a[b] implemented as *(a+b)
Both the C89 and the C99 standard draft contain the following:
> The definition of the subscript operator [] is that E1[E2] is identical to (*((E1)+(E2)))
In fact the expressions a[b] *(a + b) and b[a] are equivalent.
Here is a perfectly valid snippet of C code that will print out 't':
putchar(3["test"]);I imagine there are more optimal and less optimal ways of actually doing the indexing in machine code and the former may be better semantics, but I would think a compiler would generate identical machine code for both.
Edit: it doesn't blow up, not even with -Wall and -std=c99
Nope. For the code snippet I posted an hour ago, even with -pedantic -Wall -Wextra gcc won't issue any warnings. And why should it? It's perfectly standards conformant, because the standard actually defines the [] operator through the equivalent addition expression.
https://en.cppreference.com/w/c/language/storage_duration
It was considered pretty useless by most, so c++11 recycled the keyword to mean something different.
K&R (Second Ed). Makes no mention of the auto keyword in Section 1.10, but it does say,
> Each local variable in a function comes into existence only when the function is called, and disappears when the function is exited. This is why such variables are usually known as automatic [sic] variables[...]
The type is not given at all, I think by default it would be "int".
Yep, this is called the "implicit int" rule, and it was specifically outlawed[1] by C99 and onward.
[1] https://herbsutter.com/2015/04/16/reader-qa-why-was-implicit...
On a PDP-11 int would have been 16-bit. On x86 32 bits. But on x86_64 int is 32 bits but pointers are 64-bit. The easiest way to retain the original assumption with minimal changes to the historical source code while targeting a modern CPU is to compile in 32-bit mode.
† Or maybe some variant of BCPL -- I'm not exactly sure how functionally different the two were.
C was heavily inspired by B and I suspect written in B aswell. Alternatively, BCPL was extremely portable as it compiled to OCode (what we'd recognise today as bytecode) so that might have been another option. The assignment operators of =+ are straight from B and later changed to += due to Dennis Ritchie's personal taste.
From how I read it, it is not capable of bootstrapping itself, and an earlier C compiler in BCPL existed, this is the first C compiler written in C itself.
Because to be the first, it has to be bootstrapped in an intermediate host language… You have to get a parser running, then the syntax, then the etc… etc…
( immense plug of the Ahl book here…)
To be the first complier in a language, as was pointed out, long before I was born, the compiler has to compile itself, so before it could compile itself, it had to have other language processing programs creating the parsing, the syntax, the etc…
Porting it to GCC just means that they could compile it with GCC, the big test is to get it to compile itself, on what ever platform that is the target platform, because finally, if it cannot generate object code/machine language in the target machine’s binary, then its not really ported.
Later on, UNIX came with tools to build compilers with, YACC and LEX.
If they got it to produce PDP-7 Code, its not really much of a port, really.
It wasn't the first C compiler, it was the first self-hosted C compiler, which is different.