Make port's file descriptor available

go.mod

@@ -0,0 +1,5 @@
+module github.com/jacobsa/go-serial
+
+go 1.17
+
+require golang.org/x/sys v0.0.0-20210923061019-b8560ed6a9b7

go.sum

@@ -0,0 +1,2 @@
+golang.org/x/sys v0.0.0-20210923061019-b8560ed6a9b7 h1:c20P3CcPbopVp2f7099WLOqSNKURf30Z0uq66HpijZY=
+golang.org/x/sys v0.0.0-20210923061019-b8560ed6a9b7/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=

serial/open_darwin.go

@@ -27,11 +27,13 @@ package serial
 
 import (
 	"errors"
-	"io"
+	"fmt"
+	"os"
+	"syscall"
+	"unsafe"
+
+	"golang.org/x/sys/unix"
 )
-import "os"
-import "syscall"
-import "unsafe"
 
 // termios types
 type cc_t byte
@@ -196,7 +198,9 @@ func convertOptions(options OpenOptions) (*termios, error) {
 	return &result, nil
 }
 
-func openInternal(options OpenOptions) (io.ReadWriteCloser, error) {
+type port struct{ *os.File }
+
+func openInternal(options OpenOptions) (*port, error) {
 	// Open the serial port in non-blocking mode, since otherwise the OS will
 	// wait for the CARRIER line to be asserted.
 	file, err :=
@@ -254,5 +258,26 @@ func openInternal(options OpenOptions) (io.ReadWriteCloser, error) {
 	}
 
 	// We're done.
-	return file, nil
+	return &port{file}, nil
+}
+
+func (p *port) Flush(in, out bool) error {
+	var what int
+	if in && out {
+		what = 0x03
+	} else if in {
+		what = 0x01
+	} else if out {
+		what = 0x02
+	} else {
+		return nil
+	}
+	if err := unix.IoctlSetPointerInt(int(p.Fd()), unix.TIOCFLUSH, what); err != nil {
+		return fmt.Errorf("ioctl(TIOCFLUSH) failed: %w", err)
+	}
+	return nil
+}
+
+func (p *port) PortName() string {
+	return p.Name()
 }

serial/open_freebsd.go

@@ -16,6 +16,6 @@ package serial
 
 import "io"
 
-func openInternal(options OpenOptions) (io.ReadWriteCloser, error) {
+func openInternal(options OpenOptions) (Port, error) {
 	return nil, "Not implemented on this OS."
 }

serial/open_linux.go

@@ -2,7 +2,7 @@ package serial
 
 import (
 	"errors"
-	"io"
+	"fmt"
 	"os"
 	"syscall"
 	"unsafe"
@@ -138,7 +138,9 @@ func makeTermios2(options OpenOptions) (*termios2, error) {
 	return t2, nil
 }
 
-func openInternal(options OpenOptions) (io.ReadWriteCloser, error) {
+type port struct{ *os.File }
+
+func openInternal(options OpenOptions) (*port, error) {
 
 	file, openErr :=
 		os.OpenFile(
@@ -205,5 +207,27 @@ func openInternal(options OpenOptions) (io.ReadWriteCloser, error) {
 		}
 	}
 
-	return file, nil
+	return &port{file}, nil
+}
+
+func (p *port) Flush(in, out bool) error {
+	var queueSel int
+	if in && out {
+		queueSel = unix.TCIOFLUSH
+	} else if in {
+		queueSel = unix.TCIFLUSH
+	} else if out {
+		queueSel = unix.TCOFLUSH
+	} else {
+		return nil
+	}
+	// TCFLSH = 0x540B aka tcflush()
+	if err := unix.IoctlSetInt(int(p.Fd()), 0x540B, queueSel); err != nil {
+		return fmt.Errorf("tcflush failed: %w", err)
+	}
+	return nil
+}
+
+func (p *port) PortName() string {
+	return p.Name()
 }

serial/open_windows.go

@@ -16,7 +16,6 @@ package serial
 
 import (
 	"fmt"
-	"io"
 	"os"
 	"sync"
 	"syscall"
@@ -49,7 +48,7 @@ type structTimeouts struct {
 	WriteTotalTimeoutConstant   uint32
 }
 
-func openInternal(options OpenOptions) (io.ReadWriteCloser, error) {
+func openInternal(options OpenOptions) (*serialPort, error) {
 	if len(options.PortName) > 0 && options.PortName[0] != '\\' {
 		options.PortName = "\\\\.\\" + options.PortName
 	}
@@ -139,14 +138,39 @@ func (p *serialPort) Read(buf []byte) (int, error) {
 	return getOverlappedResult(p.fd, p.ro)
 }
 
+func (p *serialPort) Fd() uintptr {
+	return p.f.Fd()
+}
+
+func (p *serialPort) Flush(in, out bool) error {
+	var flags uintptr
+	if in {
+		flags |= 0x0008
+	}
+	if out {
+		flags |= 0x0004
+	}
+	if flags == 0 {
+		return nil
+	}
+
+	// BOOL PurgeComm(HANDLE hFile, DWORD dwFlags)
+	r, _, err := syscall.Syscall(nPurgeComm, 2, p.Fd(), flags, 0)
+	if r == 0 {
+		return fmt.Errorf("PurgeComm failed: %w", err)
+	}
+	return nil
+}
+
 var (
 	nSetCommState,
 	nSetCommTimeouts,
 	nSetCommMask,
 	nSetupComm,
 	nGetOverlappedResult,
 	nCreateEvent,
-	nResetEvent uintptr
+	nResetEvent,
+	nPurgeComm uintptr
 )
 
 func init() {
@@ -163,6 +187,7 @@ func init() {
 	nGetOverlappedResult = getProcAddr(k32, "GetOverlappedResult")
 	nCreateEvent = getProcAddr(k32, "CreateEventW")
 	nResetEvent = getProcAddr(k32, "ResetEvent")
+	nPurgeComm = getProcAddr(k32, "PurgeComm")
 }
 
 func getProcAddr(lib syscall.Handle, name string) uintptr {
@@ -320,3 +345,7 @@ func getOverlappedResult(h syscall.Handle, overlapped *syscall.Overlapped) (int,
 
 	return n, nil
 }
+
+func (p *serialPort) PortName() string {
+	return p.f.Name()
+}

serial/serial.go

@@ -159,8 +159,16 @@ type OpenOptions struct {
 	Rs485DelayRtsAfterSend int
 }
 
-// Open creates an io.ReadWriteCloser based on the supplied options struct.
-func Open(options OpenOptions) (io.ReadWriteCloser, error) {
+// Port defines the serial port.
+type Port interface {
+	io.ReadWriteCloser
+	Fd() uintptr
+	Flush(in, out bool) error
+	PortName() string
+}
+
+// Open creates a Port based on the supplied options struct.
+func Open(options OpenOptions) (Port, error) {
 	// Redirect to the OS-specific function.
 	return openInternal(options)
 }