os: add basic OS functionality

Makefile

@@ -82,7 +82,7 @@ clean:
 	@rm -rf build
 
 fmt:
-	@go fmt . ./compiler ./interp ./loader ./ir ./src/device/arm ./src/examples/* ./src/machine ./src/runtime ./src/sync
+	@go fmt . ./compiler ./interp ./loader ./ir ./src/device/arm ./src/examples/* ./src/machine ./src/os ./src/runtime ./src/sync
 	@go fmt ./testdata/*.go
 
 test:

src/os/file.go

@@ -0,0 +1,40 @@
+// Package os implements a subset of the Go "os" package. See
+// https://godoc.org/os for details.
+//
+// Note that the current implementation is blocking. This limitation should be
+// removed in a future version.
+package os
+
+import (
+	"errors"
+)
+
+// Portable analogs of some common system call errors.
+var (
+	ErrUnsupported = errors.New("operation not supported")
+)
+
+// Stdin, Stdout, and Stderr are open Files pointing to the standard input,
+// standard output, and standard error file descriptors.
+var (
+	Stdin  = &File{0, "/dev/stdin"}
+	Stdout = &File{1, "/dev/stdout"}
+	Stderr = &File{2, "/dev/stderr"}
+)
+
+// File represents an open file descriptor.
+type File struct {
+	fd   uintptr
+	name string
+}
+
+// NewFile returns a new File with the given file descriptor and name.
+func NewFile(fd uintptr, name string) *File {
+	return &File{fd, name}
+}
+
+// Fd returns the integer Unix file descriptor referencing the open file. The
+// file descriptor is valid only until f.Close is called.
+func (f *File) Fd() uintptr {
+	return f.fd
+}

src/os/file_unix.go

@@ -0,0 +1,24 @@
+// +build linux
+
+package os
+
+import (
+	"syscall"
+)
+
+// Read reads up to len(b) bytes from the File. It returns the number of bytes
+// read and any error encountered. At end of file, Read returns 0, io.EOF.
+func (f *File) Read(b []byte) (n int, err error) {
+	return syscall.Read(int(f.fd), b)
+}
+
+// Write writes len(b) bytes to the File. It returns the number of bytes written
+// and an error, if any. Write returns a non-nil error when n != len(b).
+func (f *File) Write(b []byte) (n int, err error) {
+	return syscall.Write(int(f.fd), b)
+}
+
+// Close closes the File, rendering it unusable for I/O.
+func (f *File) Close() error {
+	return syscall.Close(int(f.fd))
+}

src/os/file_wasm.go

@@ -0,0 +1,34 @@
+// +build wasm
+
+package os
+
+import (
+	_ "unsafe"
+)
+
+// Read is unsupported on this system.
+func (f *File) Read(b []byte) (n int, err error) {
+	return 0, ErrUnsupported
+}
+
+// Write writes len(b) bytes to the output. It returns the number of bytes
+// written or an error if this file is not stdout or stderr.
+func (f *File) Write(b []byte) (n int, err error) {
+	switch f.fd {
+	case Stdout.fd, Stderr.fd:
+		for _, c := range b {
+			putchar(c)
+		}
+		return len(b), nil
+	default:
+		return 0, ErrUnsupported
+	}
+}
+
+// Close is unsupported on this system.
+func (f *File) Close() error {
+	return ErrUnsupported
+}
+
+//go:linkname putchar runtime.putchar
+func putchar(c byte)

testdata/stdlib.go

@@ -0,0 +1,12 @@
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	fmt.Println("stdin: ", os.Stdin.Fd())
+	fmt.Println("stdout:", os.Stdout.Fd())
+	fmt.Println("stderr:", os.Stderr.Fd())
+}

testdata/stdlib.txt

@@ -0,0 +1,3 @@
+stdin:  0
+stdout: 1
+stderr: 2