4.2BSD function m_adj

[walsh at Bbn-Labs-B.ARPA]

From:  Bob Walsh <walsh at Bbn-Labs-B.ARPA>


This funtions does not act properly if it is passed an mbuf chain like:

	   mbuf ------> mbufs of data
	+--------+
	| tcp/ip |
	| header |
	| m_len=0|
	+--------+

and it is asked to trim some number of bytes from the end of the chain.
To correct the code,

*** uipc_mbuf.c	Fri May 11 08:04:03 1984
--- uipc_mbuf.old	Fri May 11 07:55:07 1984
***************
*** 375,383
  	struct mbuf *mp;
  	register int len;
  {
- #ifdef BUGFIXES
- 	register struct mbuf *m;
- #else
  	register struct mbuf *m, *n;
  #endif
  

--- 375,380 -----
  	struct mbuf *mp;
  	register int len;
  {
  	register struct mbuf *m, *n;
  
  	if ((m = mp) == NULL)
***************
*** 379,385
  	register struct mbuf *m;
  #else
  	register struct mbuf *m, *n;
- #endif
  
  	if ((m = mp) == NULL)
  		return;

--- 376,381 -----
  	register int len;
  {
  	register struct mbuf *m, *n;
  
  	if ((m = mp) == NULL)
  		return;
***************
*** 396,432
  			}
  		}
  	} else {
- #ifdef BUGFIXES
- 		register struct mbuf *last, *next;
- 
- 		/* step one, reverse mbuf chain */
- 		/* it is at least one long */
- 		last = 0;
- 		len = (-len);
- 		do {
- 			next = m->m_next;
- 			m->m_next = last;
- 			last = m;
- 			m = next;
- 		} while (m);
- 
- 		/* step two, reverse list and remove bytes */
- 		m = last;
- 		last = 0;
- 		do {
- 			next = m->m_next;
- 			m->m_next = last;
- 			last = m;
- 			if (len > m->m_len){		/* shave */
- 				len -= m->m_len;	/* shave */
- 				m->m_len = 0;		/* shave */
- 			} else {			/* shave */
- 				m->m_len -= len;	/* shave */
- 				len = 0;		/* shave */
- 			}
- 			m = next;
- 		} while (m);
- #else
  		/* a 2 pass algorithm might be better */
  		len = -len;
  		while (len > 0 && m->m_len != 0) {

--- 392,397 -----
  			}
  		}
  	} else {
  		/* a 2 pass algorithm might be better */
  		len = -len;
  		while (len > 0 && m->m_len != 0) {
***************
*** 443,449
  				break;
  			}
  		}
- #endif
  	}
  }
  

--- 408,413 -----
  				break;
  			}
  		}
  	}
  }
  
bob walsh
 complexity".  It clearly was not
worth the effort, was not cost effective, and any really good programmer 
could use scaled integer arithmetic anyway.  They didn't have compilers then
like we have now (most programming was in assembler), and so the argument
about the compiler doing the scaling couldn't be made, but otherwise the
current discussion sounds like a rehash of some of those old debates, with
simple string substitution for the topics).  Those who do not study history
are doomed to repeat it...
				have fun
					joe
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