C in Urbit

Under u3 is the simple c3 layer, which is just how we write C in Urbit.

When writing C in u3, please of course follow the conventions of the code around you as regards indentation, etc. It's especially important that every function have a header comment, even if it says nothing interesting.

But some of our idiosyncrasies go beyond convention. Yes, we've done awful things to C. Here's what we did and why we did.

c3: integer types

First, it's generally acknowledged that underspecified integer types are C's worst disaster. C99 fixed this, but the stdint types are wordy and annoying. We've replaced them with:

/* Good integers.
typedef uint64_t c3_d; // double-word
typedef int64_t c3_ds; // signed double-word
typedef uint32_t c3_w; // word
typedef int32_t c3_ws; // signed word
typedef uint16_t c3_s; // short
typedef int16_t c3_ss; // signed short
typedef uint8_t c3_y; // byte
typedef int8_t c3_ys; // signed byte
typedef uint8_t c3_b; // bit
typedef uint8_t c3_t; // boolean
typedef uint8_t c3_o; // loobean
typedef uint8_t c3_g; // 5-bit atom for a 32-bit log.
typedef uint32_t c3_l; // little; 31-bit unsigned integer
typedef uint32_t c3_m; // mote; also c3_l; LSB first a-z 4-char string.
/* Bad integers.
typedef char c3_c; // does not match int8_t or uint8_t
typedef int c3_i; // int - really bad
typedef uintptr_t c3_p; // pointer-length uint - really really bad
typedef intptr_t c3_ps; // pointer-length int - really really bad

Some of these need explanation. A loobean is a Nock boolean - Nock, for mysterious reasons, uses 0 as true (always say "yes") and 1 as false (always say "no").

Nock and/or Hoon cannot tell the difference between a short atom and a long one, but at the u3 level every atom under 2^31 is direct. The c3_l type is useful to annotate this. A c3_m is a mote - a string of up to 4 characters in a c3_l, least significant byte first. A c3_g should be a 5-bit atom. Of course, C cannot enforce these constraints, only document them.

Use the "bad" - ie, poorly specified - integer types only when interfacing with external code that expects them.

An enormous number of motes are defined in i/c/motes.h. There is no reason to delete motes that aren't being used, or even to modularize the definitions. Keep them alphabetical, though.

c3: variables and variable naming

The C3 style uses Hoon style TLV variable names, with a quasi Hungarian syntax. This is weird, but works really well, as long as what you're doing isn't hideously complicated. (Then it works badly, but we shouldn't need anything hideous in u3.)

A TLV variable name is a random pronounceable three-letter string, sometimes with some vague relationship to its meaning, but usually not. Usually CVC (consonant-vowel-consonant) is a good choice.

You should use TLVs much the way math people use Greek letters. The same concept should in general get the same name across different contexts. When you're working in a given area, you'll tend to remember the binding from TLV to concept by sheer power of associative memory. When you come back to it, it's not that hard to relearn. And of course, when in doubt, comment it.

Variables take pseudo-Hungarian suffixes, matching in general the suffix of the integer type:

c3_w wor_w; // 32-bit word

Unlike in standard Hungarian, there is no change for pointer variables. C structure variables take a _u suffix.

c3: loobeans

The code (from defs.h) tells the story:

# define c3y 0
# define c3n 1
# define _(x) (c3y == (x))
# define __(x) ((x) ? c3y : c3n)
# define c3a(x, y) __(_(x) && _(y))
# define c3o(x, y) __(_(x) || _(y))

In short, use _() to turn a loobean into a boolean, __ to go the other way. Use ! as usual, c3y for yes and c3n for no, c3a for and and c3o for or.