writeidx.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 */

/* Write an index record.
 * _db_writedat() is called before this function, to set the fields
 * datoff and datlen in the DB structure, which we need to write
 * the index record. */

void
_db_writeidx(DB *db, const char *key,
                     off_t offset, int whence, off_t ptrval)
{
    struct iovec    iov[2];
    char            asciiptrlen[PTR_SZ + IDXLEN_SZ +1];
    int                len;

    if ( (db->ptrval = ptrval) < 0 || ptrval > PTR_MAX)
        err_quit("invalid ptr: %d", ptrval);

    sprintf(db->idxbuf, "%s%c%d%c%d\n",
                key, SEP, db->datoff, SEP, db->datlen);
    if ( (len = strlen(db->idxbuf)) < IDXLEN_MIN || len > IDXLEN_MAX)
        err_dump("invalid length");
    sprintf(asciiptrlen, "%*d%*d", PTR_SZ, ptrval, IDXLEN_SZ, len);

        /* If we're appending, we have to lock before doing the lseek()
           and write() to make the two an atomic operation.  If we're
           overwriting an existing record, we don't have to lock. */
    if (whence == SEEK_END)        /* we're appending */
        if (writew_lock(db->idxfd, ((db->nhash+1)*PTR_SZ)+1,
                                                    SEEK_SET, 0) < 0)
            err_dump("writew_lock error");

        /* Position the index file and record the offset */
    if ( (db->idxoff = lseek(db->idxfd, offset, whence)) == -1)
        err_dump("lseek error");

    iov[0].iov_base = asciiptrlen;
    iov[0].iov_len  = PTR_SZ + IDXLEN_SZ;
    iov[1].iov_base = db->idxbuf;
    iov[1].iov_len  = len;
    if (writev(db->idxfd, &iov[0], 2) != PTR_SZ + IDXLEN_SZ + len)
        err_dump("writev error of index record");

    if (whence == SEEK_END)
        if (un_lock(db->idxfd, ((db->nhash+1)*PTR_SZ)+1, SEEK_SET, 0) < 0)
            err_dump("un_lock error");
}