std.os: add getDefaultPageSize() with smoke tests

The Kernel provides each program with the default page size, which this
function reads.

Reproduce added abi bits with:
git clone --depth=1 https://github.com/DragonFlyBSD/DragonFlyBSD
git clone --depth=1 https://git.FreeBSD.org/src.git freebsd
git clone --depth=1 https://fuchsia.googlesource.com/fuchsia
git clone --depth=1 https://github.com/haiku/haiku
git clone --depth=1 https://github.com/Stichting-MINIX-Research-Foundation/minix/
git clone --depth=1 https://github.com/NetBSD/src netbsd
git clone --depth=1 https://github.com/openbsd/src openbsd
git clone --depth=1 https://github.com/kofemann/opensolaris
rg 'define.*_SC_PAGE_SIZE' -B 1 --vimgrep

Closes #11308.

lib/std/c.zig

@@ -78,6 +78,7 @@ pub usingnamespace switch (builtin.os.tag) {
 
         pub extern "c" fn getrusage(who: c_int, usage: *c.rusage) c_int;
 
+        pub extern "c" fn sysconf(sc: c_int) c_long;
         pub extern "c" fn sched_yield() c_int;
 
         pub extern "c" fn sigaction(sig: c_int, noalias act: ?*const c.Sigaction, noalias oact: ?*c.Sigaction) c_int;

lib/std/c/dragonfly.zig

@@ -9,6 +9,10 @@ pub fn _errno() *c_int {
     return &errno;
 }
 
+pub const _SC = struct {
+    pub const PAGESIZE = 47;
+};
+
 pub extern "c" fn getdents(fd: c_int, buf_ptr: [*]u8, nbytes: usize) c_int;
 pub extern "c" fn sigaltstack(ss: ?*stack_t, old_ss: ?*stack_t) c_int;
 pub extern "c" fn getrandom(buf_ptr: [*]u8, buf_len: usize, flags: c_uint) isize;

lib/std/c/freebsd.zig

@@ -8,6 +8,10 @@ const iovec_const = std.os.iovec_const;
 extern "c" fn __error() *c_int;
 pub const _errno = __error;
 
+pub const _SC = struct {
+    pub const PAGESIZE = 47;
+};
+
 pub extern "c" fn getdents(fd: c_int, buf_ptr: [*]u8, nbytes: usize) isize;
 pub extern "c" fn sigaltstack(ss: ?*stack_t, old_ss: ?*stack_t) c_int;
 pub extern "c" fn getrandom(buf_ptr: [*]u8, buf_len: usize, flags: c_uint) isize;

lib/std/c/fuchsia.zig

@@ -1,3 +1,8 @@
+// third party ulib musl
+pub const _SC = struct {
+    pub const PAGESIZE = 30;
+};
+
 pub const pthread_mutex_t = extern struct {
     size: [__SIZEOF_PTHREAD_MUTEX_T]u8 align(@alignOf(usize)) = [_]u8{0} ** __SIZEOF_PTHREAD_MUTEX_T,
 };

lib/std/c/haiku.zig

@@ -9,6 +9,10 @@ extern "c" fn _errnop() *c_int;
 
 pub const _errno = _errnop;
 
+pub const _SC = struct {
+    pub const PAGESIZE = 27;
+};
+
 pub extern "c" fn find_directory(which: c_int, volume: i32, createIt: bool, path_ptr: [*]u8, length: i32) u64;
 
 pub extern "c" fn find_thread(thread_name: ?*anyopaque) i32;

lib/std/c/hermit.zig

@@ -1,6 +1,10 @@
 const std = @import("std");
 const maxInt = std.math.maxInt;
 
+// pub const _SC = struct {
+//     pub const PAGESIZE = TODO;
+// };
+
 pub const pthread_mutex_t = extern struct {
     inner: usize = ~@as(usize, 0),
 };

lib/std/c/linux.zig

@@ -105,6 +105,10 @@ pub const user_desc = linux.user_desc;
 pub const utsname = linux.utsname;
 pub const PR = linux.PR;
 
+pub const _SC = struct {
+    pub const PAGESIZE = 30;
+};
+
 pub const _errno = switch (native_abi) {
     .android => struct {
         extern fn __errno() *c_int;

lib/std/c/minix.zig

@@ -1,4 +1,9 @@
 const builtin = @import("builtin");
+
+pub const _SC = struct {
+    pub const PAGESIZE = 28;
+};
+
 pub const pthread_mutex_t = extern struct {
     size: [__SIZEOF_PTHREAD_MUTEX_T]u8 align(@alignOf(usize)) = [_]u8{0} ** __SIZEOF_PTHREAD_MUTEX_T,
 };

lib/std/c/netbsd.zig

@@ -10,6 +10,10 @@ const rusage = std.c.rusage;
 extern "c" fn __errno() *c_int;
 pub const _errno = __errno;
 
+pub const _SC = struct {
+    pub const PAGESIZE = 28;
+};
+
 pub const dl_iterate_phdr_callback = *const fn (info: *dl_phdr_info, size: usize, data: ?*anyopaque) callconv(.C) c_int;
 pub extern "c" fn dl_iterate_phdr(callback: dl_iterate_phdr_callback, data: ?*anyopaque) c_int;
 

lib/std/c/openbsd.zig

@@ -8,6 +8,10 @@ const iovec_const = std.os.iovec_const;
 extern "c" fn __errno() *c_int;
 pub const _errno = __errno;
 
+pub const _SC = struct {
+    pub const PAGESIZE = 28;
+};
+
 pub const dl_iterate_phdr_callback = *const fn (info: *dl_phdr_info, size: usize, data: ?*anyopaque) callconv(.C) c_int;
 pub extern "c" fn dl_iterate_phdr(callback: dl_iterate_phdr_callback, data: ?*anyopaque) c_int;
 

lib/std/c/solaris.zig

@@ -9,6 +9,11 @@ const timezone = std.c.timezone;
 extern "c" fn ___errno() *c_int;
 pub const _errno = ___errno;
 
+pub const _SC = struct {
+    pub const PAGESIZE = 11;
+    pub const NPROCESSORS_ONLN = 15;
+};
+
 pub const dl_iterate_phdr_callback = *const fn (info: *dl_phdr_info, size: usize, data: ?*anyopaque) callconv(.C) c_int;
 pub extern "c" fn dl_iterate_phdr(callback: dl_iterate_phdr_callback, data: ?*anyopaque) c_int;
 
@@ -17,7 +22,6 @@ pub extern "c" fn sigaltstack(ss: ?*stack_t, old_ss: ?*stack_t) c_int;
 pub extern "c" fn pipe2(fds: *[2]fd_t, flags: u32) c_int;
 pub extern "c" fn arc4random_buf(buf: [*]u8, len: usize) void;
 pub extern "c" fn posix_memalign(memptr: *?*anyopaque, alignment: usize, size: usize) c_int;
-pub extern "c" fn sysconf(sc: c_int) i64;
 pub extern "c" fn signalfd(fd: fd_t, mask: *const sigset_t, flags: u32) c_int;
 pub extern "c" fn madvise(address: [*]u8, len: usize, advise: u32) c_int;
 
@@ -1684,11 +1688,6 @@ pub const AF_SUN = struct {
     pub const NOPLM = 0x00000004;
 };
 
-// TODO: Add sysconf numbers when the other OSs do.
-pub const _SC = struct {
-    pub const NPROCESSORS_ONLN = 15;
-};
-
 pub const procfs = struct {
     pub const misc_header = extern struct {
         size: u32,

lib/std/os.zig

@@ -4338,6 +4338,45 @@ pub fn fork() ForkError!pid_t {
     }
 }
 
+/// Returns page size assigned by the Kernel to the process. This size is
+/// constant during process lifetime, but other page sizes might be usable.
+pub fn getDefaultPageSize() error{UnknownPageSize}!usize {
+    switch (builtin.cpu.arch) { // archs with fixed size page size
+        .emscripten, .wasm32, .wasm64 => 64 * 1024,
+        else => {},
+    }
+
+    switch (builtin.os.tag) { // kernel chooses the page size
+        .linux => {
+            if (builtin.link_libc) {
+                return @bitCast(std.c.sysconf(std.c._SC.PAGESIZE));
+            } else {
+                return linux.getauxval(std.elf.AT_PAGESZ);
+            }
+        },
+        .windows => {
+            var sbi: std.os.windows.SYSTEM_BASIC_INFORMATION = undefined;
+            const rc = std.os.windows.ntdll.NtQuerySystemInformation(
+                .SystemBasicInformation,
+                &sbi,
+                @sizeOf(std.os.windows.SYSTEM_BASIC_INFORMATION),
+                null,
+            );
+            if (rc != .SUCCESS) {
+                return error.UnknownPageSize;
+            }
+            return sbi.PageSize;
+        },
+        .macos, .ios, .watchos, .tvos => return @bitCast(darwin.getPageSize() catch unreachable),
+        // zig fmt: off
+        .dragonfly, .freebsd, .fuchsia, haiku,
+        .minix, netbsd, openbsd, solaris, => return std.c.sysconf(std.c._SC.PAGESIZE),
+        // TODO .hermit
+        // zig fmt: on
+        else => unreachable,
+    }
+}
+
 pub const MMapError = error{
     /// The underlying filesystem of the specified file does not support memory mapping.
     MemoryMappingNotSupported,

lib/std/os/test.zig

@@ -1219,3 +1219,14 @@ test "fchmodat smoke test" {
     const st = try os.fstatat(tmp.dir.fd, "foo.txt", 0);
     try expectEqual(@as(os.mode_t, 0o755), st.mode & 0b111_111_111);
 }
+
+test "getDefaultPageSize smoke test" {
+    const page_size = os.getDefaultPageSize();
+    switch (page_size) {
+        // zig fmt: off
+        1024, 4096, 8192, 16384, 16384, 32768, // 1, 4, 8, 16, 32KB
+        2097152, 4194304 => {}, // 2, 4MB
+        // zig fmt: on
+        else => return error.InvalidDefaultPageSize,
+    }
+}