os/linux: add posix_fadvise

Author: vrischmann

lib/std/os/linux.zig

@@ -1386,6 +1386,62 @@ pub fn madvise(address: [*]u8, len: usize, advice: u32) usize {
     return syscall3(.madvise, @ptrToInt(address), len, advice);
 }
 
+pub fn fadvise(fd: fd_t, offset: u64, len: u64, advice: usize) usize {
+    if (comptime std.Target.current.cpu.arch.isMIPS()) {
+        // MIPS requires a 7 argument syscall
+
+        const offset_halves = splitValue64(@bitCast(u64, offset));
+        const length_halves = splitValue64(@bitCast(u64, len));
+
+        return syscall7(
+            .fadvise64,
+            @bitCast(usize, @as(isize, fd)),
+            0,
+            offset_halves[0],
+            offset_halves[1],
+            length_halves[0],
+            length_halves[1],
+            advice,
+        );
+    } else if (comptime std.Target.current.cpu.arch.isARM()) {
+        // ARM reorders the arguments
+
+        const offset_halves = splitValue64(@bitCast(u64, offset));
+        const length_halves = splitValue64(@bitCast(u64, len));
+
+        return syscall6(
+            .fadvise64_64,
+            @bitCast(usize, @as(isize, fd)),
+            advice,
+            offset_halves[0],
+            offset_halves[1],
+            length_halves[0],
+            length_halves[1],
+        );
+    } else if (@hasField(SYS, "fadvise64_64")) {
+        const offset_halves = splitValue64(@bitCast(u64, offset));
+        const length_halves = splitValue64(@bitCast(u64, len));
+
+        return syscall6(
+            .fadvise64_64,
+            @bitCast(usize, @as(isize, fd)),
+            offset_halves[0],
+            offset_halves[1],
+            length_halves[0],
+            length_halves[1],
+            advice,
+        );
+    } else {
+        return syscall4(
+            .fadvise64,
+            @bitCast(usize, @as(isize, fd)),
+            @bitCast(usize, offset),
+            @bitCast(usize, len),
+            advice,
+        );
+    }
+}
+
 test {
     if (builtin.os.tag == .linux) {
         _ = @import("linux/test.zig");

lib/std/os/linux/test.zig

@@ -108,3 +108,24 @@ test "user and group ids" {
     expectEqual(linux.getauxval(elf.AT_EUID), linux.geteuid());
     expectEqual(linux.getauxval(elf.AT_EGID), linux.getegid());
 }
+
+test "posix_fadvise" {
+    const tmp_file_name = "temp_posix_fadvise.txt";
+    var file = try fs.cwd().createFile(tmp_file_name, .{});
+    defer {
+        file.close();
+        fs.cwd().deleteFile(tmp_file_name) catch {};
+    }
+
+    var buf: [2048]u8 = undefined;
+    _ = linux.getrandom(&buf, buf.len, 0);
+    try file.writeAll(&buf);
+
+    const ret = linux.fadvise(
+        file.handle,
+        0,
+        0,
+        linux.POSIX_FADV_SEQUENTIAL,
+    );
+    expectEqual(@as(usize, 0), ret);
+}