27 Oct
2014
27 Oct
'14
9:35 p.m.
The original UNIX system calls returned error by setting the CARRY ("C") bit on
the PSW.
It was the little wrapper to make them C callable that mapped that to -1. Yes, there was
much cleanup going to
Version 7 to get rid of using -1 with pointers in lieu of the null pointer. The
original argv list was terminated with a -1
as well.
Not that loading from *(int*) 0 was ever a good idea. On the PDP-11, you got
garbage...specifically the first few instructions of the program,
the first of which was "setd" and if you did "printf("%s\n", 0),
you got something like "p&P6" printed out.
Some idiot decided in the split I/D mode (and later on the VAX) to put a ZERO at location
zero. This led to a lot more sloppy coding.
We moved to the Gould SEL machines and like your machine, the default was to get a trap on
*(int*)0 as there was nothing mapped there.
We subsequently added a flag to the a.out format that changed this behavior (called
"braindamaged vax compatibility mode") to map a zero
at zero for the few programs we only had in binary form and we couldn't fix their
errant behavior.
The cast syntax, arguably one of the ickiest parts of C syntax, was
invented in desperation -- we needed the feature, and there was no good
syntax proposal. I blush to admit that the one Dennis chose is one that I
suggested, largely because (unlike some of the others) it was easy to
implement in Yacc.
Yep, the problem was that the book said that a cast was to perform the same operation as
assigning a value to an
invented temporary of the cast type. This of course, didn't fully explain it in the
case of some cast behavior. C++
had to devolve the C cast into FIVE different kinds of cast to make it sane.