or 16) for the device (major?) number.
+
+ Similarly, while 'units' is an int (32bits), it is unclear
+ how many of these are significant. For most devices, it seems that
+ 'units' is equivalent to 'minor'. For FH_TAPE, it's obvious that
+ only 8 bits are important. However, for FH_SOCKET...it might be
+ as high as 16 significant bits.
+
+ Let's assume that we only need 8 bits from device (major) and
+ only 8 bits from unit (minor). (On linux, only 8 bits of minor
+ are used, and none from major).
+ ---> so, we only need 0x00ff00ff (16 bits) of dev_t
+
+ ---> we MUST have all 8 bits of id.
+
+ ---> So, we only have 64 - 8 - 16 = 40 bits for ino_t. But, we
+ need 0xffff0000 for nFileIndexHigh and 0x0003ffff for nFileIndexLow
+ minimum, or 16 + 18 = 34 bits. Lucky us - we have 6 more bits
+ to distribute.
+
+ For lack of a better idea, we'll allocate 2 of the extra bits to
+ nFileIndexHigh and 4 to nFileIndexLow. */
+ /* get 8 bits from dev_t (major), put into 0xff00000000000000L */
+ tmp = (((key_t) statbuf.st_dev) & 0x0000000000ff0000LL) << 40;
+ /* get 8 bits from dev_t (minor), put into 0x00ff000000000000L */
+ tmp |= (((key_t) statbuf.st_dev) & 0x00000000000000ffLL) << 48;
+ /* get upper 16+2 bits from nFileInfoHigh, put into 0x0000ffffc0000000L
+ shift down first, then mask, to avoid sign extension on rightshift */
+ tmp |= (((key_t) statbuf.st_ino) & 0xffffc00000000000LL) >> 16;
+ /* get lower 18+4 bits from nFileInfoLow, put into 0x000000003fffff00L */
+ tmp |= (((key_t) statbuf.st_ino) & 0x00000000003fffffLL) << 8;
+ /* use all 8 bits of id, and put into 0x00000000000000ffL */
+ tmp |= (id & 0x00ff);
+ return tmp;
}
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