POSIX link() syscall only takes two arguments (no flags)

The signature is documented as:

   int link(const char *, const char *);

(see https://man7.org/linux/man-pages/man2/link.2.html or https://man.netbsd.org/link.2)

And its not some Linux extension, the [syscall
implementation](21b136cc63/fs/namei.c (L4794-L4797))
only expects two arguments too.

It probably *should* have a flags parameter, but its too late now.

I am a bit surprised that linking glibc or musl against code that invokes
a 'link' with three parameters doesn't fail (at least, I couldn't get any
local test cases to trigger a compile or link error).

The test case in std/posix/test.zig is currently disabled, but if I
manually enable it, it works with this change.

lib/std/c.zig

@@ -9060,7 +9060,7 @@ pub extern "c" fn pwrite(fd: fd_t, buf: [*]const u8, nbyte: usize, offset: off_t
 pub extern "c" fn mmap(addr: ?*align(page_size) anyopaque, len: usize, prot: c_uint, flags: MAP, fd: fd_t, offset: off_t) *anyopaque;
 pub extern "c" fn munmap(addr: *align(page_size) const anyopaque, len: usize) c_int;
 pub extern "c" fn mprotect(addr: *align(page_size) anyopaque, len: usize, prot: c_uint) c_int;
-pub extern "c" fn link(oldpath: [*:0]const u8, newpath: [*:0]const u8, flags: c_int) c_int;
+pub extern "c" fn link(oldpath: [*:0]const u8, newpath: [*:0]const u8) c_int;
 pub extern "c" fn linkat(oldfd: fd_t, oldpath: [*:0]const u8, newfd: fd_t, newpath: [*:0]const u8, flags: c_int) c_int;
 pub extern "c" fn unlink(path: [*:0]const u8) c_int;
 pub extern "c" fn unlinkat(dirfd: fd_t, path: [*:0]const u8, flags: c_uint) c_int;

lib/std/os/linux.zig

@@ -1339,13 +1339,12 @@ pub fn tgkill(tgid: pid_t, tid: pid_t, sig: i32) usize {
     return syscall3(.tgkill, @as(usize, @bitCast(@as(isize, tgid))), @as(usize, @bitCast(@as(isize, tid))), @as(usize, @bitCast(@as(isize, sig))));
 }
 
-pub fn link(oldpath: [*:0]const u8, newpath: [*:0]const u8, flags: i32) usize {
+pub fn link(oldpath: [*:0]const u8, newpath: [*:0]const u8) usize {
     if (@hasField(SYS, "link")) {
-        return syscall3(
+        return syscall2(
             .link,
             @intFromPtr(oldpath),
             @intFromPtr(newpath),
-            @as(usize, @bitCast(@as(isize, flags))),
         );
     } else {
         return syscall5(
@@ -1354,7 +1353,7 @@ pub fn link(oldpath: [*:0]const u8, newpath: [*:0]const u8, flags: i32) usize {
             @intFromPtr(oldpath),
             @as(usize, @bitCast(@as(isize, AT.FDCWD))),
             @intFromPtr(newpath),
-            @as(usize, @bitCast(@as(isize, flags))),
+            0,
         );
     }
 }

lib/std/posix.zig

@@ -2202,11 +2202,11 @@ pub const LinkError = UnexpectedError || error{
 
 /// On WASI, both paths should be encoded as valid UTF-8.
 /// On other platforms, both paths are an opaque sequence of bytes with no particular encoding.
-pub fn linkZ(oldpath: [*:0]const u8, newpath: [*:0]const u8, flags: i32) LinkError!void {
+pub fn linkZ(oldpath: [*:0]const u8, newpath: [*:0]const u8) LinkError!void {
     if (native_os == .wasi and !builtin.link_libc) {
-        return link(mem.sliceTo(oldpath, 0), mem.sliceTo(newpath, 0), flags);
+        return link(mem.sliceTo(oldpath, 0), mem.sliceTo(newpath, 0));
     }
-    switch (errno(system.link(oldpath, newpath, flags))) {
+    switch (errno(system.link(oldpath, newpath))) {
         .SUCCESS => return,
         .ACCES => return error.AccessDenied,
         .DQUOT => return error.DiskQuota,
@@ -2233,16 +2233,16 @@ pub fn linkZ(oldpath: [*:0]const u8, newpath: [*:0]const u8, flags: i32) LinkErr
 
 /// On WASI, both paths should be encoded as valid UTF-8.
 /// On other platforms, both paths are an opaque sequence of bytes with no particular encoding.
-pub fn link(oldpath: []const u8, newpath: []const u8, flags: i32) LinkError!void {
+pub fn link(oldpath: []const u8, newpath: []const u8) LinkError!void {
     if (native_os == .wasi and !builtin.link_libc) {
-        return linkat(wasi.AT.FDCWD, oldpath, wasi.AT.FDCWD, newpath, flags) catch |err| switch (err) {
+        return linkat(wasi.AT.FDCWD, oldpath, wasi.AT.FDCWD, newpath, 0) catch |err| switch (err) {
             error.NotDir => unreachable, // link() does not support directories
             else => |e| return e,
         };
     }
     const old = try toPosixPath(oldpath);
     const new = try toPosixPath(newpath);
-    return try linkZ(&old, &new, flags);
+    return try linkZ(&old, &new);
 }
 
 pub const LinkatError = LinkError || error{NotDir};

lib/std/posix/test.zig

@@ -278,7 +278,7 @@ test "link with relative paths" {
     cwd.deleteFile("new.txt") catch {};
 
     try cwd.writeFile(.{ .sub_path = "example.txt", .data = "example" });
-    try posix.link("example.txt", "new.txt", 0);
+    try posix.link("example.txt", "new.txt");
 
     const efd = try cwd.openFile("example.txt", .{});
     defer efd.close();