Why weren't new (bit width specific) printf() format option strings adoped as part of C99?

The C Rationale seems to imply that <inttypes.h> is standardizing existing practice:

<inttypes.h> was derived from the header of the same name found on several existing 64-bit systems.

but the remainder of the text doesn't write about those macros, and I don't remember they were existing practice at the time.

What follows is just speculation, but educated by experience of how standardization committees work.

One advantage of the C99 macros over standardizing additional format specifier for printf (note that C99 also did add some) is that providing <inttypes.h> and <stdint.h> when you already have an implementation supporting the required features in an implementation specific way is just writing two files with adequate typedef and macros. That reduces the cost of making existing implementation conformant, reduces the risk of breaking existing programs which made use of the existing implementation specifics features (the standard way doesn't interfere) and facilitate the porting of conformant programs to implementation who don't have these headers (they can be provided by the program). Additionally, if the implementation specific ways already varied at the time, it doesn't favorize one implementation over another.

Correct, this is how the C99 standard says you should use them. If you want truly portablt code that is 100% standards-conformant to the letter, you should always print an int using "%d" and an int32_t using "%"PRId32.

Most people won't bother, though, since there are very few cases where failure to do so would matter. Unless you're porting your code to Win16 or DOS, you can assume that sizeof(int32_t) <= sizeof(int), so it's harmless to accidentally printf an int32_t as an int. Likewise, a long long is pretty much universally 64 bits (although it is not guaranteed to be so), so printing an int64_t as a long long (e.g. with a %llx specifier) is safe as well.

The types int_fast32_t, int_least32_t, et al are hardly ever used, so you can imagine that their corresponding format specifiers are used even more rarely.

You can always cast upwards and use %jd which is the intmax_t format specifier.

printf("%jd\n", (intmax_t)(-2));

I used intmax_t to show that any intXX_t can be used, but simply casting to long is much better for the int32_t case, then use %ld.