if (scanf("%9lu",&len) < 1) barf();  /* >999999999 bytes is bad */
    if (getchar() != ':') barf();
    buf = malloc(len + 1);       /* malloc(0) is not portable */
    if (!buf) barf();
    if (fread(buf,1,len,stdin) < len) barf();
    if (getchar() != ',') barf();

Ah, the wonders of error-handling in C. Also, I wonder what's wrong with

    buf = malloc(len ? len : 1);

Tagged Netstrings (tnetstrings) was a related proposal from 15 years ago or so. It replaces the comma with a single-character type definition so you can do JSON-like objects with a couple of recursive types: you had ',', '#', '^', '!', and '~' for strings, integers, floats, booleans, and nulls, then ']' and '}' for lists and dictionaries.

Most of the links have bitrotted and I don't think it ever got much traction, but I did always like how simple it was. There's a copy someone grabbed of the original spec here: https://raw.githubusercontent.com/ged/tnetstrings.info/refs/...

Seems like a coherent, sensible proposal, as one might expect from djb. Any notable protocols use them?

   s:size:value;

(1997) -DJB