dodelete.c

unix环境高级编程的源代码

#include    "db.h"

/* Delete the current record specified by the DB structure.
 * This function is called by db_delete() and db_store(),
 * after the record has been located by _db_find(). */

int
_db_dodelete(DB *db)
{
    int        i;
    char    *ptr;
    off_t    freeptr, saveptr;

        /* Set data buffer to all blanks */
    for (ptr = db->datbuf, i = 0; i < db->datlen - 1; i++)
        *ptr++ = ' ';
    *ptr = 0;    /* null terminate for _db_writedat() */

        /* Set key to blanks */
    ptr = db->idxbuf;
    while (*ptr)
        *ptr++ = ' ';

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

        /* Write the data record with all blanks */
    _db_writedat(db, db->datbuf, db->datoff, SEEK_SET);

        /* Read the free list pointer.  Its value becomes the
           chain ptr field of the deleted index record.  This means
           the deleted record becomes the head of the free list. */
    freeptr = _db_readptr(db, FREE_OFF);

        /* Save the contents of index record chain ptr,
           before it's rewritten by _db_writeidx(). */
    saveptr = db->ptrval;

        /* Rewrite the index record.  This also rewrites the length
           of the index record, the data offset, and the data length,
           none of which has changed, but that's OK. */
    _db_writeidx(db, db->idxbuf, db->idxoff, SEEK_SET, freeptr);

        /* Write the new free list pointer */
    _db_writeptr(db, FREE_OFF, db->idxoff);

        /* Rewrite the chain ptr that pointed to this record
           being deleted.  Recall that _db_find() sets db->ptroff
           to point to this chain ptr.  We set this chain ptr
           to the contents of the deleted record's chain ptr,
           saveptr, which can be either zero or nonzero. */
    _db_writeptr(db, db->ptroff, saveptr);

    if (un_lock(db->idxfd, FREE_OFF, SEEK_SET, 1) < 0)
        err_dump("un_lock error");

    return(0);
}