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
- "
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
- "
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"?
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, presumably 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.
According to the little test I have done, "zd" is always true ,but "zu" don't work for negative numbers.
Test Code:
#include <stdio.h>
int main (void)
{ int i;
size_t uzs = 1;
ssize_t zs = -1;
for ( i= 0; i<5 ;i++, uzs <<= 16,zs <<= 16 )
{
printf ("%zu\t", uzs); /*true*/
printf ("%zd\t", uzs); /*true*/
printf ("%zu\t", zs); /* false*/
printf ("%zd\n", zs); /*true*/
}
return 0;
}
