Discussion
Studying Linux's code for evaluating format strings...
`vsnprintf()` validates that you didn't give it too large of a buffer & that we've got a valid slice before repeatedly parsing & evaluating format specifiers. It cleans up by returning the nil terminator & returning the length.
It parses a format string by handling precision & width states by updating the state & jumping ahead, setting that state to NONE, scanning ahead to the next "%" or end of string, ...
1/3?
This interpreter's mainloop branching over the result to either:
* Carefully `memcpy()` literals into place
* Possibly-reformats ints into the desired bitwidth before having a helper decode flags, possibly prepend a sign, uses micro-optimized code to output base-8/10*/16 numbers, enforces minimum precision, & assembles the padding, sign, base-prefix, preceding spaces/zeroes, digits, & trailing spaces.
...
3/4?
*Particularly tricky to micro-optimize, its not a power-of-2!
...
* Set the next format-specifier's field width.
* Set its precision.
* Carefully copies a char into the output string with padding.
* Have a helper invoke more in turn to validate & output a given string. The validation checks if the string's out of range while a wrapper outputs the error-name into the returned string. The string-output routine copies at most so many chars over before having a helper add desired padding possibly moving the text over.
...
4/5!
...
* Have a helper parse subsequent format alpha-numeric chars to possibly dispatch formatting for slightly higher-level types (beyond LibC) including dynamic-linking info, far too much of a tangential rabbithole for me to dig into...
* Carefully output "%" into the caller's string.
4.5/4.5 Fin for today! Few, done with these primitive output facilities enabling kernel-devs to do their printf-debugging!
Tomorrow: Configurable architecture & maybe more.
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... returns string literals if appropriate, iterates over subsequent flag chars parsing them into bitflags via a small-int-map, checks if a digit's next & parses that as a width, checks if a "."'s next & parses the following digits as the precision or indicates it should be taken from variadic args, looks up the following datatype-char in a lookuptable, parses following chars according to bitflags, & handles any errors.
2/3?