Zig 0.16-dev Porting Notes

This skill captures gotchas hit when porting a 0.15 project to Zig 0.16-dev (std.Io world). APIs keep moving; always check the stdlib sources for your exact build.

Std.Io replaces std.net / legacy std.posix wrappers

• std.net is gone. The new networking lives under std.Io.net (types: IpAddress, Stream, Server, UnixAddress, etc.).
• High-level functions like tcpConnectToHost, Address.parseIp, Stream.connect, Stream.read/writeAll no longer exist. You must use Io vtables (io.vtable.netConnectIp, netRead, netWrite, netListenIp, netAccept).
• If you need raw fds, call platform syscalls (std.os.linux.socket, accept4, connect, bind, listen, write, writev, read, close). Many helpers that were under std.posix are gone.
• posix.socket, posix.bind, posix.accept, posix.shutdown, posix.writev, posix.listen, posix.pipe2, posix.getrandom, std.net.has_unix_sockets, etc. are removed. Use std.os.<platform> syscalls and posix.system.* where available, or the Io vtable.
• Epoll: use std.os.linux.epoll_create1/epoll_ctl/epoll_wait directly; signatures include an explicit count param for writev/epoll_wait.

Std.Io Readers/Writers

• New writer/reader interfaces live under std.Io. TLS expects *std.Io.Reader / *std.Io.Writer (struct fields .interface). Pass pointers to interfaces, not call interface() as a function.
• std.time.sleep removed; use std.Io.sleep(io, Duration, Clock).

Time / Random / Env / Exit

• std.time.milliTimestamp removed. Use std.time.Timer or std.Io.Clock.now(clock, io) and compare Timestamp.nanoseconds.
• Random secure bytes: std.Io.randomSecure(io, buf); no std.crypto.random or std.posix.getrandom convenience.
• std.process.getEnvVarOwned is gone; use std.c.getenv and copy.
• std.posix.exit removed; use std.process.exit.

TLS Client options

• std.crypto.tls.Client.Options now requires entropy: *const [entropy_len]u8 and realtime_now_seconds: i64. Fill entropy with std.Io.randomSecure; compute seconds with Io.Clock.now(.real, io) / ns_per_s.

MemoryPool API changes

• std.heap.MemoryPool(T).initCapacity(allocator, n) returns the pool; create/destroy now require allocator. No bare init() or zero-arg deinit().

Atomic order & Thread

• Atomic orders use .monotonic / .seq_cst; .awake removed.

Format options

• std.fmt.Options replaces FormatOptions.
• std.fmt.format helper is gone; call writer.print directly (writers live in std.Io.Writer).

Randomness / crypto changes

• std.crypto.random was removed. Use an std.Io instance: const io = std.Io.Threaded.global_single_threaded.ioBasic(); io.random(&buf);.
• Ed25519.KeyPair.generate now requires an io: std.Io argument: Ed25519.KeyPair.generate(io);.

Enum conversion

• std.meta.intToEnum removed. Use std.enums.fromInt(EnumType, value) (returns ?EnumType).

Fixed-buffer writers in tests

• std.io.fixedBufferStream was removed. For in-memory writes use var w = std.Io.Writer.fixed(buf); and read bytes with std.Io.Writer.buffered(&w).
• std.ArrayList no longer has .init(allocator) shorthand; use .initBuffer(slice) for stack buffers or managed variants.

Stream/Conn helpers we added (compat pattern)

• Build a thin compat layer that wraps platform syscalls to reintroduce Stream { read, writeAll, close, readAtLeast } and address parsing/formatting, plus tcpConnectToHost/Address using raw sockets.
• For Unix sockets: build sockaddr manually and call connect.

Testing adjustments

• For in-process client/server tests, use socketpair(AF.UNIX, SOCK.STREAM|CLOEXEC, 0, &fds) to avoid relying on Io vtable network (Threaded nets return NetworkDown if not wired).

Error sets tightened

• Many functions return narrower error sets (e.g., Reader.fill no longer includes WouldBlock/Timeout). Remove unreachable branches accordingly.

syscalls return types

• std.os.linux.* return usize; cast with @as(isize, @bitCast(rc)) before checking < 0. For writev/epoll_wait include the count argument.

Use this skill when upgrading 0.15-era code to 0.16-dev to avoid common build-breakers.