findfree.c

UNIX编程高级环境（第二版）的网络套接字部分的程序源码

#include    "db.h"

/* Try to find a free index record and accompanying data record
 * of the correct sizes.  We're only called by db_store(). */

int
_db_findfree(DB *db, int keylen, int datlen)
{
    int        rc;
    off_t    offset, nextoffset, saveoffset;

        /* Lock the free list */
    if (writew_lock(db->idxfd, FREE_OFF, SEEK_SET, 1) < 0)
        err_dump("writew_lock error");

        /* Read the free list pointer */
    saveoffset = FREE_OFF;
    offset = _db_readptr(db, saveoffset);

    while (offset != 0) {
        nextoffset = _db_readidx(db, offset);
        if (strlen(db->idxbuf) == keylen && db->datlen == datlen)
            break;        /* found a match */

        saveoffset = offset;
        offset = nextoffset;
    }

    if (offset == 0)
        rc = -1;    /* no match found */
    else {
            /* Found a free record with matching sizes.
               The index record was read in by _db_readidx() above,
               which sets db->ptrval.  Also, saveoffset points to
               the chain ptr that pointed to this empty record on
               the free list.  We set this chain ptr to db->ptrval,
               which removes the empty record from the free list. */

        _db_writeptr(db, saveoffset, db->ptrval);
        rc = 0;

            /* Notice also that _db_readidx() set both db->idxoff
               and db->datoff.  This is used by the caller, db_store(),
               to write the new index record and data record. */
    }
            /* Unlock the free list */
    if (un_lock(db->idxfd, FREE_OFF, SEEK_SET, 1) < 0)
        err_dump("un_lock error");
    return(rc);
}