ssize_t
ngx_write_chain_to_file(ngx_file_t *file, ngx_chain_t *cl, off_t offset,
    ngx_pool_t *pool)
{
    u_char        *prev;
    size_t         size;
    ssize_t        total, n;
    ngx_array_t    vec;
    struct iovec  *iov, iovs[NGX_IOVS];

    /* use pwrite() if there is the only buf in a chain */

    if (cl->next == NULL) {
        return ngx_write_file(file, cl->buf->pos,
                              (size_t) (cl->buf->last - cl->buf->pos),
                              offset);
    }

    total = 0;

    vec.elts = iovs;
    vec.size = sizeof(struct iovec);
    vec.nalloc = NGX_IOVS;
    vec.pool = pool;

    do {
        prev = NULL;
        iov = NULL;
        size = 0;

        vec.nelts = 0;

        /* create the iovec and coalesce the neighbouring bufs */

        while (cl && vec.nelts < IOV_MAX) {
            if (prev == cl->buf->pos) {
                iov->iov_len += cl->buf->last - cl->buf->pos;

            } else {
                iov = ngx_array_push(&vec);
                if (iov == NULL) {
                    return NGX_ERROR;
                }

                iov->iov_base = (void *) cl->buf->pos;
                iov->iov_len = cl->buf->last - cl->buf->pos;
            }

            size += cl->buf->last - cl->buf->pos;
            prev = cl->buf->last;
            cl = cl->next;
        }

        /* use pwrite() if there is the only iovec buffer */

        if (vec.nelts == 1) {
            iov = vec.elts;

            n = ngx_write_file(file, (u_char *) iov[0].iov_base,
                               iov[0].iov_len, offset);

            if (n == NGX_ERROR) {
                return n;
            }

            return total + n;
        }

        n = ngx_writev_file(file, &vec, size, offset);

        if (n == NGX_ERROR) {
            return n;
        }

        offset += n;
        total += n;

    } while (cl);

    return total;
}