Shafik Yaghmour: ""C Portability Lessons from We…" - Hachyderm.io

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Shafik Yaghmour @shafik@hachyderm.io

"C Portability Lessons from Weird Machines": https://begriffs.com/posts/2018-11-15-c-portability.html

In this article we’ll go on a journey from 4-bit microcontrollers to room-sized mainframes and learn how porting C to each of them helped people separate the essence of the language from the environment of its birth. I’ve found technical manuals and videos for this article to help bring each computer to life.

It’s amazing that, by carefully writing portable ANSI C code and sticking to standard library functions, you can create a program that will compile and work without modification on almost any of these weird systems.

Cover of Portable C

I hope that being exposed to these examples will help you write code more portably, and dispel the belief that current computers with their multiple cores, cache hierarchy, and pipelining, are somehow too alien for C. A language tough enough to handle the diversity of old machines is tough enough to handle today’s relatively homogeneous CPUs.

To prepare this article I worked backward from the book “Portable C” (by Henry Rabinowitz), searching for architectures that illustrate each of the pitfalls he points out. You should read the book for a great explanation of what the author calls “C-World,” a semantic model of

The first unusual thing about the architecture is its word size. You may be familiar with datatypes having powers-of-two bit sizes, but these Unisys series went with multiples of 9! The word size is 18 bits, and the C compiler for the platform uses:

char — 9
short — 18
int — 36
long — 36
long long — 72

(The Honeywell 6000 was another machine with 9-bit char and 36-bit word.)

Just to make matters more interesting, the oddly sized integers use ones’ complement binary arithmetic. That’s right, in this system there are distinct values for positive and negative zero. (CDC computers also used ones’ complement.)

This machine uses a different numbering scheme for character- and integer-pointers. The same location in memory must be referred to by different addresses depending on the pointer type. A cast between char* and int* actually changes the address inside the pointer. Chris Torek recounts the details: http://c-faq.com/null/wierdptrs.ct.html

Dec 01, 2023, 12:19 AM··Web

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Shafik Yaghmour @shafik

Longer thread here: https://hachyderm.io/@shafik/111500351928261812

Ólafur Waage @olafurw@mastodon.social

@shafik shouldn't make threads run for too long though

Shafik Yaghmour @shafik

@olafurw stop trying to prematurely optimize my toots

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