RFR: 8357086: os::xxx functions returning memory size should return size_t [v3]

[David Holmes]

On Wed, 28 May 2025 12:41:44 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> I talked to Anton offline about the ptrdiff_t. That type has the correct signedness and number of bits on all our platforms, but to me that type carries a semantic meaning about what pointer diffs / indices. Because of that I found it inappropriate to use that type. 

Those were my thoughts as well when I saw that change made. I don't have a good answer either. It is not at all clear how/why the swap functions were allowed to report errors when none of the others do. It is not documented in os.hpp nor required by the `OperatingSystemMXBean` API!

> we have paths through the container code that returns -2

I couldn't spot these - do you have a link please? It is annoying this prevents use of `ssize_t`.

> An alternative could be to let these functions return a compound value [size_t size, int error_code],

Or, as is done in other code, use an out-parameter e.g:

// Returns the amount of free swap. If an error occurs then `rc` is set to a negative value
// and we return zero.; otherwise `rc` is set to zero.
static size_t free_swap_space(int& rc);

?

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PR Comment: https://git.openjdk.org/jdk/pull/25450#issuecomment-2918462323