How to use "zd" specifier with `printf()`?

Question

Looking for clarification on using "zd" with printf().

Certainly the following is correct with C99 and later.

void print_size(size_t sz) {
  printf("%zu\n", sz);
}

The C spec seems to allow printf("%zd\n", sz) depending on how it is read:

7.21.6.1 The fprintf function

z Specifies that a following d, i, o, u, x, or X conversion specifier applies to a size_t or the corresponding signed integer type argument; or that a following n conversion specifier applies to a pointer to a signed integer type corresponding to size_t argument. C11dr §7.21.6.1 7

Should this be read as

1. "z Specifies that a following d ... conversion specifier applies to a size_t or the corresponding signed integer type argument ... "(both types) and "z Specifies that a following u ... conversion specifier applies to a size_t or the corresponding signed integer type argument ..." (both types)

OR

2. "z Specifies that a following d ... conversion specifier applies to a corresponding signed integer type argument ..." (signed type only) and "z Specifies that a following u ... conversion specifier applies to a size_t" (unsigned type only).

I've been using the #2 definition, but now not so sure.

Which is correct, 1, 2, or something else?

If #2 is correct, what is an example of a type that can use "%zd"?

Answer 1

printf with a "%zd" format expects an argument of the signed type that corresponds to the unsigned type size_t.

Standard C doesn't provide a name for this type or a good way to determine what it is. If size_t is a typedef for unsigned long, for example, then "%zd" expects an argument of type long, but that's not a portable assumption.

The standard requires that corresponding signed and unsigned types use the same representation for the non-negative values that are representable in both types. A footnote says that this is meant to imply that they're interchangeable as function arguments. So this:

size_t s = 42;
printf("s = %zd\n", s);

should work, and should print "42". It will interpret the value 42, of the unsigned type size_t, as if it were of the corresponding signed type. But there's really no good reason to do that, since "%zu" is also correct and well defined, without resorting to additional language rules. And "%zu" works for all values of type size_t, including those outside the range of the corresponding signed type.

Finally, POSIX defines a type ssize_t in the headers <unistd.h> and <sys/types.h>. Though POSIX doesn't explicitly say so, it's likely that ssize_t will be the signed type corresponding to size_t. So if you're writing POSIX-specific code, "%zd" is (probably) the correct format for printing values of type ssize_t.

UPDATE: POSIX explicitly says that ssize_t isn't necessarily the signed version of size_t, so it's unwise to write code that assumes that it is:

ssize_t

This is intended to be a signed analog of size_t. The wording is such that an implementation may either choose to use a longer type or simply to use the signed version of the type that underlies size_t. All functions that return ssize_t (read() and write()) describe as "implementation-defined" the result of an input exceeding {SSIZE_MAX}. It is recognized that some implementations might have ints that are smaller than size_t. A conforming application would be constrained not to perform I/O in pieces larger than {SSIZE_MAX}, but a conforming application using extensions would be able to use the full range if the implementation provided an extended range, while still having a single type-compatible interface. The symbols size_t and ssize_t are also required in <unistd.h> to minimize the changes needed for calls to read() and write(). Implementors are reminded that it must be possible to include both <sys/types.h> and <unistd.h> in the same program (in either order) without error.