readidx.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"
#include    <sys/uio.h>        /* struct iovec */

/* Read the next index record.  We start at the specified offset in
 * the index file.  We read the index record into db->idxbuf and
 * replace the separators with null bytes.  If all is OK we set
 * db->datoff and db->datlen to the offset and length of the
 * corresponding data record in the data file.  */

off_t
_db_readidx(DB *db, off_t offset)
{
    int                i;
    char            *ptr1, *ptr2;
    char            asciiptr[PTR_SZ + 1], asciilen[IDXLEN_SZ + 1];
    struct iovec    iov[2];

        /* Position index file and record the offset.  db_nextrec()
           calls us with offset==0, meaning read from current offset.
           We still need to call lseek() to record the current offset. */
    if ( (db->idxoff = lseek(db->idxfd, offset,
                             offset == 0 ? SEEK_CUR : SEEK_SET)) == -1)
        err_dump("lseek error");

        /* Read the ascii chain ptr and the ascii length at
           the front of the index record.  This tells us the
           remaining size of the index record. */
    iov[0].iov_base = asciiptr;
    iov[0].iov_len  = PTR_SZ;
    iov[1].iov_base = asciilen;
    iov[1].iov_len  = IDXLEN_SZ;
    if ( (i = readv(db->idxfd, &iov[0], 2)) != PTR_SZ + IDXLEN_SZ) {
        if (i == 0 && offset == 0)
            return(-1);        /* EOF for db_nextrec() */
        err_dump("readv error of index record");
    }

    asciiptr[PTR_SZ] = 0;            /* null terminate */
    db->ptrval = atol(asciiptr);    /* offset of next key in chain */
                        /* this is our return value; always >= 0 */
    asciilen[IDXLEN_SZ] = 0;        /* null terminate */
    if ( (db->idxlen = atoi(asciilen)) < IDXLEN_MIN ||
                            db->idxlen > IDXLEN_MAX)
        err_dump("invalid length");

        /* Now read the actual index record.  We read it into the key
           buffer that we malloced when we opened the database. */
    if ( (i = read(db->idxfd, db->idxbuf, db->idxlen)) != db->idxlen)
        err_dump("read error of indexc record");
    if (db->idxbuf[db->idxlen-1] != '\n')
        err_dump("missing newline");    /* sanity checks */
    db->idxbuf[db->idxlen-1] = 0;        /* replace newline with null */

        /* Find the separators in the index record */
    if ( (ptr1 = strchr(db->idxbuf, SEP)) == NULL)
        err_dump("missing first separator");
    *ptr1++ = 0;                /* replace SEP with null */

    if ( (ptr2 = strchr(ptr1, SEP)) == NULL)
        err_dump("missing second separator");
    *ptr2++ = 0;                /* replace SEP with null */

    if (strchr(ptr2, SEP) != NULL)
        err_dump("too many separators");

        /* Get the starting offset and length of the data record */
    if ( (db->datoff = atol(ptr1)) < 0)
        err_dump("starting offset < 0");
    if ( (db->datlen = atol(ptr2)) <= 0 || db->datlen > DATLEN_MAX)
        err_dump("invalid length");
    return(db->ptrval);        /* return offset of next key in chain */
}