A cursed feature of C in 1972: Labels and functions were reassignable (i.e., lvalues)!
For example, this is a clever way to initialize once:
goto init;
init:
ouptr = oubuf;
init = init1;
init1:
which is compiled to:
jmp *4120
mov 4136,4144
mov 4122,4120
Note the indirect jump and assignment to that address. All gotos used indirect jumps. This apparently would have also worked with functions.
At the time, C was rapidly evolving into what we recognize today.
It started as B, an untyped and interpreted language that only had one kind of value, a word. This was a perfect fit for the PDP-7 that UNIX started on, with 18-bit words, but as they transitioned to the PDP-11, with 16-bit words and 8-bit addressing, this became a limitation.
Soon, types were added, to distinguish char, int, and pointers, and it became known as NB ("New B"). But, B wasn't particularly fast, as it was interpreted (well, threaded).
Once it was rewritten to be compiled, the language became known as C (perhaps initially as NC). For a short while, everything in C was an lvalue like B, giving the above snippet, but this was dropped a few months later, presumably for efficiency.
Some UNIX utilities had been written in B/C from the start, but efforts to rewrite the kernel itself in B/C had failed. Finally, once structs were added to C, it was powerful enough to support the kernel and it was rewritten in C over the summer of 1973, culminating in the release of UNIX V4.
This snippet appears in cvft, a compiler for translating Fortran threaded code to machine code from June 1972, which is notably derived from the early C code generator. See putchar (and also getcha) in dmr/cgd/cvft.c.
The earliest extant C compiler is last1120c from July 1972, the last C version for the PDP-11/20, before they migrated to the PDP-11/45. This version still has the label lvalue behavior of B seen in cvft. Then, it was changed to the modern behavior by the time of prestruct-c from December 1972. That version supports structures, but does not yet use them itself.
All three can be found in Dennis_Tapes: https://www.tuhs.org/Archive/Applications/Dennis_Tapes
Another strange pattern from the same program.
This one reassigns the address of an array, `int nlist[250]`, in char increments. Arrays are no longer lvalues, so this doesn't work anymore. Also, the address is unaligned every other iteration.
lbp;
nlist[250];
getnam()
{
extern nlist, lbp;
char nlist[], lbp[], c;
loop:
c = *lbp++;
if (c==';' | c=='\n')
goto el;
*nlist++ = c;
goto loop;
el:
*nlist++ = '\0';
}
Somewhat simplified from Dennis_Tapes/dmr/cgd/cg1.c:getnam.
This snippet deliberately triggers a "Bus error -- Core dumped":
int o1[];
o1 = -3;
*o1;
From Dennis_Tapes/dmr/cgd/cg1.c:expr.
@thalia wrote:
«This snippet deliberately triggers a "Bus error -- Core dumped":
int o1[];
o1 = -3;
*o1;»
Apart from the array variable not being a constant in those days,
isn't this essentially the same as
* ((int *) -3);
(with platform-dependent behavior, of course)?
Thalia Archibald
aap
Vassil Nikolov | Васил Николов