Re: [PATCHv3 iproute2 1/2] lib/libnetlink: re malloc buff if size is not enough

[Hangbin Liu]

Hi Stephen,
On Wed, Sep 20, 2017 at 09:56:05AM -0700, Stephen Hemminger wrote:
> > +realloc:
> > +   bufp = realloc(buf, buf_len);
> > +
> > +   if (bufp == NULL) {
> 
> Minor personal style issue:
> To me, blank lines are like paragraphs in writing.
> Code reads better assignment and condition check are next to
> each other.

OK, I will remove the blank lines.
> 
> > +recv:
> > +   len = recvmsg(fd, msg, flag);
> > +
> > +   if (len < 0) {
> > +           if (errno == EINTR || errno == EAGAIN)
> > +                   goto recv;
> > +           fprintf(stderr, "netlink receive error %s (%d)\n",
> > +                   strerror(errno), errno);
> > +           free(buf);
> > +           return -errno;
> > +   }
> > +
> > +   if (len == 0) {
> > +           fprintf(stderr, "EOF on netlink\n");
> > +           free(buf);
> > +           return -ENODATA;
> > +   }
> > +
> > +   if (len > buf_len) {
> > +           buf_len = len;
> > +           flag = 0;
> > +           goto realloc;
> > +   }
> > +
> > +   if (flag != 0) {
> > +           flag = 0;
> > +           goto recv;
> 
> Although I programmed in BASIC years ago. I never liked code
> with loops via goto. To me it indicates the logic is not well thought
> through.  Not sure exactly how to rearrange the control flow, but it
> should be possible to rewrite this so that it reads cleaner.

Hmm, if we remove goto. Then the logic should look like

        bufp = realloc(buf, buf_len);
        /* check bufp and set msg */

        len = recvmsg(fd, msg, flag);
        /* check len */

        if (len > buf_len) {
                buf_len = len;
                bufp = realloc(buf, buf_len);
                /* check bufp and set msg */

                len = recvmsg(fd, msg, flag);
                /* check len */
        }

        len = recvmsg(fd, msg, flag);
        /* check len */

Or maybe we can set buf_len very small first. Then it will force to realloc at
the second time. And the code would like

        int buf_len = 16;
        bufp = realloc(buf, buf_len);
        /* check bufp and set msg */

        len = recvmsg(fd, msg, flag);
        /* check len */

        buf_len = len;
        bufp = realloc(buf, buf_len);
        /* check bufp and set msg */

        len = recvmsg(fd, msg, flag);
        /* check len */

What do you think?

Thanks
Hangbin