diff --git a/lib/std/fs.zig b/lib/std/fs.zig
index 1ec16483f3..e5c2d67d67 100644
--- a/lib/std/fs.zig
+++ b/lib/std/fs.zig
@@ -39,7 +39,7 @@ pub const Watch = @import("fs/watch.zig").Watch;
 /// fit into a UTF-8 encoded array of this length.
 /// The byte count includes room for a null sentinel byte.
 pub const MAX_PATH_BYTES = switch (builtin.os.tag) {
-    .linux, .macos, .ios, .freebsd, .openbsd, .netbsd, .dragonfly, .haiku, .solaris => os.PATH_MAX,
+    .linux, .macos, .ios, .freebsd, .openbsd, .netbsd, .dragonfly, .haiku, .solaris, .plan9 => os.PATH_MAX,
     // Each UTF-16LE character may be expanded to 3 UTF-8 bytes.
     // If it would require 4 UTF-8 bytes, then there would be a surrogate
     // pair in the UTF-16LE, and we (over)account 3 bytes for it that way.
@@ -1160,7 +1160,9 @@ pub const Dir = struct {
             return self.openFileW(path_w.span(), flags);
         }
 
-        var os_flags: u32 = os.O.CLOEXEC;
+        var os_flags: u32 = 0;
+        if (@hasDecl(os.O, "CLOEXEC")) os_flags = os.O.CLOEXEC;
+
         // Use the O locking flags if the os supports them to acquire the lock
         // atomically.
         const has_flock_open_flags = @hasDecl(os.O, "EXLOCK");
@@ -1180,7 +1182,7 @@ pub const Dir = struct {
         if (@hasDecl(os.O, "LARGEFILE")) {
             os_flags |= os.O.LARGEFILE;
         }
-        if (!flags.allow_ctty) {
+        if (@hasDecl(os.O, "NOCTTY") and !flags.allow_ctty) {
             os_flags |= os.O.NOCTTY;
         }
         os_flags |= switch (flags.mode) {
@@ -1196,7 +1198,7 @@ pub const Dir = struct {
 
         // WASI doesn't have os.flock so we intetinally check OS prior to the inner if block
         // since it is not compiltime-known and we need to avoid undefined symbol in Wasm.
-        if (builtin.target.os.tag != .wasi) {
+        if (@hasDecl(os.system, "LOCK") and builtin.target.os.tag != .wasi) {
             if (!has_flock_open_flags and flags.lock != .none) {
                 // TODO: integrate async I/O
                 const lock_nonblocking = if (flags.lock_nonblocking) os.LOCK.NB else @as(i32, 0);
diff --git a/lib/std/os/plan9.zig b/lib/std/os/plan9.zig
index 3e1137c7ce..c65bafddd5 100644
--- a/lib/std/os/plan9.zig
+++ b/lib/std/os/plan9.zig
@@ -1,6 +1,12 @@
 const std = @import("../std.zig");
 const builtin = @import("builtin");
 
+pub const fd_t = i32;
+
+pub const STDIN_FILENO = 0;
+pub const STDOUT_FILENO = 1;
+pub const STDERR_FILENO = 2;
+pub const PATH_MAX = 1023;
 pub const syscall_bits = switch (builtin.cpu.arch) {
     .x86_64 => @import("plan9/x86_64.zig"),
     else => @compileError("more plan9 syscall implementations (needs more inline asm in stage2"),
@@ -57,7 +63,8 @@ pub const SIG = struct {
 };
 pub const sigset_t = c_long;
 pub const empty_sigset = 0;
-pub const siginfo_t = c_long; // TODO plan9 doesn't have sigaction_fn. Sigaction is not a union, but we incude it here to be compatible.
+pub const siginfo_t = c_long;
+// TODO plan9 doesn't have sigaction_fn. Sigaction is not a union, but we incude it here to be compatible.
 pub const Sigaction = extern struct {
     pub const handler_fn = *const fn (c_int) callconv(.C) void;
     pub const sigaction_fn = *const fn (c_int, *const siginfo_t, ?*const anyopaque) callconv(.C) void;
@@ -69,6 +76,9 @@ pub const Sigaction = extern struct {
     mask: sigset_t,
     flags: c_int,
 };
+pub const AT = struct {
+    pub const FDCWD = -100; // we just make up a constant; FDCWD and openat don't actually exist in plan9
+};
 // TODO implement sigaction
 // right now it is just a shim to allow using start.zig code
 pub fn sigaction(sig: u6, noalias act: ?*const Sigaction, noalias oact: ?*Sigaction) usize {
@@ -132,20 +142,47 @@ pub const SYS = enum(usize) {
     _NSEC = 53,
 };
 
-pub fn pwrite(fd: usize, buf: [*]const u8, count: usize, offset: usize) usize {
-    return syscall_bits.syscall4(.PWRITE, fd, @intFromPtr(buf), count, offset);
+pub fn write(fd: i32, buf: [*]const u8, count: usize) usize {
+    return syscall_bits.syscall3(._WRITE, @bitCast(@as(isize, fd)), @intFromPtr(buf), count);
+}
+pub fn pwrite(fd: i32, buf: [*]const u8, count: usize, offset: usize) usize {
+    return syscall_bits.syscall4(.PWRITE, @bitCast(@as(isize, fd)), @intFromPtr(buf), count, offset);
 }
 
-pub fn pread(fd: usize, buf: [*]const u8, count: usize, offset: usize) usize {
-    return syscall_bits.syscall4(.PREAD, fd, @intFromPtr(buf), count, offset);
+pub fn read(fd: i32, buf: [*]const u8, count: usize) usize {
+    return syscall_bits.syscall3(._READ, @bitCast(@as(isize, fd)), @intFromPtr(buf), count);
+}
+pub fn pread(fd: i32, buf: [*]const u8, count: usize, offset: usize) usize {
+    return syscall_bits.syscall4(.PREAD, @bitCast(@as(isize, fd)), @intFromPtr(buf), count, offset);
 }
 
-pub fn open(path: [*:0]const u8, omode: OpenMode) usize {
-    return syscall_bits.syscall2(.OPEN, @intFromPtr(path), @intFromEnum(omode));
+pub fn open(path: [*:0]const u8, omode: mode_t) usize {
+    return syscall_bits.syscall2(.OPEN, @intFromPtr(path), @bitCast(@as(isize, omode)));
 }
 
-pub fn create(path: [*:0]const u8, omode: OpenMode, perms: usize) usize {
-    return syscall_bits.syscall3(.CREATE, @intFromPtr(path), @intFromEnum(omode), perms);
+pub fn openat(dirfd: i32, path: [*:0]const u8, _: u32, omode: mode_t) usize {
+    if (dirfd == AT.FDCWD) { // openat(AT_FDCWD, ...) == open(...)
+        return open(path, omode);
+    }
+    var dir_path_buf: [std.fs.MAX_PATH_BYTES]u8 = undefined;
+    var total_path_buf: [std.fs.MAX_PATH_BYTES + 1]u8 = undefined;
+    const rc = fd2path(dirfd, &dir_path_buf, std.fs.MAX_PATH_BYTES);
+    if (rc != 0) return rc;
+    var fba = std.heap.FixedBufferAllocator.init(&total_path_buf);
+    var alloc = fba.allocator();
+    const dir_path = std.mem.span(@as([*:0]u8, @ptrCast(&dir_path_buf)));
+    const total_path = std.fs.path.join(alloc, &.{ dir_path, std.mem.span(path) }) catch unreachable; // the allocation shouldn't fail because it should not exceed MAX_PATH_BYTES
+    fba.reset();
+    const total_path_z = alloc.dupeZ(u8, total_path) catch unreachable; // should not exceed MAX_PATH_BYTES + 1
+    return open(total_path_z.ptr, omode);
+}
+
+pub fn fd2path(fd: i32, buf: [*]u8, nbuf: usize) usize {
+    return syscall_bits.syscall3(.FD2PATH, @bitCast(@as(isize, fd)), @intFromPtr(buf), nbuf);
+}
+
+pub fn create(path: [*:0]const u8, omode: mode_t, perms: usize) usize {
+    return syscall_bits.syscall3(.CREATE, @intFromPtr(path), @bitCast(@as(isize, omode)), perms);
 }
 
 pub fn exit(status: u8) noreturn {
@@ -163,16 +200,19 @@ pub fn exits(status: ?[*:0]const u8) noreturn {
     unreachable;
 }
 
-pub fn close(fd: usize) usize {
-    return syscall_bits.syscall1(.CLOSE, fd);
+pub fn close(fd: i32) usize {
+    return syscall_bits.syscall1(.CLOSE, @bitCast(@as(isize, fd)));
 }
-pub const OpenMode = enum(usize) {
-    OREAD = 0, //* open for read
-    OWRITE = 1, //* write
-    ORDWR = 2, //* read and write
-    OEXEC = 3, //* execute, == read but check execute permission
-    OTRUNC = 16, //* or'ed in (except for exec), truncate file first
-    OCEXEC = 32, //* or'ed in (per file descriptor), close on exec
-    ORCLOSE = 64, //* or'ed in, remove on close
-    OEXCL = 0x1000, //* or'ed in, exclusive create
+pub const mode_t = i32;
+pub const O = struct {
+    pub const READ = 0; // open for read
+    pub const RDONLY = 0;
+    pub const WRITE = 1; // write
+    pub const WRONLY = 1;
+    pub const RDWR = 2; // read and write
+    pub const EXEC = 3; // execute, == read but check execute permission
+    pub const TRUNC = 16; // or'ed in (except for exec), truncate file first
+    pub const CEXEC = 32; // or'ed in (per file descriptor), close on exec
+    pub const RCLOSE = 64; // or'ed in, remove on close
+    pub const EXCL = 0x1000; // or'ed in, exclusive create
 };
diff --git a/lib/std/os/plan9/errno.zig b/lib/std/os/plan9/errno.zig
index 94197beca3..47a232e67c 100644
--- a/lib/std/os/plan9/errno.zig
+++ b/lib/std/os/plan9/errno.zig
@@ -73,4 +73,12 @@ pub const E = enum(u16) {
     // These added in 1003.1b-1993
     CANCELED = 61,
     INPROGRESS = 62,
+
+    // We just add these to be compatible with std.os, which uses them,
+    // They should never get used.
+    DQUOT,
+    CONNRESET,
+    OVERFLOW,
+    LOOP,
+    TXTBSY,
 };