os_fid.c

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/*-
 * See the file LICENSE for redistribution information.
 *
 * Copyright (c) 1996, 1997, 1998, 1999, 2000
 *      Sleepycat Software.  All rights reserved.
 */

#include "config.h"

#ifndef lint
static const char revid[] = "$Id: os__fid_8c-source.html,v 1.1 2008/06/08 10:21:13 sebdiaz Exp $";
#endif /* not lint */

#ifndef NO_SYSTEM_INCLUDES
#include <sys/types.h>
#include <sys/stat.h>

#if TIME_WITH_SYS_TIME
#include <sys/time.h>
#include <time.h>
#else
#if HAVE_SYS_TIME_H
#include <sys/time.h>
#else
#include <time.h>
#endif
#endif

#include <string.h>
#include <unistd.h>
#endif

#include "db_int.h"

#define SERIAL_INIT     0
static u_int32_t fid_serial = SERIAL_INIT;

/*
 * CDB___os_fileid --
 *      Return a unique identifier for a file.
 *
 * PUBLIC: int CDB___os_fileid __P((DB_ENV *, const char *, int, u_int8_t *));
 */
int
CDB___os_fileid(dbenv, fname, unique_okay, fidp)
        DB_ENV *dbenv;
        const char *fname;
        int unique_okay;
        u_int8_t *fidp;
{
        struct stat sb;
        size_t i;
        int ret;
        u_int32_t tmp;
        u_int8_t *p;

        /* Clear the buffer. */
        memset(fidp, 0, DB_FILE_ID_LEN);

        /* On POSIX/UNIX, use a dev/inode pair. */
        if (stat(fname, &sb)) {
                ret = CDB___os_get_errno();
                CDB___db_err(dbenv, "%s: %s", fname, strerror(ret));
                return (ret);
        }
        
        /* 
         * Initialize/increment the serial number we use to help avoid
         * fileid collisions.  Note that we don't bother with locking;
         * it's unpleasant to do from down in here, and if we race on
         * this no real harm will be done, since the finished fileid
         * has so many other components.
         * 
         * We increment by 100000 on each call as a simple way of
         * randomizing;  simply incrementing seems potentially less useful
         * if pids are also simply incremented, since this is process-local
         * and we may be one of a set of processes starting up.  100000
         * pushes us out of pid space on most platforms, and has few 
         * interesting properties in base 2.
         */
        if (fid_serial == SERIAL_INIT)
                fid_serial = (u_int32_t)getpid();
        else
                fid_serial += 100000;

        /*
         * !!!
         * Nothing is ever big enough -- on Sparc V9, st_ino, st_dev and the
         * time_t types are all 8 bytes.  As DB_FILE_ID_LEN is only 20 bytes,
         * we convert to a (potentially) smaller fixed-size type and use it.
         *
         * We don't worry about byte sexing or the actual variable sizes.
         *
         * When this routine is called from the DB access methods, it's only
         * called once -- whatever ID is generated when a database is created
         * is stored in the database file's metadata, and that is what is
         * saved in the mpool region's information to uniquely identify the
         * file.
         *
         * When called from the mpool layer this routine will be called each
         * time a new thread of control wants to share the file, which makes
         * things tougher.  As far as byte sexing goes, since the mpool region
         * lives on a single host, there's no issue of that -- the entire
         * region is byte sex dependent.  As far as variable sizes go, we make
         * the simplifying assumption that 32-bit and 64-bit processes will
         * get the same 32-bit values if we truncate any returned 64-bit value
         * to a 32-bit value.  When we're called from the mpool layer, though,
         * we need to be careful not to include anything that isn't 
         * reproducible for a given file, such as the timestamp or serial 
         * number.
         */
        tmp = (u_int32_t)sb.st_ino;
        for (p = (u_int8_t *)&tmp, i = sizeof(u_int32_t); i > 0; --i)
                *fidp++ = *p++;

        tmp = (u_int32_t)sb.st_dev;
        for (p = (u_int8_t *)&tmp, i = sizeof(u_int32_t); i > 0; --i)
                *fidp++ = *p++;

        if (unique_okay) {
                /*
                 * We want the number of seconds, not the high-order 0 bits,
                 * so convert the returned time_t to a (potentially) smaller
                 * fixed-size type.
                 */
                tmp = (u_int32_t)time(NULL);
                for (p = (u_int8_t *)&tmp, i = sizeof(u_int32_t); i > 0; --i)
                        *fidp++ = *p++;
                
                for (p = (u_int8_t *)&fid_serial, i = sizeof(u_int32_t); 
                    i > 0; --i)
                        *fidp++ = *p++;
        }

        return (0);
}

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