diff --git a/src/lib/Makefile b/src/lib/Makefile
index a6b0c4782b7..780aa3a4311 100644
--- a/src/lib/Makefile
+++ b/src/lib/Makefile
@@ -7,8 +7,8 @@ all: install
 GC=6g
 
 DIRS=\
-	container/array\
 	container\
+	container/array\
 	fmt\
 	hash\
 	http\
@@ -26,7 +26,9 @@ DIRS=\
 	unicode\
 
 FILES=\
+	bignum\
 	bufio\
+	exec\
 	flag\
 	log\
 	malloc\
@@ -36,15 +38,15 @@ FILES=\
 	strings\
 	testing\
 	utf8\
-	bignum\
 
 TEST=\
+	bignum\
 	bufio\
+	exec\
 	once\
 	sort\
 	strings\
 	utf8\
-	bignum\
 
 clean.dirs: $(addsuffix .dirclean, $(DIRS))
 install.dirs: $(addsuffix .dirinstall, $(DIRS))
@@ -96,6 +98,7 @@ flag.6: fmt.dirinstall
 log.6: fmt.dirinstall io.dirinstall os.dirinstall time.dirinstall
 testing.6: flag.install fmt.dirinstall
 strings.6: utf8.install
+exec.6: os.dirinstall
 
 fmt.dirinstall: io.dirinstall reflect.dirinstall strconv.dirinstall
 hash.dirinstall: os.dirinstall
@@ -112,4 +115,5 @@ strconv.dirinstall: math.dirinstall os.dirinstall utf8.install
 tabwriter.dirinstall: os.dirinstall io.dirinstall container/array.dirinstall
 time.dirinstall: once.install os.dirinstall io.dirinstall
 sync.dirinstall:
+syscall.dirinstall: sync.dirinstall
 
diff --git a/src/lib/exec.go b/src/lib/exec.go
new file mode 100644
index 00000000000..ec48801f778
--- /dev/null
+++ b/src/lib/exec.go
@@ -0,0 +1,160 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package exec
+
+import (
+	"os";
+	"syscall";
+)
+
+const (
+	DevNull = iota;
+	Passthru;
+	Pipe;
+	MergeWithStdout;
+)
+
+type Cmd struct {
+	Stdin *os.FD;
+	Stdout *os.FD;
+	Stderr *os.FD;
+	Pid int;
+}
+
+// Given mode (DevNull, etc), return fd for child
+// and fd to record in Cmd structure.
+func modeToFDs(mode, fd int) (*os.FD, *os.FD, *os.Error) {
+	switch mode {
+	case DevNull:
+		rw := os.O_WRONLY;
+		if fd == 0 {
+			rw = os.O_RDONLY;
+		}
+		f, err := os.Open("/dev/null", rw, 0);
+		return f, nil, err;
+	case Passthru:
+		switch fd {
+		case 0:
+			return os.Stdin, nil, nil;
+		case 1:
+			return os.Stdout, nil, nil;
+		case 2:
+			return os.Stderr, nil, nil;
+		}
+	case Pipe:
+		r, w, err := os.Pipe();
+		if err != nil {
+			return nil, nil, err;
+		}
+		if fd == 0 {
+			return r, w, nil;
+		}
+		return w, r, nil;
+	}
+	return nil, nil, os.EINVAL;
+}
+
+// Start command running with pipes possibly
+// connected to stdin, stdout, stderr.
+// TODO(rsc): Should the stdin,stdout,stderr args
+// be [3]int instead?
+func OpenCmd(argv0 string, argv, envv []string, stdin, stdout, stderr int)
+	(p *Cmd, err *os.Error)
+{
+	p = new(Cmd);
+	var fd [3]*os.FD;
+
+	if fd[0], p.Stdin, err = modeToFDs(stdin, 0); err != nil {
+		goto Error;
+	}
+	if fd[1], p.Stdout, err = modeToFDs(stdout, 1); err != nil {
+		goto Error;
+	}
+	if stderr == MergeWithStdout {
+		p.Stderr = p.Stdout;
+	} else if fd[2], p.Stderr, err = modeToFDs(stderr, 2); err != nil {
+		goto Error;
+	}
+
+	// Run command.
+	p.Pid, err = os.ForkExec(argv0, argv, envv, fd);
+	if err != nil {
+		goto Error;
+	}
+	if fd[0] != os.Stdin {
+		fd[0].Close();
+	}
+	if fd[1] != os.Stdout {
+		fd[1].Close();
+	}
+	if fd[2] != os.Stderr && fd[2] != fd[1] {
+		fd[2].Close();
+	}
+	return p, nil;
+
+Error:
+	if fd[0] != os.Stdin && fd[0] != nil {
+		fd[0].Close();
+	}
+	if fd[1] != os.Stdout && fd[1] != nil {
+		fd[1].Close();
+	}
+	if fd[2] != os.Stderr && fd[2] != nil && fd[2] != fd[1] {
+		fd[2].Close();
+	}
+	if p.Stdin != nil {
+		p.Stdin.Close();
+	}
+	if p.Stdout != nil {
+		p.Stdout.Close();
+	}
+	if p.Stderr != nil {
+		p.Stderr.Close();
+	}
+	return nil, err;
+}
+
+func (p *Cmd) Wait(options uint64) (*os.Waitmsg, *os.Error) {
+	if p.Pid < 0 {
+		return nil, os.EINVAL;
+	}
+	w, err := os.Wait(p.Pid, options);
+	if w != nil && (w.Exited() || w.Signaled()) {
+		p.Pid = -1;
+	}
+	return w, err;
+}
+
+func (p *Cmd) Close() *os.Error {
+	if p.Pid >= 0 {
+		// Loop on interrupt, but
+		// ignore other errors -- maybe
+		// caller has already waited for pid.
+		w, err := p.Wait(0);
+		for err == os.EINTR {
+			w, err = p.Wait(0);
+		}
+	}
+
+	// Close the FDs that are still open.
+	var err *os.Error;
+	if p.Stdin != nil && p.Stdin.Fd() >= 0 {
+		if err1 := p.Stdin.Close(); err1 != nil {
+			err = err1;
+		}
+	}
+	if p.Stdout != nil && p.Stdout.Fd() >= 0 {
+		if err1 := p.Stdout.Close(); err1 != nil && err != nil {
+			err = err1;
+		}
+	}
+	if p.Stderr != nil && p.Stderr != p.Stdout && p.Stderr.Fd() >= 0 {
+		if err1 := p.Stderr.Close(); err1 != nil && err != nil {
+			err = err1;
+		}
+	}
+	return err;
+}
+
diff --git a/src/lib/exec_test.go b/src/lib/exec_test.go
new file mode 100644
index 00000000000..740dfa7654d
--- /dev/null
+++ b/src/lib/exec_test.go
@@ -0,0 +1,51 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package exec
+
+import (
+	"exec";
+	"io";
+	"testing";
+)
+
+func TestOpenCmdCat(t *testing.T) {
+	cmd, err := exec.OpenCmd("/bin/cat", []string("cat"), nil,
+		exec.Pipe, exec.Pipe, exec.DevNull);
+	if err != nil {
+		t.Fatalf("opencmd /bin/cat: %v", err);
+	}
+	io.WriteString(cmd.Stdin, "hello, world\n");
+	cmd.Stdin.Close();
+	var buf [64]byte;
+	n, err1 := io.Readn(cmd.Stdout, buf);
+	if err1 != nil && err1 != io.ErrEOF {
+		t.Fatalf("reading from /bin/cat: %v", err1);
+	}
+	if string(buf[0:n]) != "hello, world\n" {
+		t.Fatalf("reading from /bin/cat: got %q", buf[0:n]);
+	}
+	if err1 = cmd.Close(); err1 != nil {
+		t.Fatalf("closing /bin/cat: %v", err1);
+	}
+}
+
+func TestOpenCmdEcho(t *testing.T) {
+	cmd, err := OpenCmd("/bin/echo", []string("echo", "hello", "world"), nil,
+		exec.DevNull, exec.Pipe, exec.DevNull);
+	if err != nil {
+		t.Fatalf("opencmd /bin/echo: %v", err);
+	}
+	var buf [64]byte;
+	n, err1 := io.Readn(cmd.Stdout, buf);
+	if err1 != nil && err1 != io.ErrEOF {
+		t.Fatalf("reading from /bin/echo: %v", err1);
+	}
+	if string(buf[0:n]) != "hello world\n" {
+		t.Fatalf("reading from /bin/echo: got %q", buf[0:n]);
+	}
+	if err1 = cmd.Close(); err1 != nil {
+		t.Fatalf("closing /bin/echo: %v", err1);
+	}
+}
diff --git a/src/lib/net/fd.go b/src/lib/net/fd.go
index 1ec0d8af9e8..0c7770c77f6 100644
--- a/src/lib/net/fd.go
+++ b/src/lib/net/fd.go
@@ -280,17 +280,26 @@ func (fd *netFD) Accept(sa *syscall.Sockaddr) (nfd *netFD, err *os.Error) {
 		return nil, os.EINVAL
 	}
 
+	// See ../syscall/exec.go for description of ForkLock.
+	// It is okay to hold the lock across syscall.Accept
+	// because we have put fd.fd into non-blocking mode.
+	syscall.ForkLock.RLock();
 	var s, e int64;
 	for {
 		s, e = syscall.Accept(fd.fd, sa);
 		if e != syscall.EAGAIN {
 			break;
 		}
+		syscall.ForkLock.RUnlock();
 		pollserver.WaitRead(fd);
+		syscall.ForkLock.RLock();
 	}
 	if e != 0 {
+		syscall.ForkLock.RUnlock();
 		return nil, os.ErrnoToError(e)
 	}
+	syscall.CloseOnExec(s);
+	syscall.ForkLock.RUnlock();
 
 	raddr, err1 := sockaddrToHostPort(sa);
 	if err1 != nil {
diff --git a/src/lib/net/net.go b/src/lib/net/net.go
index b81e99268a1..db708191b11 100644
--- a/src/lib/net/net.go
+++ b/src/lib/net/net.go
@@ -143,10 +143,15 @@ func boolint(b bool) int {
 func socket(net, laddr, raddr string, f, p, t int64, la, ra *syscall.Sockaddr)
 	(fd *netFD, err *os.Error)
 {
+	// See ../syscall/exec.go for description of ForkLock.
+	syscall.ForkLock.RLock();
 	s, e := syscall.Socket(f, p, t);
 	if e != 0 {
+		syscall.ForkLock.RUnlock();
 		return nil, os.ErrnoToError(e)
 	}
+	syscall.CloseOnExec(s);
+	syscall.ForkLock.RUnlock();
 
 	// Allow reuse of recently-used addresses.
 	syscall.Setsockopt_int(s, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1);
diff --git a/src/lib/os/Makefile b/src/lib/os/Makefile
index ac0d3394d87..413a8d1b1bc 100644
--- a/src/lib/os/Makefile
+++ b/src/lib/os/Makefile
@@ -3,8 +3,8 @@
 # license that can be found in the LICENSE file.
 
 # DO NOT EDIT.  Automatically generated by gobuild.
-# gobuild -m dir_amd64_linux.go env.go error.go file.go stat_amd64_linux.go\
-#    time.go types.go >Makefile
+# gobuild -m dir_${GOARCH}_${GOOS}.go env.go error.go file.go\
+#    stat_${GOARCH}_${GOOS}.go time.go types.go exec.go >Makefile
 O=6
 GC=$(O)g
 CC=$(O)c -w
@@ -46,6 +46,7 @@ O3=\
 
 O4=\
 	dir_$(GOARCH)_$(GOOS).$O\
+	exec.$O\
 
 os.a: a1 a2 a3 a4
 
@@ -62,7 +63,7 @@ a3:	$(O3)
 	rm -f $(O3)
 
 a4:	$(O4)
-	$(AR) grc os.a dir_$(GOARCH)_$(GOOS).$O
+	$(AR) grc os.a dir_$(GOARCH)_$(GOOS).$O exec.$O
 	rm -f $(O4)
 
 newpkg: clean
diff --git a/src/lib/os/exec.go b/src/lib/os/exec.go
new file mode 100644
index 00000000000..0ce51773c50
--- /dev/null
+++ b/src/lib/os/exec.go
@@ -0,0 +1,70 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package os
+
+import (
+	"os";
+	"syscall";
+)
+
+func ForkExec(argv0 string, argv []string, envv []string, fd []*FD)
+	(pid int, err *Error)
+{
+	// Create array of integer (system) fds.
+	intfd := make([]int64, len(fd));
+	for i, f := range(fd) {
+		if f == nil {
+			intfd[i] = -1;
+		} else {
+			intfd[i] = f.Fd();
+		}
+	}
+
+	p, e := syscall.ForkExec(argv0, argv, envv, intfd);
+	return int(p), ErrnoToError(e);
+}
+
+func Exec(argv0 string, argv []string, envv []string) *Error {
+	e := syscall.Exec(argv0, argv, envv);
+	return ErrnoToError(e);
+}
+
+// TODO(rsc): Should os implement its own syscall.WaitStatus
+// wrapper with the methods, or is exposing the underlying one enough?
+//
+// TODO(rsc): Certainly need to have os.Rusage struct,
+// since syscall one might have different field types across
+// different OS.
+
+type Waitmsg struct {
+	Pid int;
+	syscall.WaitStatus;
+	Rusage *syscall.Rusage;
+}
+
+const (
+	WNOHANG = syscall.WNOHANG;
+	WSTOPPED = syscall.WSTOPPED;
+	WRUSAGE = 1<<60;
+)
+
+func Wait(pid int, options uint64) (w *Waitmsg, err *Error) {
+	var status syscall.WaitStatus;
+	var rusage *syscall.Rusage;
+	if options & WRUSAGE != 0 {
+		rusage = new(syscall.Rusage);
+		options ^= WRUSAGE;
+	}
+	pid1, e := syscall.Wait4(int64(pid), &status, int64(options), rusage);
+	if e != 0 {
+		return nil, ErrnoToError(e);
+	}
+	w = new(Waitmsg);
+	w.Pid = pid;
+	w.WaitStatus = status;
+	w.Rusage = rusage;
+	return w, nil;
+}
+
diff --git a/src/lib/os/file.go b/src/lib/os/file.go
index d4725760de3..24aab1256dc 100644
--- a/src/lib/os/file.go
+++ b/src/lib/os/file.go
@@ -4,8 +4,10 @@
 
 package os
 
-import syscall "syscall"
-import os "os"
+import (
+	"os";
+	"syscall";
+)
 
 // Auxiliary information if the FD describes a directory
 type dirInfo struct {	// TODO(r): 6g bug means this can't be private
@@ -57,7 +59,17 @@ const (
 )
 
 func Open(name string, mode int, flags int) (fd *FD, err *Error) {
-	r, e := syscall.Open(name, int64(mode), int64(flags));
+	r, e := syscall.Open(name, int64(mode), int64(flags | syscall.O_CLOEXEC));
+	if e != 0 {
+		return nil, ErrnoToError(e);
+	}
+
+	// There's a race here with fork/exec, which we are
+	// content to live with.  See ../syscall/exec.go
+	if syscall.O_CLOEXEC == 0 {	// O_CLOEXEC not supported
+		syscall.CloseOnExec(r);
+	}
+
 	return NewFD(r, name), ErrnoToError(e)
 }
 
@@ -122,10 +134,18 @@ func (fd *FD) WriteString(s string) (ret int, err *Error) {
 
 func Pipe() (fd1 *FD, fd2 *FD, err *Error) {
 	var p [2]int64;
+
+	// See ../syscall/exec.go for description of lock.
+	syscall.ForkLock.RLock();
 	r, e := syscall.Pipe(&p);
 	if e != 0 {
+		syscall.ForkLock.RUnlock();
 		return nil, nil, ErrnoToError(e)
 	}
+	syscall.CloseOnExec(p[0]);
+	syscall.CloseOnExec(p[1]);
+	syscall.ForkLock.RUnlock();
+
 	return NewFD(p[0], "|0"), NewFD(p[1], "|1"), nil
 }
 
diff --git a/src/lib/sync/mutex.go b/src/lib/sync/mutex.go
index 1269027e11e..8e05cd819d2 100644
--- a/src/lib/sync/mutex.go
+++ b/src/lib/sync/mutex.go
@@ -39,3 +39,20 @@ func (m *Mutex) Unlock() {
 	semrelease(&m.sema);
 }
 
+// Stub implementation of r/w locks.
+// This satisfies the semantics but
+// is not terribly efficient.
+// TODO(rsc): Real r/w locks.
+
+type RWMutex struct {
+	Mutex;
+}
+
+func (m *RWMutex) RLock() {
+	m.Lock();
+}
+
+func (m *RWMutex) RUnlock() {
+	m.Unlock();
+}
+
diff --git a/src/lib/syscall/Makefile b/src/lib/syscall/Makefile
index a9975b5c6f9..194fcca7646 100644
--- a/src/lib/syscall/Makefile
+++ b/src/lib/syscall/Makefile
@@ -3,9 +3,10 @@
 # license that can be found in the LICENSE file.
 
 # DO NOT EDIT.  Automatically generated by gobuild.
-# gobuild -m errstr_darwin.go file_darwin.go socket_darwin.go\
-#    syscall_amd64_darwin.go time_amd64_darwin.go types_amd64_darwin.go\
-#    asm_amd64_darwin.s syscall.go signal_amd64_darwin.go >Makefile
+# gobuild -m errstr_${GOOS}.go file_${GOOS}.go socket_${GOOS}.go\
+#    syscall_${GOARCH}_${GOOS}.go time_${GOARCH}_${GOOS}.go types_${GOARCH}_${GOOS}.go\
+#    asm_${GOARCH}_${GOOS}.s syscall.go signal_${GOARCH}_${GOOS}.go\
+#    exec.go >Makefile
 O=6
 GC=$(O)g
 CC=$(O)c -w
@@ -36,31 +37,46 @@ coverage: packages
 O1=\
 	errstr_$(GOOS).$O\
 	syscall_$(GOARCH)_$(GOOS).$O\
-	types_$(GOARCH)_$(GOOS).$O\
 	asm_$(GOARCH)_$(GOOS).$O\
 	syscall.$O\
 	signal_$(GOARCH)_$(GOOS).$O\
 
 O2=\
+	types_$(GOARCH)_$(GOOS).$O\
+
+O3=\
 	file_$(GOOS).$O\
 	socket_$(GOOS).$O\
 	time_$(GOARCH)_$(GOOS).$O\
 
-syscall.a: a1 a2
+O4=\
+	exec.$O\
+
+syscall.a: a1 a2 a3 a4
 
 a1:	$(O1)
-	$(AR) grc syscall.a errstr_$(GOOS).$O syscall_$(GOARCH)_$(GOOS).$O types_$(GOARCH)_$(GOOS).$O asm_$(GOARCH)_$(GOOS).$O syscall.$O signal_$(GOARCH)_$(GOOS).$O
+	$(AR) grc syscall.a errstr_$(GOOS).$O syscall_$(GOARCH)_$(GOOS).$O asm_$(GOARCH)_$(GOOS).$O syscall.$O signal_$(GOARCH)_$(GOOS).$O
 	rm -f $(O1)
 
 a2:	$(O2)
-	$(AR) grc syscall.a file_$(GOOS).$O socket_$(GOOS).$O time_$(GOARCH)_$(GOOS).$O
+	$(AR) grc syscall.a types_$(GOARCH)_$(GOOS).$O
 	rm -f $(O2)
 
+a3:	$(O3)
+	$(AR) grc syscall.a file_$(GOOS).$O socket_$(GOOS).$O time_$(GOARCH)_$(GOOS).$O
+	rm -f $(O3)
+
+a4:	$(O4)
+	$(AR) grc syscall.a exec.$O
+	rm -f $(O4)
+
 newpkg: clean
 	$(AR) grc syscall.a
 
 $(O1): newpkg
 $(O2): a1
+$(O3): a2
+$(O4): a3
 
 nuke: clean
 	rm -f $(GOROOT)/pkg/syscall.a
diff --git a/src/lib/syscall/exec.go b/src/lib/syscall/exec.go
new file mode 100644
index 00000000000..809cd8c578d
--- /dev/null
+++ b/src/lib/syscall/exec.go
@@ -0,0 +1,291 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Fork, exec, wait, etc.
+
+package syscall
+
+import (
+	"sync";
+	"syscall";
+	"unsafe";
+)
+
+// Lock synchronizing creation of new file descriptors with fork.
+//
+// We want the child in a fork/exec sequence to inherit only the
+// file descriptors we intend.  To do that, we mark all file
+// descriptors close-on-exec and then, in the child, explicitly
+// unmark the ones we want the exec'ed program to keep.
+// Unix doesn't make this easy: there is, in general, no way to
+// allocate a new file descriptor close-on-exec.  Instead you
+// have to allocate the descriptor and then mark it close-on-exec.
+// If a fork happens between those two events, the child's exec
+// will inherit an unwanted file descriptor.
+//
+// This lock solves that race: the create new fd/mark close-on-exec
+// operation is done holding ForkLock for reading, and the fork itself
+// is done holding ForkLock for writing.  At least, that's the idea.
+// There are some complications.
+//
+// Some system calls that create new file descriptors can block
+// for arbitrarily long times: open on a hung NFS server or named
+// pipe, accept on a socket, and so on.  We can't reasonably grab
+// the lock across those operations.
+//
+// It is worse to inherit some file descriptors than others.
+// If a non-malicious child accidentally inherits an open ordinary file,
+// that's not a big deal.  On the other hand, if a long-lived child
+// accidentally inherits the write end of a pipe, then the reader
+// of that pipe will not see EOF until that child exits, potentially
+// causing the parent program to hang.  This is a common problem
+// in threaded C programs that use popen.
+//
+// Luckily, the file descriptors that are most important not to
+// inherit are not the ones that can take an arbitrarily long time
+// to create: pipe returns instantly, and the net package uses
+// non-blocking I/O to accept on a listening socket.
+// The rules for which file descriptor-creating operations use the
+// ForkLock are as follows:
+//
+// 1) Pipe.    Does not block.  Use the ForkLock.
+// 2) Socket.  Does not block.  Use the ForkLock.
+// 3) Accept.  If using non-blocking mode, use the ForkLock.
+//             Otherwise, live with the race.
+// 4) Open.    Can block.  Use O_CLOEXEC if available (Linux).
+//             Otherwise, live with the race.
+// 5) Dup.     Does not block.  Use the ForkLock.
+//             On Linux, could use fcntl F_DUPFD_CLOEXEC
+//             instead of the ForkLock, but only for dup(fd, -1).
+
+var ForkLock sync.RWMutex
+
+func CloseOnExec(fd int64) {
+	Fcntl(fd, F_SETFD, FD_CLOEXEC);
+}
+
+// Convert array of string to array
+// of NUL-terminated byte pointer.
+func StringArrayPtr(ss []string) []*byte {
+	bb := make([]*byte, len(ss)+1);
+	for i := 0; i < len(ss); i++ {
+		bb[i] = StringBytePtr(ss[i]);
+	}
+	bb[len(ss)] = nil;
+	return bb;
+}
+
+func Wait4(pid int64, wstatus *WaitStatus, options int64, rusage *Rusage)
+	(wpid, err int64)
+{
+	var s WaitStatus;
+	r1, r2, err1 := Syscall6(SYS_WAIT4,
+		pid,
+		int64(uintptr(unsafe.Pointer(&s))),
+		options,
+		int64(uintptr(unsafe.Pointer(rusage))), 0, 0);
+	if wstatus != nil {
+		*wstatus = s;
+	}
+	return r1, err1;
+}
+
+// Fork, dup fd onto 0..len(fd), and exec(argv0, argvv, envv) in child.
+// If a dup or exec fails, write the errno int64 to pipe.
+// (Pipe is close-on-exec so if exec succeeds, it will be closed.)
+// In the child, this function must not acquire any locks, because
+// they might have been locked at the time of the fork.  This means
+// no rescheduling, no malloc calls, and no new stack segments.
+// The calls to RawSyscall are okay because they are assembly
+// functions that do not grow the stack.
+func forkAndExecInChild(argv0 *byte, argv []*byte, envv []*byte, fd []int64, pipe int64)
+	(pid int64, err int64)
+{
+	// Declare all variables at top in case any
+	// declarations require heap allocation (e.g., err1).
+	var r1, r2, err1 int64;
+	var nextfd int64;
+	var i int;
+
+	darwin := OS == "darwin";
+
+	// About to call fork.
+	// No more allocation or calls of non-assembly functions.
+	r1, r2, err1 = RawSyscall(SYS_FORK, 0, 0, 0);
+	if err1 != 0 {
+		return 0, err1
+	}
+
+	// On Darwin:
+	//	r1 = child pid in both parent and child.
+	//	r2 = 0 in parent, 1 in child.
+	// Convert to normal Unix r1 = 0 in child.
+	if darwin && r2 == 1 {
+		r1 = 0;
+	}
+
+	if r1 != 0 {
+		// parent; return PID
+		return r1, 0
+	}
+
+	// Fork succeeded, now in child.
+
+	// Pass 1: look for fd[i] < i and move those up above len(fd)
+	// so that pass 2 won't stomp on an fd it needs later.
+	nextfd = int64(len(fd));
+	if pipe < nextfd {
+		r1, r2, err = RawSyscall(SYS_DUP2, pipe, nextfd, 0);
+		if err != 0 {
+			goto childerror;
+		}
+		RawSyscall(SYS_FCNTL, nextfd, F_SETFD, FD_CLOEXEC);
+		pipe = nextfd;
+		nextfd++;
+	}
+	for i = 0; i < len(fd); i++ {
+		if fd[i] >= 0 && fd[i] < int64(i) {
+			r1, r2, err = RawSyscall(SYS_DUP2, fd[i], nextfd, 0);
+			if err != 0 {
+				goto childerror;
+			}
+			RawSyscall(SYS_FCNTL, nextfd, F_SETFD, FD_CLOEXEC);
+			fd[i] = nextfd;
+			nextfd++;
+			if nextfd == pipe {	// don't stomp on pipe
+				nextfd++;
+			}
+		}
+	}
+
+	// Pass 2: dup fd[i] down onto i.
+	for i = 0; i < len(fd); i++ {
+		if fd[i] == -1 {
+			RawSyscall(SYS_CLOSE, int64(i), 0, 0);
+			continue;
+		}
+		if fd[i] == int64(i) {
+			// dup2(i, i) won't clear close-on-exec flag on Linux,
+			// probably not elsewhere either.
+			r1, r2, err = RawSyscall(SYS_FCNTL, fd[i], F_SETFD, 0);
+			if err != 0 {
+				goto childerror;
+			}
+			continue;
+		}
+		// The new fd is created NOT close-on-exec,
+		// which is exactly what we want.
+		r1, r2, err = RawSyscall(SYS_DUP2, fd[i], int64(i), 0);
+		if err != 0 {
+			goto childerror;
+		}
+	}
+
+	// By convention, we don't close-on-exec the fds we are
+	// started with, so if len(fd) < 3, close 0, 1, 2 as needed.
+	// Programs that know they inherit fds >= 3 will need
+	// to set them close-on-exec.
+	for i = len(fd); i < 3; i++ {
+		RawSyscall(SYS_CLOSE, int64(i), 0, 0);
+	}
+
+	// Time to exec.
+	r1, r2, err1 = RawSyscall(SYS_EXECVE,
+		int64(uintptr(unsafe.Pointer(argv0))),
+		int64(uintptr(unsafe.Pointer(&argv[0]))),
+		int64(uintptr(unsafe.Pointer(&envv[0]))));
+
+childerror:
+	// send error code on pipe
+	RawSyscall(SYS_WRITE, pipe, int64(uintptr(unsafe.Pointer(&err1))), 8);
+	for {
+		RawSyscall(SYS_EXIT, 253, 0, 0);
+	}
+
+	// Calling panic is not actually safe,
+	// but the for loop above won't break
+	// and this shuts up the compiler.
+	panic("unreached");
+}
+
+// Combination of fork and exec, careful to be thread safe.
+func ForkExec(argv0 string, argv []string, envv []string, fd []int64)
+	(pid int64, err int64)
+{
+	var p [2]int64;
+	var r1 int64;
+	var n, err1 int64;
+	var wstatus WaitStatus;
+
+	p[0] = -1;
+	p[1] = -1;
+
+	// Convert args to C form.
+	argv0p := StringBytePtr(argv0);
+	argvp := StringArrayPtr(argv);
+	envvp := StringArrayPtr(envv);
+
+	// Acquire the fork lock so that no other threads
+	// create new fds that are not yet close-on-exec
+	// before we fork.
+	ForkLock.Lock();
+
+	// Allocate child status pipe close on exec.
+	if r1, err = Pipe(&p); err != 0 {
+		goto error;
+	}
+	if r1, err = Fcntl(p[0], F_SETFD, FD_CLOEXEC); err != 0 {
+		goto error;
+	}
+	if r1, err = Fcntl(p[1], F_SETFD, FD_CLOEXEC); err != 0 {
+		goto error;
+	}
+
+	// Kick off child.
+	pid, err = forkAndExecInChild(argv0p, argvp, envvp, fd, p[1]);
+	if err != 0 {
+	error:
+		if p[0] >= 0 {
+			Close(p[0]);
+			Close(p[1]);
+		}
+		ForkLock.Unlock();
+		return 0, err
+	}
+	ForkLock.Unlock();
+
+	// Read child error status from pipe.
+	Close(p[1]);
+	n, r1, err = Syscall(SYS_READ, p[0], int64(uintptr(unsafe.Pointer(&err1))), 8);
+	Close(p[0]);
+	if err != 0 || n != 0 {
+		if n == 8 {
+			err = err1;
+		}
+		if err == 0 {
+			err = EPIPE;
+		}
+
+		// Child failed; wait for it to exit, to make sure
+		// the zombies don't accumulate.
+		pid1, err1 := Wait4(pid, &wstatus, 0, nil);
+		for err1 == EINTR {
+			pid1, err1 = Wait4(pid, &wstatus, 0, nil);
+		}
+		return 0, err
+	}
+
+	// Read got EOF, so pipe closed on exec, so exec succeeded.
+	return pid, 0
+}
+
+// Ordinary exec.
+func Exec(argv0 string, argv []string, envv []string) (err int64) {
+	r1, r2, err1 := RawSyscall(SYS_EXECVE,
+		int64(uintptr(unsafe.Pointer(StringBytePtr(argv0)))),
+		int64(uintptr(unsafe.Pointer(&StringArrayPtr(argv)[0]))),
+		int64(uintptr(unsafe.Pointer(&StringArrayPtr(envv)[0]))));
+	return err1;
+}
+
diff --git a/src/lib/syscall/types_amd64_darwin.go b/src/lib/syscall/types_amd64_darwin.go
index 101f223b855..36ced7ab617 100644
--- a/src/lib/syscall/types_amd64_darwin.go
+++ b/src/lib/syscall/types_amd64_darwin.go
@@ -7,6 +7,9 @@
 
 package syscall
 
+import "syscall"
+
+const OS = "darwin"
 
 // Time
 
@@ -57,6 +60,7 @@ const (
 	O_NDELAY = O_NONBLOCK;
 	O_SYNC = 0x80;
 	O_TRUNC = 0x400;
+	O_CLOEXEC = 0;  // not supported
 
 	F_GETFD = 1;
 	F_SETFD = 2;
@@ -239,3 +243,88 @@ type Kevent_t struct {
 	Udata int64;
 }
 
+
+// Wait status.
+// See /usr/include/bits/waitstatus.h
+
+const (
+	WNOHANG = 1;
+	WUNTRACED = 2;
+	WEXITED = 4;
+	WSTOPPED = 8;
+	WCONTINUED = 0x10;
+	WNOWAIT = 0x20;
+)
+
+type WaitStatus uint32;
+
+// TODO(rsc): should be method on WaitStatus,
+// not *WaitStatus, but causes problems when
+// embedding in a *Waitmsg in package os.
+// Need to find the 6g bug.
+
+// Wait status is 7 bits at bottom, either 0 (exited),
+// 0x7F (stopped), or a signal number that caused an exit.
+// The 0x80 bit is whether there was a core dump.
+// An extra number (exit code, signal causing a stop)
+// is in the high bits.
+
+const (
+	mask = 0x7F;
+	core = 0x80;
+	shift = 8;
+
+	exited = 0;
+	stopped = 0x7F;
+)
+
+func (wp *WaitStatus) Exited() bool {
+	w := *wp;  // TODO(rsc): no pointer
+	return w&mask == exited;
+}
+
+func (wp *WaitStatus) ExitStatus() int {
+	w := *wp;  // TODO(rsc): no pointer
+	if w&mask != exited {
+		return -1;
+	}
+	return int(w >> shift);
+}
+
+func (wp *WaitStatus) Signaled() bool {
+	w := *wp;  // TODO(rsc): no pointer
+	return w&mask != stopped && w&mask != 0;
+}
+
+func (wp *WaitStatus) Signal() int {
+	w := *wp;  // TODO(rsc): no pointer
+	sig := int(w & mask);
+	if sig == stopped || sig == 0 {
+		return -1;
+	}
+	return sig;
+}
+
+func (wp *WaitStatus) CoreDump() bool {
+	w := *wp;  // TODO(rsc): no pointer
+	return w.Signaled() && w&core != 0;
+}
+
+func (wp *WaitStatus) Stopped() bool {
+	w := *wp;  // TODO(rsc): no pointer
+	return w&mask == stopped && w>>shift != SIGSTOP;
+}
+
+func (wp *WaitStatus) Continued() bool {
+	w := *wp;  // TODO(rsc): no pointer
+	return w&mask == stopped && w>>shift == SIGSTOP;
+}
+
+func (wp *WaitStatus) StopSignal() int {
+	w := *wp;  // TODO(rsc): no pointer
+	if !w.Stopped() {
+		return -1;
+	}
+	return int(w >> shift) & 0xFF;
+}
+
diff --git a/src/lib/syscall/types_amd64_linux.go b/src/lib/syscall/types_amd64_linux.go
index ebfe21c8666..cbb65eeab06 100644
--- a/src/lib/syscall/types_amd64_linux.go
+++ b/src/lib/syscall/types_amd64_linux.go
@@ -7,6 +7,9 @@
 
 package syscall
 
+import "syscall"
+
+const OS = "linux"
 
 // Time
 
@@ -57,6 +60,7 @@ const (
 	O_NDELAY = O_NONBLOCK;
 	O_SYNC = 0x1000;
 	O_TRUNC = 0x200;
+	O_CLOEXEC = 0x80000;
 
 	F_GETFD = 1;
 	F_SETFD = 2;
@@ -218,3 +222,98 @@ type EpollEvent struct {
 	Fd int32;
 	Pad int32;
 }
+
+
+// Wait status.
+// See /usr/include/bits/waitstatus.h
+
+const (
+	WNOHANG = 1;
+	WUNTRACED = 2;
+	WSTOPPED = 2;	// same as WUNTRACED
+	WEXITED = 4;
+	WCONTINUED = 8;
+	WNOWAIT = 0x01000000;
+	WNOTHREAD = 0x20000000;
+	WALL = 0x40000000;
+	WCLONE = 0x80000000;
+)
+
+type WaitStatus uint32;
+
+// TODO(rsc): should be method on WaitStatus,
+// not *WaitStatus, but causes problems when
+// embedding in a *Waitmsg in package os.
+// Need to find the 6g bug.
+
+// Wait status is 7 bits at bottom, either 0 (exited),
+// 0x7F (stopped), or a signal number that caused an exit.
+// The 0x80 bit is whether there was a core dump.
+// An extra number (exit code, signal causing a stop)
+// is in the high bits.  At least that's the idea.
+// There are various irregularities.  For example, the
+// "continued" status is 0xFFFF, distinguishing itself
+// from stopped via the core dump bit.
+
+const (
+	mask = 0x7F;
+	core = 0x80;
+	exited = 0x00;
+	stopped = 0x7F;
+	shift = 8;
+
+	// types_amd64_darwin.go refers to SIGSTOP.
+	// do the same here so the dependencies are
+	// the same on Linux as on Darwin.
+	__unused = SIGSTOP;
+)
+
+func (wp *WaitStatus) Exited() bool {
+	w := *wp;  // TODO(rsc): no pointer
+	return w&mask == exited;
+}
+
+func (wp *WaitStatus) Signaled() bool {
+	w := *wp;  // TODO(rsc): no pointer
+	return w&mask != stopped && w&mask != exited;
+}
+
+func (wp *WaitStatus) Stopped() bool {
+	w := *wp;  // TODO(rsc): no pointer
+	return w&0xFF == stopped;
+}
+
+func (wp *WaitStatus) Continued() bool {
+	w := *wp;  // TODO(rsc): no pointer
+	return w == 0xFFFF;
+}
+
+func (wp *WaitStatus) CoreDump() bool {
+	w := *wp;  // TODO(rsc): no pointer
+	return w.Signaled() && w&core != 0;
+}
+
+func (wp *WaitStatus) ExitStatus() int {
+	w := *wp;  // TODO(rsc): no pointer
+	if !w.Exited() {
+		return -1;
+	}
+	return int(w >> shift) & 0xFF;
+}
+
+func (wp *WaitStatus) Signal() int {
+	w := *wp;  // TODO(rsc): no pointer
+	if !w.Signaled() {
+		return -1;
+	}
+	return int(w & mask);
+}
+
+func (wp *WaitStatus) StopSignal() int {
+	w := *wp;  // TODO(rsc): no pointer
+	if !w.Stopped() {
+		return -1;
+	}
+	return int(w >> shift) & 0xFF;
+}
+