What is the purpose of the h and hh modifiers for printf?
c, format-specifiers, printf, promotions, variadic-functions
Solution
One possible reason: for symmetry with the use of those modifiers in the formatted input functions? I know it wouldn't be strictly necessary, but maybe there was value seen for that?
Although they don't mention the importance of symmetry for the "h" and "hh" modifiers in the C99 Rationale document, the committee does mention it as a consideration for why the "%p" conversion specifier is supported for `fscanf()` (even though that wasn't new for C99 - "%p" support is in C90):
Input pointer conversion with %p was added to C89, although it is obviously risky, for symmetry with fprintf.
In the section on `fprintf()`, the C99 rationale document does discuss that "hh" was added, but merely refers the reader to the `fscanf()` section:
The %hh and %ll length modifiers were added in C99 (see §7.19.6.2).
I know it's a tenuous thread, but I'm speculating anyway, so I figured I'd give whatever argument there might be.
Also, for completeness, the "h" modifier was in the original C89 standard - presumably it would be there even if it wasn't strictly necessary because of widespread existing use, even if there might not have been a technical requirement to use the modifier.
Problem
Aside from `%hn` and `%hhn` (where the `h` or `hh` specifies the size of the pointed-to object), what is the point of the `h` and `hh` modifiers for `printf` format specifiers? Due to default promotions which are required by the standard to be applied for variadic functions, it is impossible to pass arguments of type `char` or `short` (or any signed/unsigned variants thereof) to `printf`. According to 7.19.6.1(7), the `h` modifier: Specifies that a following d, i, o, u, x, or X conversion specifier applies to a short int or unsigned short int argument (the argument will have been promoted according to the integer promotions, but its value shall be converted to short int or unsigned short int before printing); or that a following n conversion specifier applies to a pointer to a short int argument. If the argument was actually of type `short` or `unsigned short`, then promotion to `int` followed by a conversion back to `short` or `unsigned short` will yield the same value as promotion to `int` without any conversion back. Thus, for arguments of type `short` or `unsigned short`, `%d`, `%u`, etc. should give identical results to `%hd`, `%hu`, etc. (and likewise for `char` types and `hh`). As far as I can tell, the only situation where the `h` or `hh` modifier could possibly be useful is when the argument passed it an `int` outside the range of `short` or `unsigned short`, e.g. ``` printf("%hu", 0x10000); ``` but my understanding is that passing the wrong type like this results in undefined behavior anyway, so that you could not expect it to print 0. One real world case I've seen is code like this: ``` char c = 0xf0; printf("%hhx", c); ``` where the author expects it to print `f0` despite the implementation having a plain `char` type that's signed (in which case, `printf("%x", c)` would print `fffffff0` or similar). But is this expectation warranted? (Note: What's going on is that the original type was `char`, which gets promoted to `int` and converted back to `unsigned char` instead of `char`, thus changing the value that gets printed. But does the standard specify this behavior, or is it an implementation detail that broken software might be relying on?)