[TUHS] A few comments on porting the Bourne shell

[segaloco via TUHS]

I'll have to double check later but I'm fairly certain the remaining L/R cheats are gone by SysV.  From what I can tell much of that portability work may have been done prior to the V7 release code base we're familiar with, as I did some comparison and found only one significant change between V7 and 32V code as I know it at least.  Either the claims of portability issues came between 32V and System III (meaning the shell was accepted as "broken"? in 32V) or the code we actually see in V7 has already been tidied up significantly and doesn't represent the "non-portable" version lamented in the famous quote.  Does this observation hold with reality?  Is there an earlier, more PDP-11 bound version of the Bourne Shell out there?  I seem to recall reading something about some bits of it even being in assembly at one point, but can't remember the quote source.

- Matt G.

------- Original Message -------
On Friday, December 30th, 2022 at 10:25 AM, Paul Ruizendaal <pnr at planet.nl> wrote:


> London and Reiser report about porting the shell that “it required by far the largest conversion effort of any supposedly portable program, for the simple reason that it is not portable.” By the time of SysIII this is greatly improved, but also in porting the SysIII user land it was the most complex of the set so far.
> 
> There were three aspects that I found noteworthy:
> 
> 1. London/Reiser apparently felt strongly about a property of casts. The code argues that casting an l-value should not convert it into a r-value:
> 
> <quote from "mode.h">
> 
> /* the following nonsense is required
> * because casts turn an Lvalue
> * into an Rvalue so two cheats
> * are necessary, one for each context.
> */
> union { int _cheat;};
> #define Lcheat(a) ((a)._cheat)
> #define Rcheat(a) ((int)(a))
> <endquote>
> 
> 
> However, Lcheat is only used in two places (in service.c), to set and to clear a flag in a pointer. Interestingly, the 32V code already replaces one of these instances with a regular r-value cast. So far, I’d never thought about this aspect of casts. I stumbled across it, because the Plan 9 compiler did not accept the Lcheat expansion as valid C.
> 
> 2. On the history of dup2
> 
> The shell code includes the following:
> 
> <quote from “io.c”>
> 
> rename(f1,f2)
> REG INT f1, f2;
> {
> #ifdef RES /* research has different sys calls from TS */
> IF f1!=f2
> THEN dup(f1|DUPFLG, f2);
> close(f1);
> IF f2==0 THEN ioset|=1 FI
> FI
> #else
> INT fs;
> IF f1!=f2
> THEN fs = fcntl(f2,1,0);
> close(f2);
> fcntl(f1,0,f2);
> close(f1);
> IF fs==1 THEN fcntl(f2,2,1) FI
> IF f2==0 THEN ioset|=1 FI
> FI
> #endif
> }
> <endquote>
> 
> 
> I’ve check the 8th edition source, and indeed it supports using DUPFLG to signal to dup() that it really is dup2(). I had earlier wondered why dup2() did not appear in research until 10th edition, but now that is clear. It would seem that the dup of 8th edition is a direct ancestor to dup() in Plan 9. I wonder why this way of doing things never caught on in the other Unices.
> 
> 3. Halfway to demand paging
> 
> I stumbled across this one because I had a bug in my signal handling. From early days onwards, Unix supported dynamically growing the stack allocation, which arguably is a first step towards building the mechanisms for demand paging. It appears that the Bourne shell made another step, catching page faults and expanding the data/bss allocation dynamically:
> 
> <quote from “fault.c”>
> 
> VOID fault(sig)
> REG INT sig;
> {
> signal(sig, fault);
> IF sig==MEMF
> THEN IF setbrk(brkincr) == -1
> THEN error(nospace);
> FI
> ELIF ...
> <endquote>
> 
> 
> This was already present in 7th edition, so it is by no means new in 32V or SysIII -- it had just escaped my attention as a conceptual step in the development of Unix memory handling.
>