diff --git a/proctl/arch.go b/proctl/arch.go
index 62cdcba9..99be3af0 100644
--- a/proctl/arch.go
+++ b/proctl/arch.go
@@ -1,22 +1,45 @@
 package proctl
 
+import "runtime"
+
 type Arch interface {
 	PtrSize() int
 	BreakpointInstruction() []byte
 	BreakpointSize() int
+	CurgInstructions() []byte
 }
 
 type AMD64 struct {
 	ptrSize             int
 	breakInstruction    []byte
 	breakInstructionLen int
+	curgInstructions    []byte
 }
 
 func AMD64Arch() *AMD64 {
+	var (
+		curg       []byte
+		breakInstr = []byte{0xCC}
+	)
+
+	switch runtime.GOOS {
+	case "darwin":
+		curg = []byte{
+			0x65, 0x48, 0x8b, 0x0C, 0x25, 0xA0, 0x08, // mov %gs:0x8a0,%rcx
+			0x0, 0x0,
+		}
+	case "linux":
+		curg = []byte{
+			0x64, 0x48, 0x8b, 0x0c, 0x25, 0xf0, 0xff, 0xff, 0xff, // mov %fs:0xfffffffffffffff0,%rcx
+		}
+	}
+	curg = append(curg, breakInstr[0])
+
 	return &AMD64{
 		ptrSize:             8,
-		breakInstruction:    []byte{0xCC},
+		breakInstruction:    breakInstr,
 		breakInstructionLen: 1,
+		curgInstructions:    curg,
 	}
 }
 
@@ -31,3 +54,7 @@ func (a *AMD64) BreakpointInstruction() []byte {
 func (a *AMD64) BreakpointSize() int {
 	return a.breakInstructionLen
 }
+
+func (a *AMD64) CurgInstructions() []byte {
+	return a.curgInstructions
+}
diff --git a/proctl/proctl.go b/proctl/proctl.go
index ead2e1f5..accbee9c 100644
--- a/proctl/proctl.go
+++ b/proctl/proctl.go
@@ -248,13 +248,13 @@ func (dbp *DebuggedProcess) next() error {
 		return err
 	}
 
-	curg, err := dbp.CurrentThread.curG()
+	g, err := dbp.CurrentThread.getG()
 	if err != nil {
 		return err
 	}
 
-	if curg.DeferPC != 0 {
-		_, err = dbp.TempBreak(curg.DeferPC)
+	if g.DeferPC != 0 {
+		_, err = dbp.TempBreak(g.DeferPC)
 		if err != nil {
 			return err
 		}
@@ -273,7 +273,7 @@ func (dbp *DebuggedProcess) next() error {
 		if err = th.Next(); err != nil {
 			if err, ok := err.(GoroutineExitingError); ok {
 				waitCount = waitCount - 1 + chanRecvCount
-				if err.goid == curg.Id {
+				if err.goid == g.Id {
 					goroutineExiting = true
 				}
 				if err := th.Continue(); err != nil {
@@ -290,12 +290,12 @@ func (dbp *DebuggedProcess) next() error {
 		if err != nil {
 			return err
 		}
-		tg, err := thread.curG()
+		tg, err := thread.getG()
 		if err != nil {
 			return err
 		}
 		// Make sure we're on the same goroutine, unless it has exited.
-		if tg.Id == curg.Id || goroutineExiting {
+		if tg.Id == g.Id || goroutineExiting {
 			if dbp.CurrentThread != thread {
 				dbp.SwitchThread(thread.Id)
 			}
@@ -427,7 +427,7 @@ func (dbp *DebuggedProcess) GoroutinesInfo() ([]*G, error) {
 	allgptr := binary.LittleEndian.Uint64(faddr)
 
 	for i := uint64(0); i < allglen; i++ {
-		g, err := parseG(dbp.CurrentThread, allgptr+(i*uint64(dbp.arch.PtrSize())))
+		g, err := parseG(dbp.CurrentThread, allgptr+(i*uint64(dbp.arch.PtrSize())), true)
 		if err != nil {
 			return nil, err
 		}
@@ -467,10 +467,6 @@ func (dbp *DebuggedProcess) EvalSymbol(name string) (*Variable, error) {
 	return dbp.CurrentThread.EvalSymbol(name)
 }
 
-func (dbp *DebuggedProcess) CallFn(name string, fn func() error) error {
-	return dbp.CurrentThread.CallFn(name, fn)
-}
-
 // Returns a reader for the dwarf data
 func (dbp *DebuggedProcess) DwarfReader() *reader.Reader {
 	return reader.New(dbp.dwarf)
diff --git a/proctl/proctl_linux.go b/proctl/proctl_linux.go
index 9ff87017..93787be9 100644
--- a/proctl/proctl_linux.go
+++ b/proctl/proctl_linux.go
@@ -234,7 +234,6 @@ func (dbp *DebuggedProcess) trapWait(pid int) (*ThreadContext, error) {
 		if ok {
 			th.Status = status
 		}
-
 		if status.Exited() {
 			if wpid == dbp.Pid {
 				dbp.exited = true
@@ -249,30 +248,29 @@ func (dbp *DebuggedProcess) trapWait(pid int) (*ThreadContext, error) {
 			if err != nil {
 				return nil, fmt.Errorf("could not get event message: %s", err)
 			}
-
 			th, err = dbp.addThread(int(cloned), false)
 			if err != nil {
 				return nil, err
 			}
-
-			err = th.Continue()
-			if err != nil {
+			if err = th.Continue(); err != nil {
 				return nil, fmt.Errorf("could not continue new thread %d %s", cloned, err)
 			}
-
-			err = dbp.Threads[int(wpid)].Continue()
-			if err != nil {
-				return nil, fmt.Errorf("could not continue new thread %d %s", cloned, err)
+			if err = dbp.Threads[int(wpid)].Continue(); err != nil {
+				return nil, fmt.Errorf("could not continue existing thread %d %s", cloned, err)
 			}
 			continue
 		}
 		if status.StopSignal() == sys.SIGTRAP {
 			return dbp.handleBreakpointOnThread(wpid)
 		}
+		if status.StopSignal() == sys.SIGTRAP && dbp.halt {
+			return th, nil
+		}
 		if status.StopSignal() == sys.SIGSTOP && dbp.halt {
 			return nil, ManualStopError{}
 		}
 		if th != nil {
+			// TODO(dp) alert user about unexpected signals here.
 			if err := th.Continue(); err != nil {
 				return nil, err
 			}
diff --git a/proctl/proctl_test.go b/proctl/proctl_test.go
index 3b6198fc..f9bc49fd 100644
--- a/proctl/proctl_test.go
+++ b/proctl/proctl_test.go
@@ -411,59 +411,3 @@ func TestSwitchThread(t *testing.T) {
 		}
 	})
 }
-
-func TestFunctionCall(t *testing.T) {
-	withTestProcess("testprog", t, func(p *DebuggedProcess, fixture protest.Fixture) {
-		pc, err := p.FindLocation("main.main")
-		if err != nil {
-			t.Fatal(err)
-		}
-		_, err = p.Break(pc)
-		if err != nil {
-			t.Fatal(err)
-		}
-		err = p.Continue()
-		if err != nil {
-			t.Fatal(err)
-		}
-		pc, err = p.PC()
-		if err != nil {
-			t.Fatal(err)
-		}
-		fn := p.goSymTable.PCToFunc(pc)
-		if fn == nil {
-			t.Fatalf("Could not find func for PC: %#v", pc)
-		}
-		if fn.Name != "main.main" {
-			t.Fatal("Program stopped at incorrect place")
-		}
-		if err = p.CallFn("runtime.getg", func() error {
-			th := p.CurrentThread
-			pc, err := th.PC()
-			if err != nil {
-				t.Fatal(err)
-			}
-			f := th.dbp.goSymTable.LookupFunc("runtime.getg")
-			if f == nil {
-				t.Fatalf("could not find function %s", "runtime.getg")
-			}
-			if pc-1 != f.End-2 && pc != f.End-2 {
-				t.Fatalf("wrong pc expected %#v got %#v", f.End-2, pc-1)
-			}
-			return nil
-		}); err != nil {
-			t.Fatal(err)
-		}
-		pc, err = p.PC()
-		if err != nil {
-			t.Fatal(err)
-		}
-		fn = p.goSymTable.PCToFunc(pc)
-		if fn == nil {
-			t.Fatalf("Could not find func for PC: %#v", pc)
-		}
-		if fn.Name != "main.main" {
-			t.Fatal("Program stopped at incorrect place")
-		}
-	})
-}
diff --git a/proctl/registers.go b/proctl/registers.go
index ab9f2700..522870ca 100644
--- a/proctl/registers.go
+++ b/proctl/registers.go
@@ -9,6 +9,7 @@ import "fmt"
 type Registers interface {
 	PC() uint64
 	SP() uint64
+	CX() uint64
 	SetPC(*ThreadContext, uint64) error
 }
 
diff --git a/proctl/registers_darwin_amd64.go b/proctl/registers_darwin_amd64.go
index 80feb3da..29630277 100644
--- a/proctl/registers_darwin_amd64.go
+++ b/proctl/registers_darwin_amd64.go
@@ -5,7 +5,7 @@ import "C"
 import "fmt"
 
 type Regs struct {
-	pc, sp uint64
+	pc, sp, cx uint64
 }
 
 func (r *Regs) PC() uint64 {
@@ -16,6 +16,10 @@ func (r *Regs) SP() uint64 {
 	return r.sp
 }
 
+func (r *Regs) CX() uint64 {
+	return r.cx
+}
+
 func (r *Regs) SetPC(thread *ThreadContext, pc uint64) error {
 	kret := C.set_pc(thread.os.thread_act, C.uint64_t(pc))
 	if kret != C.KERN_SUCCESS {
@@ -30,6 +34,6 @@ func registers(thread *ThreadContext) (Registers, error) {
 	if kret != C.KERN_SUCCESS {
 		return nil, fmt.Errorf("could not get registers")
 	}
-	regs := &Regs{pc: uint64(state.__rip), sp: uint64(state.__rsp)}
+	regs := &Regs{pc: uint64(state.__rip), sp: uint64(state.__rsp), cx: uint64(state.__rcx)}
 	return regs, nil
 }
diff --git a/proctl/registers_linux_amd64.go b/proctl/registers_linux_amd64.go
index 2a2793b6..0cb0a487 100644
--- a/proctl/registers_linux_amd64.go
+++ b/proctl/registers_linux_amd64.go
@@ -14,6 +14,10 @@ func (r *Regs) SP() uint64 {
 	return r.regs.Rsp
 }
 
+func (r *Regs) CX() uint64 {
+	return r.regs.Rcx
+}
+
 func (r *Regs) SetPC(thread *ThreadContext, pc uint64) error {
 	r.regs.SetPC(pc)
 	return sys.PtraceSetRegs(thread.Id, r.regs)
diff --git a/proctl/threads.go b/proctl/threads.go
index 3840f136..bc991b13 100644
--- a/proctl/threads.go
+++ b/proctl/threads.go
@@ -85,47 +85,6 @@ func (thread *ThreadContext) Step() (err error) {
 	return nil
 }
 
-// Call a function named `name`. This is currently _NOT_ safe.
-func (thread *ThreadContext) CallFn(name string, fn func() error) error {
-	f := thread.dbp.goSymTable.LookupFunc(name)
-	if f == nil {
-		return fmt.Errorf("could not find function %s", name)
-	}
-
-	// Set breakpoint at the end of the function (before it returns).
-	bp, err := thread.TempBreak(f.End - 2)
-	if err != nil {
-		return err
-	}
-	defer thread.dbp.Clear(bp.Addr)
-
-	regs, err := thread.saveRegisters()
-	if err != nil {
-		return err
-	}
-
-	previousFrame := make([]byte, f.FrameSize)
-	frameSize := uintptr(regs.SP() + uint64(f.FrameSize))
-	if _, err := readMemory(thread, frameSize, previousFrame); err != nil {
-		return err
-	}
-	defer func() { writeMemory(thread, frameSize, previousFrame) }()
-
-	if err = thread.SetPC(f.Entry); err != nil {
-		return err
-	}
-	defer thread.restoreRegisters()
-	if err = thread.Continue(); err != nil {
-		return err
-	}
-	th, err := thread.dbp.trapWait(-1)
-	if err != nil {
-		return err
-	}
-	th.CurrentBreakpoint = nil
-	return fn()
-}
-
 // Set breakpoint using this thread.
 func (thread *ThreadContext) Break(addr uint64) (*BreakPoint, error) {
 	return thread.dbp.setBreakpoint(thread.Id, addr, false)
@@ -228,7 +187,7 @@ func (thread *ThreadContext) next(curpc uint64, fde *frame.FrameDescriptionEntry
 	if !covered {
 		fn := thread.dbp.goSymTable.PCToFunc(ret)
 		if fn != nil && fn.Name == "runtime.goexit" {
-			g, err := thread.curG()
+			g, err := thread.getG()
 			if err != nil {
 				return err
 			}
@@ -275,15 +234,77 @@ func (thread *ThreadContext) SetPC(pc uint64) error {
 	return regs.SetPC(thread, pc)
 }
 
-func (thread *ThreadContext) curG() (*G, error) {
-	var g *G
-	err := thread.CallFn("runtime.getg", func() error {
-		regs, err := thread.Registers()
-		if err != nil {
-			return err
+// Returns information on the G (goroutine) that is executing on this thread.
+//
+// The G structure for a thread is stored in thread local memory. Execute instructions
+// that move the *G structure into a CPU register (we use rcx here), and then grab
+// the new registers and parse the G structure.
+//
+// We cannot simply use the allg linked list in order to find the M that represents
+// the given OS thread and follow its G pointer because on Darwin mach ports are not
+// universal, so our port for this thread would not map to the `id` attribute of the M
+// structure. Also, when linked against libc, Go prefers the libc version of clone as
+// opposed to the runtime version. This has the consequence of not setting M.id for
+// any thread, regardless of OS.
+//
+// In order to get around all this craziness, we write the instructions to retrieve the G
+// structure running on this thread (which is stored in thread local memory) into the
+// current instruction stream. The instructions are obviously arch/os dependant, as they
+// vary on how thread local storage is implemented, which MMU register is used and
+// what the offset into thread local storage is.
+func (thread *ThreadContext) getG() (g *G, err error) {
+	var pcInt uint64
+	pcInt, err = thread.PC()
+	if err != nil {
+		return
+	}
+	pc := uintptr(pcInt)
+	// Read original instructions.
+	originalInstructions := make([]byte, len(thread.dbp.arch.CurgInstructions()))
+	if _, err = readMemory(thread, pc, originalInstructions); err != nil {
+		return
+	}
+	// Write new instructions.
+	if _, err = writeMemory(thread, pc, thread.dbp.arch.CurgInstructions()); err != nil {
+		return
+	}
+	// We're going to be intentionally modifying the registers
+	// once we execute the code we inject into the instruction stream,
+	// so save them off here so we can restore them later.
+	if _, err = thread.saveRegisters(); err != nil {
+		return
+	}
+	// Ensure original instructions and PC are both restored.
+	defer func() {
+		// Do not shadow previous error, if there was one.
+		originalErr := err
+		// Restore the original instructions and register contents.
+		if _, err = writeMemory(thread, pc, originalInstructions); err != nil {
+			return
 		}
-		g, err = parseG(thread, regs.SP()+uint64(thread.dbp.arch.PtrSize()))
-		return err
-	})
-	return g, err
+		if err = thread.restoreRegisters(); err != nil {
+			return
+		}
+		err = originalErr
+		return
+	}()
+	// Execute new instructions.
+	if err = thread.resume(); err != nil {
+		return
+	}
+	// Set the halt flag so that trapWait will ignore the fact that
+	// we hit a breakpoint that isn't captured in our list of
+	// known breakpoints.
+	thread.dbp.halt = true
+	defer func(dbp *DebuggedProcess) { dbp.halt = false }(thread.dbp)
+	if _, err = thread.dbp.trapWait(-1); err != nil {
+		return
+	}
+	// Grab *G from RCX.
+	regs, err := thread.Registers()
+	if err != nil {
+		return nil, err
+	}
+	g, err = parseG(thread, regs.CX(), false)
+	return
 }
diff --git a/proctl/threads_darwin.c b/proctl/threads_darwin.c
index 4ae4bfe5..3879ac8e 100644
--- a/proctl/threads_darwin.c
+++ b/proctl/threads_darwin.c
@@ -3,9 +3,6 @@
 int
 write_memory(mach_port_name_t task, mach_vm_address_t addr, void *d, mach_msg_type_number_t len) {
 	kern_return_t kret;
-	pointer_t data;
-	memcpy((void *)&data, d, len);
-
 	vm_region_submap_short_info_data_64_t info;
 	mach_msg_type_number_t count = VM_REGION_SUBMAP_SHORT_INFO_COUNT_64;
 	mach_vm_size_t l = len;
@@ -19,7 +16,7 @@ write_memory(mach_port_name_t task, mach_vm_address_t addr, void *d, mach_msg_ty
 	kret = mach_vm_protect(task, addr, len, FALSE, VM_PROT_WRITE|VM_PROT_COPY|VM_PROT_READ);
 	if (kret != KERN_SUCCESS) return -1;
 
-	kret = mach_vm_write((vm_map_t)task, addr, (vm_offset_t)&data, len);
+	kret = mach_vm_write((vm_map_t)task, addr, (vm_offset_t)d, len);
 	if (kret != KERN_SUCCESS) return -1;
 
 	// Restore virtual memory permissions
diff --git a/proctl/threads_darwin.go b/proctl/threads_darwin.go
index f6aee81b..7cd18453 100644
--- a/proctl/threads_darwin.go
+++ b/proctl/threads_darwin.go
@@ -57,12 +57,14 @@ func (t *ThreadContext) blocked() bool {
 }
 
 func writeMemory(thread *ThreadContext, addr uintptr, data []byte) (int, error) {
+	if len(data) == 0 {
+		return 0, nil
+	}
 	var (
 		vm_data = unsafe.Pointer(&data[0])
 		vm_addr = C.mach_vm_address_t(addr)
 		length  = C.mach_msg_type_number_t(len(data))
 	)
-
 	if ret := C.write_memory(thread.dbp.os.task, vm_addr, vm_data, length); ret < 0 {
 		return 0, fmt.Errorf("could not write memory")
 	}
@@ -70,6 +72,9 @@ func writeMemory(thread *ThreadContext, addr uintptr, data []byte) (int, error)
 }
 
 func readMemory(thread *ThreadContext, addr uintptr, data []byte) (int, error) {
+	if len(data) == 0 {
+		return 0, nil
+	}
 	var (
 		vm_data = unsafe.Pointer(&data[0])
 		vm_addr = C.mach_vm_address_t(addr)
diff --git a/proctl/threads_darwin.h b/proctl/threads_darwin.h
index eb4d6b44..93310f71 100644
--- a/proctl/threads_darwin.h
+++ b/proctl/threads_darwin.h
@@ -1,3 +1,4 @@
+#include <stdlib.h>
 #include <sys/types.h>
 #include <mach/mach.h>
 #include <mach/mach_vm.h>
diff --git a/proctl/threads_linux.go b/proctl/threads_linux.go
index f7e5dca7..aa4dfd05 100644
--- a/proctl/threads_linux.go
+++ b/proctl/threads_linux.go
@@ -62,9 +62,15 @@ func (thread *ThreadContext) restoreRegisters() error {
 }
 
 func writeMemory(thread *ThreadContext, addr uintptr, data []byte) (int, error) {
+	if len(data) == 0 {
+		return 0, nil
+	}
 	return sys.PtracePokeData(thread.Id, addr, data)
 }
 
 func readMemory(thread *ThreadContext, addr uintptr, data []byte) (int, error) {
+	if len(data) == 0 {
+		return 0, nil
+	}
 	return sys.PtracePeekData(thread.Id, addr, data)
 }
diff --git a/proctl/variables.go b/proctl/variables.go
index 93e2c166..36852b02 100644
--- a/proctl/variables.go
+++ b/proctl/variables.go
@@ -79,15 +79,20 @@ func (ng NoGError) Error() string {
 	return fmt.Sprintf("no G executing on thread %d", ng.tid)
 }
 
-func parseG(thread *ThreadContext, addr uint64) (*G, error) {
-	gaddrbytes, err := thread.readMemory(uintptr(addr), thread.dbp.arch.PtrSize())
-	if err != nil {
-		return nil, fmt.Errorf("error derefing *G %s", err)
-	}
-	initialInstructions := append([]byte{op.DW_OP_addr}, gaddrbytes...)
-	gaddr := binary.LittleEndian.Uint64(gaddrbytes)
-	if gaddr == 0 {
-		return nil, NoGError{tid: thread.Id}
+func parseG(thread *ThreadContext, gaddr uint64, deref bool) (*G, error) {
+	initialInstructions := make([]byte, thread.dbp.arch.PtrSize()+1)
+	initialInstructions[0] = op.DW_OP_addr
+	binary.LittleEndian.PutUint64(initialInstructions[1:], gaddr)
+	if deref {
+		gaddrbytes, err := thread.readMemory(uintptr(gaddr), thread.dbp.arch.PtrSize())
+		if err != nil {
+			return nil, fmt.Errorf("error derefing *G %s", err)
+		}
+		initialInstructions = append([]byte{op.DW_OP_addr}, gaddrbytes...)
+		gaddr = binary.LittleEndian.Uint64(gaddrbytes)
+		if gaddr == 0 {
+			return nil, NoGError{tid: thread.Id}
+		}
 	}
 
 	rdr := thread.dbp.DwarfReader()
@@ -97,9 +102,6 @@ func parseG(thread *ThreadContext, addr uint64) (*G, error) {
 		return nil, err
 	}
 
-	// Let's parse all of the members we care about in order so that
-	// we don't have to spend any extra time seeking.
-
 	// Parse defer
 	deferAddr, err := rdr.AddrForMember("_defer", initialInstructions)
 	if err != nil {
@@ -109,7 +111,7 @@ func parseG(thread *ThreadContext, addr uint64) (*G, error) {
 	// Dereference *defer pointer
 	deferAddrBytes, err := thread.readMemory(uintptr(deferAddr), thread.dbp.arch.PtrSize())
 	if err != nil {
-		return nil, fmt.Errorf("error derefing *G %s", err)
+		return nil, fmt.Errorf("error derefing defer %s", err)
 	}
 	if binary.LittleEndian.Uint64(deferAddrBytes) != 0 {
 		initialDeferInstructions := append([]byte{op.DW_OP_addr}, deferAddrBytes...)
@@ -126,11 +128,16 @@ func parseG(thread *ThreadContext, addr uint64) (*G, error) {
 		if err != nil {
 			return nil, err
 		}
-		err = rdr.SeekToEntry(entry)
-		if err != nil {
-			return nil, err
-		}
 	}
+
+	// Let's parse all of the members we care about in order so that
+	// we don't have to spend any extra time seeking.
+
+	err = rdr.SeekToEntry(entry)
+	if err != nil {
+		return nil, err
+	}
+
 	// Parse sched
 	schedAddr, err := rdr.AddrForMember("sched", initialInstructions)
 	if err != nil {
@@ -530,14 +537,14 @@ func (thread *ThreadContext) readString(addr uintptr) (string, error) {
 	// read len
 	val, err := thread.readMemory(addr+uintptr(thread.dbp.arch.PtrSize()), thread.dbp.arch.PtrSize())
 	if err != nil {
-		return "", err
+		return "", fmt.Errorf("could not read string len %s", err)
 	}
 	strlen := int(binary.LittleEndian.Uint64(val))
 
 	// read addr
 	val, err = thread.readMemory(addr, thread.dbp.arch.PtrSize())
 	if err != nil {
-		return "", err
+		return "", fmt.Errorf("could not read string pointer %s", err)
 	}
 	addr = uintptr(binary.LittleEndian.Uint64(val))
 	if addr == 0 {
@@ -546,7 +553,7 @@ func (thread *ThreadContext) readString(addr uintptr) (string, error) {
 
 	val, err = thread.readMemory(addr, strlen)
 	if err != nil {
-		return "", err
+		return "", fmt.Errorf("could not read string at %#v due to %s", addr, err)
 	}
 
 	return *(*string)(unsafe.Pointer(&val)), nil