std.os: add INVALID_SOCKET

[yyny]

This constant returns an invalid socket_t that can be used as a sentinel value.

[daurnimator]

I don't think this belongs here. -1 is not universal for other operating systems.
Also consider that in zig we frequently store file descriptors in unsigned integers.

IMO the name/concept really is windows specific, and hence should continue to live only there.

[yyny]

-1 is not universal for other operating systems.

First of, I don't think this is even true, could you provide a counterexample? Second, any hypothetical socket implementations aside, Zig itself only defines a socket implementation for POSIX systems (And for Windows), and POSIX requires -1 to be an invalid file descriptor (Any negative number, actually, as well as any value above OPEN_MAX, -1 is just the value that is typically returned). Using -1 will be perfectly fine for the foreseeable future.
Moving the definition to the system files (like os/linux.zig) might be a good idea, though.

Also consider that in Zig we frequently store file descriptors in unsigned integers.

Storing INVALID_SOCKET in an unsigned integer would be user error, it also contradicts your earlier concern, if all file descriptors can universally be assumed to be unsigned, then -1 is by definition an invalid file descriptor value. It won't even compile anyway, since INVALID_SOCKET has type socket_t, which is signed. And even if it was unsigned, a different value could be chosen. I don't think this concern is relevant.

IMO the name/concept really is windows specific, and hence should continue to live only there.

The concept is not Windows specific, POSIX also defines what constitutes an invalid socket/file descriptor value, and as mentioned above it is the only other socket implementation Zig currently supports.
This is exactly why I would like this change, the concept is universal, I need a value for it, and could use a portable constant for this in the standard library so I don't have to define it myself.

[kprotty]

Are there any use cases of -1 in the stdlib that could be replaced by INVALID_SOCKET before it gets merged?

[arBmind]

@kprotty Are there any use cases of -1 in the stdlib that could be replaced by INVALID_SOCKET before it gets merged?

After a check of all relevant zig files in the std library, I found no use of -1 to initialize a socket_t.

We have some places where we use -1 to initialize a fd_t, that someone might want to inspect deeper (possibly in a different issue)

• zig/lib/std/child_process.zig

  Line 258 in cff5d9c

  poll_fds[0].fd = -1;

• zig/lib/std/child_process.zig

  Line 262 in cff5d9c

  poll_fds[1].fd = -1;

• some calls to std.os.mmap
• and quite a lot in io_uring.zig

Maybe io_uring.zig is using the fd_t because for linux it's the same as socket_t? I have not enough experience with io_uring to decide this.