diff --git a/macos/choose_directory.c b/macos/choose_directory.c
new file mode 100644
index 0000000..54ff82f
--- /dev/null
+++ b/macos/choose_directory.c
@@ -0,0 +1,105 @@
+#include "choose_directory.h"
+
+#include "error.h"
+#include "resources.h"
+#include "strutil.h"
+
+#include <string.h>
+
+typedef enum {
+	kCDNone,
+	kCDChild,
+	kCDCurrent,
+} ChooseDirStatus;
+
+struct ChooseDirReply {
+	ChooseDirStatus status;
+	FSSpec *directory;
+};
+
+// FIXME: Don't use pascal on PowerMac.
+static pascal Boolean ChooseDirectoryFileFilter(CInfoPBPtr pb,
+                                                void *yourDataPtr)
+{
+	(void)yourDataPtr;
+	return (pb->hFileInfo.ioFlAttrib & ioDirMask) == 0;
+}
+
+enum {
+	kCDItemChoose = 10,
+	kCDItemChooseCurrent = 11,
+};
+
+static void SelectCurrentDirectory(struct ChooseDirReply *reply)
+{
+	CInfoPBRec ci;
+	OSStatus err;
+	Str63 name;
+	FSSpec *directory;
+
+	memset(&ci, 0, sizeof(ci));
+	ci.dirInfo.ioNamePtr = name;
+	ci.dirInfo.ioVRefNum = -LMGetSFSaveDisk();
+	ci.dirInfo.ioFDirIndex = -1;
+	ci.dirInfo.ioDrDirID = LMGetCurDirStore();
+	err = PBGetCatInfoSync(&ci);
+	if (err != 0) {
+		ExitErrorOS(kErrUnknown, err);
+	}
+	directory = reply->directory;
+	directory->vRefNum = ci.dirInfo.ioVRefNum;
+	directory->parID = ci.dirInfo.ioDrParID;
+	StrCopy(directory->name, sizeof(directory->name), name);
+	reply->status = kCDCurrent;
+}
+
+static pascal short ChooseDirectoryDialogHook(short item, DialogRef theDialog,
+                                              void *yourDataPtr)
+{
+	struct ChooseDirReply *reply;
+	if (GetWRefCon((WindowRef)theDialog) != sfMainDialogRefCon) {
+		return item;
+	}
+	reply = yourDataPtr;
+	switch (item) {
+	case kCDItemChoose:
+		reply->status = kCDChild;
+		return sfItemCancelButton;
+	case kCDItemChooseCurrent:
+		SelectCurrentDirectory(yourDataPtr);
+		return sfItemCancelButton;
+	default:
+		return item;
+	}
+}
+
+Boolean ChooseDirectory(FSSpec *directory)
+{
+	struct ChooseDirReply cdreply;
+	FileFilterYDUPP fileFilter;
+	DlgHookYDUPP dlgHook;
+	StandardFileReply sfreply;
+	Point where;
+
+	cdreply.status = kCDNone;
+	cdreply.directory = directory;
+	fileFilter = NewFileFilterYDProc(ChooseDirectoryFileFilter);
+	ASSERT(fileFilter != NULL);
+	dlgHook = NewDlgHookYDProc(ChooseDirectoryDialogHook);
+	ASSERT(dlgHook != NULL);
+	where.v = -1;
+	where.h = -1;
+	CustomGetFile(fileFilter, -1, NULL, &sfreply, rDLOG_OpenFolder, where,
+	              dlgHook, NULL /* filterProc */, 0, NULL, &cdreply);
+	// DisposeFileFilterYDUPP(fileFilter);
+	// DisposeDlgHookYDUPP(dlgHook);
+	switch (cdreply.status) {
+	case kCDChild:
+		BlockMoveData(&sfreply.sfFile, directory, sizeof(FSSpec));
+		return 1;
+	case kCDCurrent:
+		return 1;
+	default:
+		return 0;
+	}
+}
diff --git a/macos/choose_directory.h b/macos/choose_directory.h
new file mode 100644
index 0000000..b5da516
--- /dev/null
+++ b/macos/choose_directory.h
@@ -0,0 +1,8 @@
+#ifndef MACOS_CHOOSE_DIRECTORY_H
+#define MACOS_CHOOSE_DIRECTORY_H
+
+// Present a dialog box to choose a directory. Returns true and initializes
+// directory if a directory is selected.
+Boolean ChooseDirectory(FSSpec *directory);
+
+#endif
diff --git a/macos/derez.sh b/macos/derez.sh
new file mode 100644
index 0000000..fb52c9f
--- /dev/null
+++ b/macos/derez.sh
@@ -0,0 +1 @@
+DeRez icons.rsrc | iconv -f MACINTOSH -t UTF-8 | sed 's# */.*##' > icons.r
diff --git a/macos/error.c b/macos/error.c
new file mode 100644
index 0000000..4a3590b
--- /dev/null
+++ b/macos/error.c
@@ -0,0 +1,72 @@
+#include "error.h"
+
+#include "main.h"
+#include "resources.h"
+#include "strutil.h"
+
+#include <Dialogs.h>
+#include <TextUtils.h>
+
+void ExitError(ErrorCode errCode)
+{
+	Str255 message;
+
+	GetIndString(message, rSTRS_Errors, errCode);
+	if (message[0] == 0) {
+		GetIndString(message, rSTRS_Errors, kErrUnknown);
+	}
+	ParamText(message, NULL, NULL, NULL);
+	Alert(rAlrtError, NULL);
+	QuitApp();
+}
+
+void ExitErrorOS(ErrorCode errCode, short osErr)
+{
+	Str255 message;
+
+	GetIndString(message, rSTRS_Errors, errCode);
+	if (message[0] == 0) {
+		GetIndString(message, rSTRS_Errors, kErrUnknown);
+	}
+	StrAppendFormat(message, "\p\rError code: %d", osErr);
+	ParamText(message, NULL, NULL, NULL);
+	Alert(rAlrtError, NULL);
+	QuitApp();
+}
+
+void ExitMemError(void)
+{
+	ExitErrorOS(kErrOutOfMemory, MemError());
+}
+
+void ExitAssert(const unsigned char *file, int line,
+                const unsigned char *message)
+{
+	Str255 dmessage;
+
+	GetIndString(dmessage, rSTRS_Errors, kErrInternal);
+	StrAppendFormat(dmessage, "\p\rError at: %S:%d", file, line);
+	if (message != NULL) {
+		StrAppendFormat(dmessage, "\p: %S", message);
+	}
+	ParamText(dmessage, NULL, NULL, NULL);
+	Alert(rAlrtError, NULL);
+	QuitApp();
+}
+
+void ShowError(const struct ErrorParams *p)
+{
+	Str255 message;
+
+	GetIndString(message, rSTRS_Errors, p->err);
+	if (message[0] == 0) {
+		GetIndString(message, rSTRS_Errors, kErrUnknown);
+	} else if (p->strParam != NULL) {
+		StrSubstitute(message, p->strParam);
+	}
+	if (p->osErr != NULL) {
+		StrAppendFormat(message, "\p\rError code: %d", p->osErr);
+	}
+	ParamText(message, NULL, NULL, NULL);
+	Alert(rAlrtError, NULL);
+}
diff --git a/macos/error.h b/macos/error.h
new file mode 100644
index 0000000..a70badc
--- /dev/null
+++ b/macos/error.h
@@ -0,0 +1,48 @@
+// error.h - Error handling.
+#ifndef MACOS_ERROR_H
+#define MACOS_ERROR_H
+
+// Error codes, corresponding to messages in a STR# resource.
+typedef enum {
+	kErrUnknown = 1,
+	kErrInternal,
+	kErrOutOfMemory,
+	kErrCouldNotSaveProject,
+} ErrorCode;
+
+struct Error {
+	ErrorCode code;
+	short osErr;
+};
+
+// Show an error dialog with the given error message, then quit the program.
+void ExitError(ErrorCode errCode);
+
+// Show an error dialog with the given error message and an OS error code, then
+// quit the program.
+void ExitErrorOS(ErrorCode errCode, short osErr);
+
+// Show an out of memory error and quit the program.
+void ExitMemError(void);
+
+// Show an assertion error and quit the program.
+void ExitAssert(const unsigned char *file, int line,
+                const unsigned char *message);
+
+#define EXIT_ASSERT(str) ExitAssert("\p" __FILE__, __LINE__, str)
+
+#define ASSERT(p)                                         \
+	do {                                                  \
+		if (!(p))                                         \
+			ExitAssert("\p" __FILE__, __LINE__, "\p" #p); \
+	} while (0)
+
+struct ErrorParams {
+	ErrorCode err;
+	OSErr osErr;
+	const unsigned char *strParam;
+};
+
+void ShowError(const struct ErrorParams *p);
+
+#endif
diff --git a/macos/icons.r b/macos/icons.r
new file mode 100644
index 0000000..9275e70
--- /dev/null
+++ b/macos/icons.r
@@ -0,0 +1,316 @@
+data 'icl8' (128) {
+	$"0000 0000 0000 0000 0000 0000 0000 00FF"
+	$"0000 0000 0000 00FF 0000 0000 0000 0000"
+	$"0000 0000 0000 0000 0000 0000 0000 FF9F"
+	$"FF00 0000 0000 FFC0 FF00 0000 0000 0000"
+	$"0000 0000 0000 0000 0000 0000 00FF 9F92"
+	$"9FFF 0000 00FF C0C0 C0FF 0000 0000 0000"
+	$"0000 0000 0000 0000 0000 0000 FF9F 929F"
+	$"929F FF00 FFC0 C0C0 C0C0 FF00 0000 0000"
+	$"0000 0000 0000 00FF FFFF FFFF FFFF FFFF"
+	$"9F92 9FFF C0C0 C0C0 C0C0 C0FF 0000 0000"
+	$"0000 0000 0000 FFFF F5F5 F5F5 F5F5 F5FF"
+	$"929F FFFF FFFF C0C0 C0FF FFFF FF00 0000"
+	$"0000 0000 00FF 2BFF F5F5 F5FF FFFF FFFF"
+	$"FFFF FFFF 9FFF C0C0 C0FF 0000 0000 0000"
+	$"0000 0000 FFFF FFFF F5F5 FFFF F5F5 F5F5"
+	$"F5F5 F5FF 92FF C0C0 C0FF 0000 0000 0000"
+	$"0000 0000 FFF5 F5F5 F5FF 2BFF F5F5 F5F5"
+	$"F5F5 F5FF 9FFF C0C0 C0FF 0000 0000 0000"
+	$"0000 0000 FFF5 F5F5 FFFF FFFF F5F5 F5F5"
+	$"F5F5 F5FF 92FF C0C0 C0FF 0000 0000 0000"
+	$"0000 0000 FFF5 F5F5 FFF5 F5F5 F5F5 F5F5"
+	$"F5F5 F5FF 9FFF C0C0 C0FF 0000 0000 0000"
+	$"0000 0000 FFF5 F5F5 FFF5 F5F5 F5F5 FFFF"
+	$"FFFF F5FF 92FF C0C0 C0FF FF00 0000 0000"
+	$"0000 00FF FFF5 F5F5 FFF5 F5F5 F5FF 2A2A"
+	$"2A2A FFFF 9FFF C0C0 C0FF 9FFF 0000 0000"
+	$"0000 FF9F FFF5 F5F5 FFF5 F5F5 FF2A 2A2A"
+	$"2A2A 2AFF FFFF FFFF FFFF FF9F FF00 0000"
+	$"00FF 9F92 FFF5 F5F5 FFF5 F5F5 FF00 2A00"
+	$"2A00 2A00 2A00 2A00 2A00 54FF 9FFF 0000"
+	$"FF9F 929F FFF5 F5F5 FFF5 F5F5 FF2A 002A"
+	$"002A 002A 002A 002A 002A 54FF 929F FF00"
+	$"00FF 9F92 FFF5 F5F5 FFF5 F5F5 FF00 2A00"
+	$"2A00 2A00 2A00 2A00 2A00 54FF 9F92 9FFF"
+	$"0000 FF9F FFF5 F5F5 FFF5 F5F5 FF2A 002A"
+	$"002A 002A 002A 002A 002A 54FF 929F FF00"
+	$"0000 00FF FFF5 F5F5 FFF5 F5F5 FF00 2A00"
+	$"2A00 2A00 2A00 2A00 2A00 54FF 9FFF 0000"
+	$"0000 0000 FFFF FFFF FFF5 F5F5 FF2A 002A"
+	$"002A 002A 002A 002A 002A 54FF FF00 0000"
+	$"0000 0000 00FF FF69 FFF5 F5F5 FF00 2A00"
+	$"2A00 2A00 2A00 2A00 2A00 54FF 0000 0000"
+	$"0000 0000 0000 FF69 FFFF FFFF FF2A 002A"
+	$"002A 002A 002A 002A 002A 54FF 0000 0000"
+	$"0000 0000 0000 FF69 6969 FF92 FF00 2A00"
+	$"2A00 2A00 2A00 2A00 2A00 54FF 0000 0000"
+	$"0000 0000 0000 FF69 6969 FF9F FFFF FFFF"
+	$"FFFF FFFF FFFF FFFF FFFF FFFF 0000 0000"
+	$"0000 0000 0000 FF69 6969 FF92 9F92 9F92"
+	$"9F92 9F92 9F92 9FFF 0000 0000 0000 0000"
+	$"0000 0000 0000 FF69 6969 FF9F 929F 929F"
+	$"929F 929F 929F FF00 0000 0000 0000 0000"
+	$"0000 00FF FFFF FF69 6969 FFFF FFFF 9F92"
+	$"9F92 9F92 9FFF 0000 0000 0000 0000 0000"
+	$"0000 0000 FF69 6969 6969 6969 FF9F 929F"
+	$"929F 929F FF00 0000 0000 0000 0000 0000"
+	$"0000 0000 00FF 6969 6969 69FF 00FF 9F92"
+	$"9F92 9FFF 0000 0000 0000 0000 0000 0000"
+	$"0000 0000 0000 FF69 6969 FF00 0000 FF9F"
+	$"929F FF00 0000 0000 0000 0000 0000 0000"
+	$"0000 0000 0000 00FF 69FF 0000 0000 00FF"
+	$"9FFF 0000 0000 0000 0000 0000 0000 0000"
+	$"0000 0000 0000 0000 FF00 0000 0000 0000"
+	$"FF00 0000 0000 0000 0000 0000 0000 0000"
+};
+
+data 'icl8' (129) {
+	$"0000 00FF FFFF FFFF FFFF FFFF FFFF FFFF"
+	$"FFFF FFFF FFFF 0000 0000 0000 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5F5 F5F5 F5F5 F5F5"
+	$"F5F5 F5F5 F5FF FF00 0000 0000 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5F5 F5F5 F5F5 F5F5"
+	$"F5F5 F5F5 F5FF 2BFF 0000 0000 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5F5 F5FF F5F5 F5F5"
+	$"F5F5 F5F5 F5FF 2B2B FF00 0000 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5F5 FFC0 FFF5 F5F5"
+	$"F5F5 F5F5 F5FF 2B2B 2BFF 0000 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5FF C0C0 C0FF F5F5"
+	$"F5F5 F5F5 F5FF 2B2B 2B2B FF00 0000 0000"
+	$"0000 00FF F5F5 F5F5 FFC0 C0C0 C0C0 FFF5"
+	$"F5F5 F5F5 F5FF FFFF FFFF FFFF 0000 0000"
+	$"0000 00FF F5F5 F5FF C0C0 C0C0 C0C0 C0FF"
+	$"F5F5 F5F5 F5F5 F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 FFFF FFFF C0C0 C0FF FFFF"
+	$"FFF5 F5F5 F5F5 F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5FF C0C0 C0FF F5F5"
+	$"F5F5 F5F5 F5F5 F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5FF C0C0 C0FF F5F5"
+	$"F5F5 F5F5 F5F5 F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5FF C0C0 C0FF F5F5"
+	$"F5FF FFFF FFFF F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5FF C0C0 C0FF F5F5"
+	$"F5FF 6969 69FF F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5FF C0C0 C0FF F5F5"
+	$"F5FF 6969 69FF F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5FF C0C0 C0FF F5F5"
+	$"F5FF 6969 69FF F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5FF C0C0 C0FF F5F5"
+	$"F5FF 6969 69FF F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5FF C0C0 C0FF F5F5"
+	$"F5FF 6969 69FF F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5FF C0C0 C0FF F5F5"
+	$"F5FF 6969 69FF F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5FF C0C0 C0FF F5F5"
+	$"F5FF 6969 69FF F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5FF C0C0 C0FF F5F5"
+	$"F5FF 6969 69FF F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5FF FFFF FFFF F5F5"
+	$"F5FF 6969 69FF F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5F5 F5F5 F5F5 F5F5"
+	$"F5FF 6969 69FF F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5F5 F5F5 F5F5 F5F5"
+	$"F5FF 6969 69FF F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5F5 F5F5 F5F5 FFFF"
+	$"FFFF 6969 69FF FFFF FFF5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5F5 F5F5 F5F5 F5FF"
+	$"6969 6969 6969 69FF F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5F5 F5F5 F5F5 F5F5"
+	$"FF69 6969 6969 FFF5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5F5 F5F5 F5F5 F5F5"
+	$"F5FF 6969 69FF F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5F5 F5F5 F5F5 F5F5"
+	$"F5F5 FF69 FFF5 F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5F5 F5F5 F5F5 F5F5"
+	$"F5F5 F5FF F5F5 F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5F5 F5F5 F5F5 F5F5"
+	$"F5F5 F5F5 F5F5 F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF F5F5 F5F5 F5F5 F5F5 F5F5 F5F5"
+	$"F5F5 F5F5 F5F5 F5F5 F5F5 F5FF 0000 0000"
+	$"0000 00FF FFFF FFFF FFFF FFFF FFFF FFFF"
+	$"FFFF FFFF FFFF FFFF FFFF FFFF 0000 0000"
+};
+
+data 'ICN#' (128) {
+	$"0001 0100 0002 8280 0004 4440 0008 2820"
+	$"01FF 1010 0301 3C78 051F F440 0F30 1440"
+	$"0850 1440 08F0 1440 0880 1440 0883 D460"
+	$"1884 3450 288F FFE8 4888 0014 8888 0012"
+	$"4888 0011 2888 0012 1888 0014 0F88 0018"
+	$"0688 0010 02F8 0010 0228 0010 022F FFF0"
+	$"0220 0100 0220 0200 1E3C 0400 0808 0800"
+	$"0414 1000 0222 2000 0141 4000 0080 8000"
+	$"0001 0100 0003 8380 0007 C7C0 000F EFE0"
+	$"01FF FFF0 03FF FFF8 07FF FFC0 0FFF FFC0"
+	$"0FFF FFC0 0FFF FFC0 0FFF FFC0 0FFF FFE0"
+	$"1FFF FFF0 3FFF FFF8 7FFF FFFC FFFF FFFE"
+	$"7FFF FFFF 3FFF FFFE 1FFF FFFC 0FFF FFF8"
+	$"07FF FFF0 03FF FFF0 03FF FFF0 03FF FFF0"
+	$"03FF FF00 03FF FE00 1FFF FC00 0FFF F800"
+	$"07F7 F000 03E3 E000 01C1 C000 0080 8000"
+};
+
+data 'ICN#' (129) {
+	$"1FFF FC00 1000 0600 1000 0500 1010 0480"
+	$"1028 0440 1044 0420 1082 07F0 1101 0010"
+	$"13C7 8010 1044 0010 1044 0010 1044 7C10"
+	$"1044 4410 1044 4410 1044 4410 1044 4410"
+	$"1044 4410 1044 4410 1044 4410 1044 4410"
+	$"107C 4410 1000 4410 1000 4410 1003 C790"
+	$"1001 0110 1000 8210 1000 4410 1000 2810"
+	$"1000 1010 1000 0010 1000 0010 1FFF FFF0"
+	$"1FFF FC00 1FFF FE00 1FFF FF00 1FFF FF80"
+	$"1FFF FFC0 1FFF FFE0 1FFF FFF0 1FFF FFF0"
+	$"1FFF FFF0 1FFF FFF0 1FFF FFF0 1FFF FFF0"
+	$"1FFF FFF0 1FFF FFF0 1FFF FFF0 1FFF FFF0"
+	$"1FFF FFF0 1FFF FFF0 1FFF FFF0 1FFF FFF0"
+	$"1FFF FFF0 1FFF FFF0 1FFF FFF0 1FFF FFF0"
+	$"1FFF FFF0 1FFF FFF0 1FFF FFF0 1FFF FFF0"
+	$"1FFF FFF0 1FFF FFF0 1FFF FFF0 1FFF FFF0"
+};
+
+data 'icl4' (128) {
+	$"0000 0000 0000 000F 0000 000F 0000 0000"
+	$"0000 0000 0000 00F8 F000 00F7 F000 0000"
+	$"0000 0000 0000 0F88 8F00 0F77 7F00 0000"
+	$"0000 0000 0000 F888 88F0 F777 77F0 0000"
+	$"0000 000F FFFF FFFF 888F 7777 777F 0000"
+	$"0000 00FF 0C0C 0C0F 88FF FF77 7FFF F000"
+	$"0000 0FCF C0CF FFFF FFFF 8F77 7F00 0000"
+	$"0000 FFFF 0CFF 0C0C 0C0F 8F77 7F00 0000"
+	$"0000 F0C0 CFCF C0C0 C0CF 8F77 7F00 0000"
+	$"0000 FC0C FFFF 0C0C 0C0F 8F77 7F00 0000"
+	$"0000 F0C0 F0C0 C0C0 C0CF 8F77 7F00 0000"
+	$"0000 FC0C FC0C 0CFF FF0F 8F77 7FF0 0000"
+	$"000F F0C0 F0C0 CFCC CCFF 8F77 7F8F 0000"
+	$"00F8 FC0C FC0C FCCC CCCF FFFF FFF8 F000"
+	$"0F88 F0C0 F0C0 F0C0 C0C0 C0C0 C0CF 8F00"
+	$"F888 FC0C FC0C FC0C 0C0C 0C0C 0CCF 88F0"
+	$"0F88 F0C0 F0C0 F0C0 C0C0 C0C0 C0CF 888F"
+	$"00F8 FC0C FC0C FC0C 0C0C 0C0C 0CCF 88F0"
+	$"000F F0C0 F0C0 F0C0 C0C0 C0C0 C0CF 8F00"
+	$"0000 FFFF FC0C FC0C 0C0C 0C0C 0CCF F000"
+	$"0000 0FF4 F0C0 F0C0 C0C0 C0C0 C0CF 0000"
+	$"0000 00F4 FFFF FC0C 0C0C 0C0C 0CCF 0000"
+	$"0000 00F4 44F8 F0C0 C0C0 C0C0 C0CF 0000"
+	$"0000 00F4 44F8 FFFF FFFF FFFF FFFF 0000"
+	$"0000 00F4 44F8 8888 8888 888F 0000 0000"
+	$"0000 00F4 44F8 8888 8888 88F0 0000 0000"
+	$"000F FFF4 44FF FF88 8888 8F00 0000 0000"
+	$"0000 F444 4444 F888 8888 F000 0000 0000"
+	$"0000 0F44 444F 0F88 888F 0000 0000 0000"
+	$"0000 00F4 44F0 00F8 88F0 0000 0000 0000"
+	$"0000 000F 4F00 000F 8F00 0000 0000 0000"
+	$"0000 0000 F000 0000 F000 0000 0000 0000"
+};
+
+data 'icl4' (129) {
+	$"000F FFFF FFFF FFFF FFFF FF00 0000 0000"
+	$"000F 0C0C 0C0C 0C0C 0C0C 0FF0 0000 0000"
+	$"000F C0C0 C0C0 C0C0 C0C0 CFCF 0000 0000"
+	$"000F 0C0C 0C0F 0C0C 0C0C 0FCC F000 0000"
+	$"000F C0C0 C0F7 F0C0 C0C0 CFCC CF00 0000"
+	$"000F 0C0C 0F77 7F0C 0C0C 0FCC CCF0 0000"
+	$"000F C0C0 F777 77F0 C0C0 CFFF FFFF 0000"
+	$"000F 0C0F 7777 777F 0C0C 0C0C 0C0F 0000"
+	$"000F C0FF FF77 7FFF F0C0 C0C0 C0CF 0000"
+	$"000F 0C0C 0F77 7F0C 0C0C 0C0C 0C0F 0000"
+	$"000F C0C0 CF77 7FC0 C0C0 C0C0 C0CF 0000"
+	$"000F 0C0C 0F77 7F0C 0FFF FF0C 0C0F 0000"
+	$"000F C0C0 CF77 7FC0 CF44 4FC0 C0CF 0000"
+	$"000F 0C0C 0F77 7F0C 0F44 4F0C 0C0F 0000"
+	$"000F C0C0 CF77 7FC0 CF44 4FC0 C0CF 0000"
+	$"000F 0C0C 0F77 7F0C 0F44 4F0C 0C0F 0000"
+	$"000F C0C0 CF77 7FC0 CF44 4FC0 C0CF 0000"
+	$"000F 0C0C 0F77 7F0C 0F44 4F0C 0C0F 0000"
+	$"000F C0C0 CF77 7FC0 CF44 4FC0 C0CF 0000"
+	$"000F 0C0C 0F77 7F0C 0F44 4F0C 0C0F 0000"
+	$"000F C0C0 CFFF FFC0 CF44 4FC0 C0CF 0000"
+	$"000F 0C0C 0C0C 0C0C 0F44 4F0C 0C0F 0000"
+	$"000F C0C0 C0C0 C0C0 CF44 4FC0 C0CF 0000"
+	$"000F 0C0C 0C0C 0CFF FF44 4FFF FC0F 0000"
+	$"000F C0C0 C0C0 C0CF 4444 444F C0CF 0000"
+	$"000F 0C0C 0C0C 0C0C F444 44FC 0C0F 0000"
+	$"000F C0C0 C0C0 C0C0 CF44 4FC0 C0CF 0000"
+	$"000F 0C0C 0C0C 0C0C 0CF4 FC0C 0C0F 0000"
+	$"000F C0C0 C0C0 C0C0 C0CF C0C0 C0CF 0000"
+	$"000F 0C0C 0C0C 0C0C 0C0C 0C0C 0C0F 0000"
+	$"000F C0C0 C0C0 C0C0 C0C0 C0C0 C0CF 0000"
+	$"000F FFFF FFFF FFFF FFFF FFFF FFFF 0000"
+};
+
+data 'ics#' (128) {
+	$"0110 02A8 0444 08EE 1028 2028 5C2C 942A"
+	$"5429 343A 1404 1408 7710 2220 1540 0880"
+	$"0110 03B8 07FC 0FFE 1FF8 3FF8 7FFC FFFE"
+	$"7FFF 3FFE 1FFC 1FF8 7FF0 3FE0 1DC0 0880"
+};
+
+data 'ics#' (129) {
+	$"7FE0 4430 4A28 513C 7B84 4A04 4AE4 4AA4"
+	$"4AA4 4EA4 40A4 43BC 4114 40A4 4044 7FFC"
+	$"7FE0 7FF0 7FF8 7FFC 7FFC 7FFC 7FFC 7FFC"
+	$"7FFC 7FFC 7FFC 7FFC 7FFC 7FFC 7FFC 7FFC"
+};
+
+data 'ics8' (128) {
+	$"0000 0000 0000 00FF 0000 00FF 0000 0000"
+	$"0000 0000 0000 FF9F FF00 FFC0 FF00 0000"
+	$"0000 0000 00FF 9F92 9FFF C0C0 C0FF 0000"
+	$"0000 0000 FF9F 929F FFFF FFC0 FFFF FF00"
+	$"0000 00FF 9F92 9F92 9F92 FFC0 FF00 0000"
+	$"0000 FF9F 929F 929F 929F FFC0 FF00 0000"
+	$"00FF 9FFF FFFF 9F92 9F92 FFC0 FFFF 0000"
+	$"FF9F 92FF 69FF 929F 929F FFC0 FF9F FF00"
+	$"00FF 9FFF 69FF 9F92 9F92 FFC0 FF92 9FFF"
+	$"0000 FFFF 69FF 929F 929F FFFF FF9F FF00"
+	$"0000 00FF 69FF 9F92 9F92 9F92 9FFF 0000"
+	$"0000 00FF 69FF 929F 929F 929F FF00 0000"
+	$"00FF FFFF 69FF FFFF 9F92 9FFF 0000 0000"
+	$"0000 FF69 6969 FF9F 929F FF00 0000 0000"
+	$"0000 00FF 69FF 00FF 9FFF 0000 0000 0000"
+	$"0000 0000 FF00 0000 FF00 0000 0000 0000"
+};
+
+data 'ics8' (129) {
+	$"00FF FFFF FFFF FFFF FFFF FF00 0000 0000"
+	$"00FF F5F5 F5FF F5F5 F5F5 FFFF 0000 0000"
+	$"00FF F5F5 FFC0 FFF5 F5F5 FF2B FF00 0000"
+	$"00FF F5FF C0C0 C0FF F5F5 FFFF FFFF 0000"
+	$"00FF FFFF FFC0 FFFF FFF5 F5F5 F5FF 0000"
+	$"00FF F5F5 FFC0 FFF5 F5F5 F5F5 F5FF 0000"
+	$"00FF F5F5 FFC0 FFF5 FFFF FFF5 F5FF 0000"
+	$"00FF F5F5 FFC0 FFF5 FF69 FFF5 F5FF 0000"
+	$"00FF F5F5 FFC0 FFF5 FF69 FFF5 F5FF 0000"
+	$"00FF F5F5 FFFF FFF5 FF69 FFF5 F5FF 0000"
+	$"00FF F5F5 F5F5 F5F5 FF69 FFF5 F5FF 0000"
+	$"00FF F5F5 F5F5 FFFF FF69 FFFF FFFF 0000"
+	$"00FF F5F5 F5F5 F5FF 6969 69FF F5FF 0000"
+	$"00FF F5F5 F5F5 F5F5 FF69 FFF5 F5FF 0000"
+	$"00FF F5F5 F5F5 F5F5 F5FF F5F5 F5FF 0000"
+	$"00FF FFFF FFFF FFFF FFFF FFFF FFFF 0000"
+};
+
+data 'ics4' (128) {
+	$"0000 000F 000F 0000 0000 00F8 F0F7 F000"
+	$"0000 0F88 8F77 7F00 0000 F888 FFF7 FFF0"
+	$"000F 8888 88F7 F000 00F8 8888 88F7 F000"
+	$"0F8F FF88 88F7 FF00 F88F 4F88 88F7 F8F0"
+	$"0F8F 4F88 88F7 F88F 00FF 4F88 88FF F8F0"
+	$"000F 4F88 8888 8F00 000F 4F88 8888 F000"
+	$"0FFF 4FFF 888F 0000 00F4 44F8 88F0 0000"
+	$"000F 4F0F 8F00 0000 0000 F000 F000 0000"
+};
+
+data 'ics4' (129) {
+	$"0FFF FFFF FFF0 0000 0F0C 0F0C 0CFF 0000"
+	$"0FC0 F7F0 C0FC F000 0F0F 777F 0CFF FF00"
+	$"0FFF F7FF F0C0 CF00 0F0C F7FC 0C0C 0F00"
+	$"0FC0 F7F0 FFF0 CF00 0F0C F7FC F4FC 0F00"
+	$"0FC0 F7F0 F4F0 CF00 0F0C FFFC F4FC 0F00"
+	$"0FC0 C0C0 F4F0 CF00 0F0C 0CFF F4FF FF00"
+	$"0FC0 C0CF 444F CF00 0F0C 0C0C F4FC 0F00"
+	$"0FC0 C0C0 CFC0 CF00 0FFF FFFF FFFF FF00"
+};
+
diff --git a/macos/main.c b/macos/main.c
new file mode 100644
index 0000000..e92c2b0
--- /dev/null
+++ b/macos/main.c
@@ -0,0 +1,278 @@
+#include "main.h"
+
+#include "error.h"
+#include "project.h"
+#include "resources.h"
+
+#ifndef __MWERKS__
+// Link error if defined with CodeWarrior.
+QDGlobals qd;
+#endif
+
+void QuitApp(void)
+{
+	ExitToShell();
+}
+
+static void HandleClose(WindowRef window)
+{
+	ProjectHandle project;
+
+	if (window == NULL) {
+		return;
+	}
+	switch (GetWindowKind(window)) {
+	case kApplicationWindowKind:
+		project = (ProjectHandle)GetWRefCon(window);
+		ASSERT(project != NULL);
+		ProjectCommand(window, project, rMENU_File, iFile_Close);
+		break;
+	}
+}
+
+static void AdjustMenus(void)
+{
+	WindowRef window;
+	MenuHandle fileMenu;
+	ProjectHandle project;
+	Boolean hasDocument = FALSE;
+
+	fileMenu = GetMenuHandle(rMENU_File);
+	window = FrontWindow();
+	if (window != NULL) {
+		switch (GetWindowKind(window)) {
+		case kApplicationWindowKind:
+			project = (ProjectHandle)GetWRefCon(window);
+			ASSERT(project != NULL);
+			hasDocument = TRUE;
+			ProjectAdjustMenus(project, fileMenu);
+			break;
+		}
+	}
+
+	if (fileMenu != NULL) {
+		EnableItem(fileMenu, iFile_New);
+		DisableItem(fileMenu, iFile_Open);
+		if (!hasDocument) {
+			DisableItem(fileMenu, iFile_Close);
+			DisableItem(fileMenu, iFile_Save);
+			DisableItem(fileMenu, iFile_SaveAs);
+			DisableItem(fileMenu, iFile_Revert);
+		}
+		EnableItem(fileMenu, iFile_Quit);
+	}
+}
+
+static void HandleMenuCommand(long command)
+{
+	int menuID = command >> 16;
+	int item = command & 0xffff;
+	WindowRef window;
+	ProjectHandle project;
+
+	switch (menuID) {
+	case rMENU_Apple:
+		switch (item) {
+		case iApple_Info:
+			return;
+		default: {
+			Str255 itemName;
+			GetMenuItemText(GetMenuHandle(rMENU_Apple), item, itemName);
+			OpenDeskAcc(itemName);
+		}
+			return;
+		}
+		return;
+	case rMENU_File:
+		switch (item) {
+		case iFile_New:
+			ProjectNew();
+			return;
+		case iFile_Quit:
+			QuitApp();
+			return;
+		}
+		break;
+	case rMENU_Edit:
+		break;
+	}
+
+	window = FrontWindow();
+	if (window != NULL) {
+		switch (GetWindowKind(window)) {
+		case kApplicationWindowKind:
+			project = (ProjectHandle)GetWRefCon(window);
+			ASSERT(project != NULL);
+			ProjectCommand(window, project, menuID, item);
+			return;
+		}
+	}
+
+	SysBeep(1);
+}
+
+// Handle a mouse down event.
+static void HandleMouseDown(EventRecord *event)
+{
+	short clickPart;
+	WindowPtr window;
+	ProjectHandle project;
+
+	clickPart = FindWindow(event->where, &window);
+	switch (clickPart) {
+		// case inDesk:
+
+	case inMenuBar:
+		AdjustMenus();
+		HandleMenuCommand(MenuSelect(event->where));
+		HiliteMenu(0);
+		break;
+
+	case inSysWindow:
+		SystemClick(event, window);
+		break;
+
+	case inContent:
+		if (window != FrontWindow()) {
+			SelectWindow(window);
+		} else {
+			switch (GetWindowKind(window)) {
+			case kApplicationWindowKind:
+				project = (ProjectHandle)GetWRefCon(window);
+				ASSERT(project != NULL);
+				ProjectMouseDown(window, project, event);
+				break;
+			}
+		}
+		break;
+
+	case inDrag:
+		if ((event->modifiers & cmdKey) != 0 && window != FrontWindow()) {
+			SelectWindow(window);
+		}
+		DragWindow(window, event->where, &(*GetGrayRgn())->rgnBBox);
+		break;
+
+		// case inGrow:
+
+	case inGoAway:
+		if (TrackGoAway(window, event->where)) {
+			HandleClose(window);
+		}
+		break;
+
+		// case inZoomIn:
+		// case inZoomOut:
+		// case inCollapseBox:
+		// case inProxyIcon:
+	}
+}
+
+static void HandleKeyDown(EventRecord *event)
+{
+	long command;
+
+	if ((event->modifiers & cmdKey) != 0) {
+		AdjustMenus();
+		command = MenuKey(event->message & charCodeMask);
+		HandleMenuCommand(command);
+		HiliteMenu(0);
+	}
+}
+
+static void HandleActivate(EventRecord *event)
+{
+	ProjectHandle project;
+	WindowRef window;
+	GrafPtr savedPort;
+	int isActive;
+
+	window = (WindowRef)event->message;
+	GetPort(&savedPort);
+	SetPort(window);
+	isActive = (event->modifiers & activeFlag) != 0;
+	switch (GetWindowKind(window)) {
+	case kApplicationWindowKind:
+		project = (ProjectHandle)GetWRefCon(window);
+		ASSERT(project != NULL);
+		ProjectActivate(window, project, isActive);
+		break;
+	}
+	SetPort(savedPort);
+}
+
+static void HandleUpdate(EventRecord *event)
+{
+	ProjectHandle project;
+	WindowPtr window;
+	GrafPtr savedPort;
+
+	window = (WindowPtr)event->message;
+	GetPort(&savedPort);
+	SetPort(window);
+	BeginUpdate(window);
+	switch (GetWindowKind(window)) {
+	case kApplicationWindowKind:
+		project = (ProjectHandle)GetWRefCon(window);
+		ASSERT(project != NULL);
+		ProjectUpdate(window, project);
+		break;
+	}
+	EndUpdate(window);
+	SetPort(savedPort);
+}
+
+void main(void)
+{
+	MaxApplZone();
+	InitGraf(&qd.thePort);
+	InitFonts();
+	InitWindows();
+	InitMenus();
+	TEInit();
+	InitDialogs(NULL);
+	InitCursor();
+
+	// Set up menu bar.
+	{
+		Handle menuBar;
+		menuBar = GetNewMBar(rMBAR_Main);
+		if (menuBar == NULL) {
+			EXIT_ASSERT(NULL);
+		}
+		SetMenuBar(menuBar);
+		DisposeHandle(menuBar);
+		AppendResMenu(GetMenuHandle(rMENU_Apple), 'DRVR');
+		DrawMenuBar();
+	}
+
+	for (;;) {
+		EventRecord event;
+		WaitNextEvent(everyEvent, &event, 1, NULL);
+		switch (event.what) {
+		case mouseDown:
+			HandleMouseDown(&event);
+			break;
+
+			// case mouseUp:
+
+		case keyDown:
+			HandleKeyDown(&event);
+			break;
+
+			// case autoKey:
+
+		case activateEvt:
+			HandleActivate(&event);
+			break;
+
+		case updateEvt:
+			HandleUpdate(&event);
+			break;
+
+			// case diskEvt:
+			// osEvt:
+			// case kHighLevelEvent:
+		}
+	}
+}
diff --git a/macos/main.h b/macos/main.h
new file mode 100644
index 0000000..c5b0e1e
--- /dev/null
+++ b/macos/main.h
@@ -0,0 +1,21 @@
+// main.h - Main entry point.
+#ifndef MACOS_MAIN_H
+#define MACOS_MAIN_H
+
+// Menu item IDs.
+enum {
+	iApple_Info = 1,
+
+	iFile_New = 1,
+	iFile_Open = 2,
+	iFile_Close = 4,
+	iFile_Save = 5,
+	iFile_SaveAs = 6,
+	iFile_Revert = 7,
+	iFile_Quit = 9,
+};
+
+// Quit the application.
+void QuitApp(void);
+
+#endif
diff --git a/macos/menu.r b/macos/menu.r
new file mode 100644
index 0000000..e737a6a
--- /dev/null
+++ b/macos/menu.r
@@ -0,0 +1,56 @@
+#include "Menus.r"
+
+#include "resources.h"
+
+resource 'MBAR' (rMBAR_Main) {{
+	rMENU_Apple,
+	rMENU_File,
+	rMENU_Edit,
+}};
+
+resource 'MENU' (rMENU_Apple, preload) {
+	rMENU_Apple,
+	textMenuProc,
+	0b01111111111111111111111111111101,
+	enabled,
+	apple,
+	{
+		"About " APPNAME "…", noIcon, noKey, noMark, plain,
+		"-",                noIcon, noKey, noMark, plain,
+	}
+};
+
+resource 'MENU' (rMENU_File, preload) {
+	rMENU_File,
+	textMenuProc,
+	0b01111111111111111111111101111011,
+	enabled,
+	"File",
+	{
+		"New",              noIcon, "N",   noMark, plain,
+		"Open…",            noIcon, "O",   noMark, plain,
+		"-",                noIcon, noKey, noMark, plain,
+		"Close",            noIcon, "W",   noMark, plain,
+		"Save",             noIcon, "S",   noMark, plain,
+		"Save As…",         noIcon, noKey, noMark, plain,
+		"Revert",           noIcon, noKey, noMark, plain,
+		"-",                noIcon, noKey, noMark, plain,
+		"Quit",             noIcon, "Q",   noMark, plain,
+	}
+};
+
+resource 'MENU' (rMENU_Edit, preload) {
+	rMENU_Edit,
+	textMenuProc,
+	0b01111111111111111111111111111101,
+	enabled,
+	"Edit",
+	{
+		"Undo",             noIcon, "Z",   noMark, plain,
+		"-",                noIcon, noKey, noMark, plain,
+		"Cut",              noIcon, "X",   noMark, plain,
+		"Copy",             noIcon, "C",   noMark, plain,
+		"Paste",            noIcon, "V",   noMark, plain,
+		"Clear",            noIcon, noKey, noMark, plain,
+	}
+};
diff --git a/macos/path.c b/macos/path.c
new file mode 100644
index 0000000..0d6b6a4
--- /dev/null
+++ b/macos/path.c
@@ -0,0 +1,90 @@
+#include "path.h"
+
+#include "error.h"
+
+#include <string.h>
+
+enum {
+	kMaxPath = 512,
+};
+
+OSErr GetDirPath(FSSpec *spec, int *pathLength, Handle *path)
+{
+	Handle segments; // Packed sequence of segments, Pascal strings.
+	int textLen;     // Total length of segments, including length byte.
+	Handle result;
+	unsigned char *name;
+	CInfoPBRec ci;
+	OSErr err;
+	unsigned char *op;
+	const unsigned char *ip, *ie;
+	int length;
+
+	// Query target.
+	memset(&ci, 0, sizeof(ci));
+	ci.dirInfo.ioNamePtr = spec->name;
+	ci.dirInfo.ioVRefNum = spec->vRefNum;
+	ci.dirInfo.ioDrDirID = spec->parID;
+	err = PBGetCatInfoSync(&ci);
+	if (err != noErr) {
+		return err;
+	}
+	// FIXME: kioFlAttribDirMask
+	if ((ci.hFileInfo.ioFlAttrib & ioDirMask) == 0) {
+		return fnfErr;
+	}
+	segments = NULL;
+	textLen = spec->name[0] + 1;
+	PtrToHand(spec->name, &segments, textLen);
+	if (segments == NULL) {
+		return MemError();
+	}
+
+	// Add name of parent.
+	while (ci.dirInfo.ioDrParID != fsRtParID) {
+		ASSERT(textLen < kMaxPath);
+		SetHandleSize(segments, textLen + 256);
+		err = MemError();
+		if (err != noErr) {
+			goto done;
+		}
+		HLock(segments);
+		name = (unsigned char *)*segments + textLen;
+		ci.dirInfo.ioNamePtr = name;
+		ci.dirInfo.ioDrDirID = ci.dirInfo.ioDrParID;
+		ci.dirInfo.ioFDirIndex = -1;
+		err = PBGetCatInfoSync(&ci);
+		HUnlock(segments);
+		if (err != 0) {
+			goto done;
+		}
+		textLen += name[0] + 1;
+	}
+
+	// Copy segments into handle in reverse order.
+	result = NewHandle(textLen);
+	if (result == NULL) {
+		err = MemError();
+		goto done;
+	}
+	op = (unsigned char *)*result + textLen;
+	ip = (const unsigned char *)*segments;
+	ie = ip + textLen;
+	while (ip < ie) {
+		op--;
+		*op = ':';
+		length = *ip;
+		op -= length;
+		BlockMoveData(ip + 1, op, length);
+		ip += length + 1;
+	}
+	*pathLength = textLen;
+	*path = result;
+	err = 0;
+
+done:
+	if (segments != NULL) {
+		DisposeHandle(segments);
+	}
+	return err;
+}
diff --git a/macos/path.h b/macos/path.h
new file mode 100644
index 0000000..ef3a18c
--- /dev/null
+++ b/macos/path.h
@@ -0,0 +1,8 @@
+// path.h - Path manipulation utilities.
+#ifndef MACOS_PATH_H
+#define MACOS_PATH_H
+
+// Get the full path to a directory.
+OSErr GetDirPath(FSSpec *spec, int *pathLength, Handle *path);
+
+#endif
diff --git a/macos/project.c b/macos/project.c
new file mode 100644
index 0000000..9e091a7
--- /dev/null
+++ b/macos/project.c
@@ -0,0 +1,530 @@
+#include "project.h"
+
+#include "choose_directory.h"
+#include "crc32.h"
+#include "error.h"
+#include "main.h"
+#include "path.h"
+#include "resources.h"
+#include "tempfile.h"
+
+#include <Files.h>
+#include <MacMemory.h>
+#include <MacWindows.h>
+
+#include <string.h>
+
+/*
+Project format:
+
+Header:
+byte[32]	magic
+uint32		version
+uint32		file size
+uint32		data crc32
+uint32		chunk count
+ckinfo[]	chunk info
+byte[]		chunk data
+
+Chunk info:
+byte[8]		chunk name
+uint32		byte offset
+uint32		byte length
+
+Chunks: (names zero-padded)
+"s.alias"	source directory alias
+"d.alias"	destination directory alias
+
+Chunks are aligned to 16 byte boundaries, and the file is padded to a 16 byte
+boundary. This is just to make it easier to read hexdumps.
+*/
+
+// clang-format off
+
+// Magic cookie that identifies project files.
+static const kMagic[32] = {
+	// Identify the file type.
+	'S', 'y', 'n', 'c', 'F', 'i', 'l', 'e', 's', ' ',
+	'P', 'r', 'o', 'j', 'e', 'c', 't',
+	// Detect newline conversions.
+	0, 10, 0, 13, 0, 13, 10, 0,
+	// Detect encoding conversions (infinity in Roman and UTF-8).
+	0xb0, 0, 0xe2, 0x88, 0x9e, 0, 0,
+};
+
+// clang-format on
+
+static const kChunkAlias[2][8] = {
+	{'l', '.', 'a', 'l', 'i', 'a', 's', 0},
+	{'r', '.', 'a', 'l', 'i', 'a', 's', 0},
+};
+
+struct ProjectHeader {
+	UInt8 magic[32];
+	UInt32 version;
+	UInt32 size;
+	UInt32 crc32;
+	UInt32 chunkCount;
+};
+
+struct ProjectChunk {
+	UInt8 name[8];
+	UInt32 offset;
+	UInt32 size;
+};
+
+// Dimensions of controls.
+enum {
+	kVBorder = 10,
+	kHBorder = 10,
+	kDirVSize = 62,
+};
+
+enum {
+	// Base ID for aliases.
+	rAliasBase = 128,
+};
+
+enum {
+	kControlChooseDirLocal,
+	kControlChooseDirRemote,
+};
+
+// FIXME: this is redundant.
+enum {
+	kDirLocal,
+	kDirRemote,
+};
+
+struct ProjectDir {
+	int pathLength;
+	Handle path;
+	AliasHandle alias;
+};
+
+// A synchronization project.
+struct Project {
+	int windowWidth;
+	int isActive;
+
+	// File reference to the project file, or fileRef == 0 if file not saved.
+	short fileRef;
+	FSSpec fileSpec;
+
+	struct ProjectDir dirs[2];
+};
+
+void ProjectNew(void)
+{
+	ProjectHandle project;
+	WindowRef window;
+	ControlRef control;
+	struct Project *projectp;
+	int i, windowWidth, controlWidth;
+
+	project = (ProjectHandle)NewHandle(sizeof(struct Project));
+	if (project == NULL) {
+		EXIT_ASSERT(NULL);
+	}
+	window = GetNewCWindow(rWIND_Project, NULL, (WindowPtr)-1);
+	if (window == NULL) {
+		EXIT_ASSERT(NULL);
+	}
+	windowWidth = window->portRect.right - window->portRect.left;
+	SetWRefCon(window, (long)project);
+	projectp = *project;
+	memset(projectp, 0, sizeof(*projectp));
+	projectp->windowWidth = windowWidth;
+
+	for (i = 0; i < 2; i++) {
+		control = GetNewControl(rCNTL_ChooseFolder, window);
+		if (control == NULL) {
+			EXIT_ASSERT(NULL);
+		}
+		controlWidth =
+			(*control)->contrlRect.right - (*control)->contrlRect.left;
+		MoveControl(control, windowWidth - controlWidth - kHBorder,
+		            kVBorder + kDirVSize * i + 22);
+		SetControlReference(control, kControlChooseDirLocal + i);
+	}
+
+	ShowWindow(window);
+}
+
+void ProjectAdjustMenus(ProjectHandle project, MenuHandle fileMenu)
+{
+	(void)project;
+	if (fileMenu != NULL) {
+		EnableItem(fileMenu, iFile_Close);
+		EnableItem(fileMenu, iFile_Save);
+		EnableItem(fileMenu, iFile_SaveAs);
+		EnableItem(fileMenu, iFile_Revert);
+	}
+}
+
+static void ProjectClose(WindowRef window, ProjectHandle project)
+{
+	struct Project *projectp;
+	int i;
+
+	KillControls(window);
+	DisposeWindow(window);
+	HLock((Handle)project);
+	projectp = *project;
+	if (projectp->fileRef != 0) {
+		FSClose(projectp->fileRef);
+	}
+	for (i = 0; i < 2; i++) {
+		if (projectp->dirs[i].path != NULL) {
+			DisposeHandle(projectp->dirs[i].path);
+		}
+	}
+	DisposeHandle((Handle)project);
+}
+
+#define kHeaderSize 48
+#define kChunkInfoSize 16
+#define kChunkCount 2
+
+#define ALIGN(x) (((x) + 15) + ~(UInt32)15)
+
+struct ProjectData {
+	Handle data;
+	Size size;
+};
+
+static OSErr ProjectMarshal(ProjectHandle project, struct ProjectData *datap)
+{
+	struct ProjectChunk ckInfo[kChunkCount];
+	Handle ckData[kChunkCount];
+	UInt32 size, pos, ckOff, ckSize;
+	Handle h;
+	int i;
+	struct ProjectHeader *hdr;
+
+	size = kHeaderSize + kChunkCount * kChunkInfoSize;
+	for (i = 0; i < 2; i++) {
+		h = (Handle)(*project)->dirs[i].alias;
+		ckSize = GetHandleSize(h);
+		memcpy(ckInfo[i].name, kChunkAlias[i], 8);
+		ckInfo[i].offset = size;
+		ckInfo[i].size = ckSize;
+		ckData[i] = h;
+		size = ALIGN(size + ckSize);
+	}
+
+	h = NewHandle(size);
+	if (h == NULL) {
+		return MemError();
+	}
+	hdr = (void *)*h;
+	memcpy(hdr->magic, kMagic, sizeof(kMagic));
+	hdr->version = 0x10000;
+	hdr->size = size;
+	hdr->chunkCount = kChunkCount;
+	memcpy(*h + kHeaderSize, ckInfo, sizeof(ckInfo));
+	pos = kHeaderSize + kChunkCount * kChunkInfoSize;
+	for (i = 0; i < kChunkCount; i++) {
+		ckOff = ckInfo[i].offset;
+		ckSize = ckInfo[i].size;
+		memset(*h + pos, 0, ckOff - pos);
+		memcpy(*h + ckOff, *ckData[i], ckSize);
+		pos = ckOff + ckSize;
+	}
+	memcpy(*h + pos, 0, size - pos);
+
+	datap->data = h;
+	datap->size = size;
+	return noErr;
+}
+
+// Prompt the user for a location to save the project.
+static void ProjectSaveAs(WindowRef window, ProjectHandle project)
+{
+	// See listing 1-12 in IM: Files. This has some differences.
+	Str255 name;
+	StandardFileReply reply;
+	short refNum;
+	struct ProjectData data;
+	struct Project *projectp;
+	OSErr err;
+	Boolean hasfile;
+	ParamBlockRec pb;
+	struct ErrorParams errp;
+	long size;
+
+	data.data = NULL;
+	refNum = 0;
+	hasfile = FALSE;
+
+	GetWTitle(window, name);
+	StandardPutFile("\pSave project as:", name, &reply);
+	if (!reply.sfGood) {
+		return;
+	}
+
+	err = ProjectMarshal(project, &data);
+	if (err != noErr) {
+		goto error;
+	}
+
+	// Delete file if replacing. This way, we get a new creation date (makes
+	// sense) and the correct creator and type codes.
+	if (reply.sfReplacing) {
+		err = FSpDelete(&reply.sfFile);
+		if (err != noErr) {
+			goto error;
+		}
+	}
+
+	// Write out the new file.
+	err = FSpCreate(&reply.sfFile, kCreator, kTypeProject, reply.sfScript);
+	if (err != noErr) {
+		goto error;
+	}
+	hasfile = TRUE;
+	err = FSpOpenDF(&reply.sfFile, fsWrPerm, &refNum);
+	if (err != noErr) {
+		goto error;
+	}
+	size = data.size;
+	err = FSWrite(refNum, &size, *data.data);
+	if (err != noErr) {
+		goto error;
+	}
+	DisposeHandle(data.data);
+	data.data = NULL;
+	memset(&pb, 0, sizeof(pb));
+	pb.fileParam.ioFRefNum = refNum;
+	PBFlushFile(&pb, FALSE);
+	err = pb.fileParam.ioResult;
+	if (err != noErr) {
+		goto error;
+	}
+
+	// Done writing, update the window and project.
+	SetWTitle(window, reply.sfFile.name);
+	projectp = *project;
+	if (projectp->fileRef != 0) {
+		FSClose(projectp->fileRef);
+	}
+	projectp->fileRef = refNum;
+	projectp->fileSpec = reply.sfFile;
+	return;
+
+error:
+	if (data.data != NULL) {
+		DisposeHandle(data.data);
+	}
+	if (refNum != 0) {
+		FSClose(refNum);
+	}
+	if (hasfile) {
+		FSpDelete(&reply.sfFile);
+	}
+	memset(&errp, 0, sizeof(errp));
+	errp.err = kErrCouldNotSaveProject;
+	errp.osErr = err;
+	errp.strParam = reply.sfFile.name;
+	ShowError(&errp);
+}
+
+static void ProjectSaveImpl(WindowRef window, ProjectHandle project)
+{
+	struct ProjectData data;
+	short refNum;
+	FSSpec temp;
+	OSErr err;
+	struct ErrorParams errp;
+	Boolean hasfile;
+	long size;
+
+	(void)window;
+
+	data.data = NULL;
+	refNum = 0;
+	hasfile = FALSE;
+
+	err = ProjectMarshal(project, &data);
+	if (err != noErr) {
+		goto error;
+	}
+	err = MakeTempFile((*project)->fileSpec.vRefNum, &temp);
+	if (err != noErr) {
+		goto error;
+	}
+	err = FSpCreate(&temp, 'trsh', 'trsh', smSystemScript);
+	if (err != noErr) {
+		goto error;
+	}
+	hasfile = TRUE;
+	err = FSpOpenDF(&temp, fsWrPerm, &refNum);
+	if (err != noErr) {
+		goto error;
+	}
+	size = data.size;
+	err = FSWrite(refNum, &size, *data.data);
+	if (err != noErr) {
+		goto error;
+	}
+	DisposeHandle(data.data);
+	data.data = NULL;
+	err = FSClose(refNum);
+	refNum = 0;
+	if (err != noErr) {
+		goto error;
+	}
+	err = FSpExchangeFiles(&temp, &(*project)->fileSpec);
+	if (err != noErr) {
+		goto error;
+	}
+	FSpDelete(&temp);
+	return;
+
+error:
+	if (data.data) {
+		DisposeHandle(data.data);
+	}
+	if (refNum != 0) {
+		FSClose(refNum);
+	}
+	if (hasfile) {
+		FSpDelete(&temp);
+	}
+	memcpy(&temp.name, &(*project)->fileSpec.name, sizeof(temp.name));
+	memset(&errp, 0, sizeof(errp));
+	errp.err = kErrCouldNotSaveProject;
+	errp.osErr = err;
+	errp.strParam = temp.name;
+	ShowError(&errp);
+}
+
+// Save the project to disk in response to a Save command.
+static void ProjectSave(WindowRef window, ProjectHandle project)
+{
+	if ((*project)->fileRef == 0) {
+		ProjectSaveAs(window, project);
+	} else {
+		ProjectSaveImpl(window, project);
+	}
+}
+
+void ProjectCommand(WindowRef window, ProjectHandle project, int menuID,
+                    int item)
+{
+	switch (menuID) {
+	case rMENU_File:
+		switch (item) {
+		case iFile_Close:
+			ProjectClose(window, project);
+			break;
+		case iFile_Save:
+			ProjectSave(window, project);
+			break;
+		case iFile_SaveAs:
+			ProjectSaveAs(window, project);
+			break;
+		}
+		break;
+	}
+}
+
+void ProjectUpdate(WindowRef window, ProjectHandle project)
+{
+	struct Project *projectp;
+	Rect rect;
+	Handle text;
+	int i;
+
+	HLock((Handle)project);
+
+	projectp = *project;
+	rect = window->portRect;
+	EraseRect(&rect);
+	MoveTo(0, kVBorder + kDirVSize - 10);
+	LineTo(projectp->windowWidth, kVBorder + kDirVSize - 10);
+
+	for (i = 0; i < 2; i++) {
+		MoveTo(kHBorder, kVBorder + kDirVSize * i + 12);
+		DrawString(i == 0 ? "\pLocal Folder" : "\pRemote Folder");
+		text = projectp->dirs[i].path;
+		if (text != NULL) {
+			HLock(text);
+			MoveTo(kHBorder, kVBorder + kDirVSize * i + 36);
+			DrawText(*text, 0, projectp->dirs[i].pathLength);
+			HUnlock(text);
+		}
+	}
+	UpdateControls(window, window->visRgn);
+
+	HUnlock((Handle)project);
+}
+
+void ProjectActivate(WindowRef window, ProjectHandle project, int isActive)
+{
+	struct Project *projectp;
+
+	projectp = *project;
+	if (projectp->isActive != isActive) {
+		projectp->isActive = isActive;
+		InvalRect(&window->portRect);
+	}
+}
+
+static void ProjectChooseDir(WindowRef window, ProjectHandle project,
+                             int whichDir)
+{
+	struct Project *projectp;
+	struct ProjectDir *dirp;
+	FSSpec directory;
+	Handle path;
+	int pathLength;
+	OSErr err;
+
+	if (ChooseDirectory(&directory)) {
+		err = GetDirPath(&directory, &pathLength, &path);
+		if (err != noErr) {
+			ExitErrorOS(kErrUnknown, err);
+		}
+		projectp = *project;
+		dirp = &projectp->dirs[whichDir];
+		if (dirp->path != NULL) {
+			DisposeHandle(dirp->path);
+		}
+		dirp->pathLength = pathLength;
+		dirp->path = path;
+		InvalRect(&window->portRect);
+	}
+}
+
+void ProjectMouseDown(WindowRef window, ProjectHandle project,
+                      EventRecord *event)
+{
+	Point mouse;
+	GrafPtr savedPort;
+	ControlRef control;
+	ControlPartCode part;
+	long reference;
+
+	GetPort(&savedPort);
+	SetPort(window);
+	mouse = event->where;
+	GlobalToLocal(&mouse);
+	part = FindControl(mouse, window, &control);
+	switch (part) {
+	case kControlButtonPart:
+		reference = GetControlReference(control);
+		if (TrackControl(control, mouse, NULL) != 0) {
+			switch (reference) {
+			case kControlChooseDirLocal:
+				ProjectChooseDir(window, project, kDirLocal);
+				break;
+			case kControlChooseDirRemote:
+				ProjectChooseDir(window, project, kDirRemote);
+				break;
+			}
+		}
+		break;
+	}
+	SetPort(savedPort);
+}
diff --git a/macos/project.h b/macos/project.h
new file mode 100644
index 0000000..a0503a4
--- /dev/null
+++ b/macos/project.h
@@ -0,0 +1,28 @@
+// project.h - Synchronization projects.
+#ifndef MACOS_PROJECT_H
+#define MACOS_PROJECT_H
+
+// Handle to a synchronization project.
+typedef struct Project **ProjectHandle;
+
+// Create a new empty synchronization project.
+void ProjectNew(void);
+
+// Adjust menus before selecting a menu item.
+void ProjectAdjustMenus(ProjectHandle project, MenuHandle fileMenu);
+
+// Handle a menu command or equivalent.
+void ProjectCommand(WindowRef window, ProjectHandle project, int menuID,
+                    int item);
+
+// Handle an update event for a project window.
+void ProjectUpdate(WindowRef window, ProjectHandle project);
+
+// Set whether a project window is active.
+void ProjectActivate(WindowRef window, ProjectHandle project, int isActive);
+
+// Handle a mouse down event in a window.
+void ProjectMouseDown(WindowRef window, ProjectHandle project,
+                      EventRecord *event);
+
+#endif
diff --git a/macos/resources.h b/macos/resources.h
new file mode 100644
index 0000000..53fabd4
--- /dev/null
+++ b/macos/resources.h
@@ -0,0 +1,32 @@
+#ifndef MACOS_RESOURCES_H
+#define MACOS_RESOURCES_H
+
+#define APPNAME "SyncFiles"
+
+#define kCreator 'SyFl'
+#define kTypeProject 'SyPr'
+
+/* ALRT Error dialog */
+#define rAlrtError 128
+
+/* CNTL Controls */
+#define rCNTL_ChooseFolder 128
+
+/* DLOG Dialogs */
+#define rDLOG_OpenFolder 128
+
+/* MBAR Menu bar */
+#define rMBAR_Main 128
+
+/* MENU Menus */
+#define rMENU_Apple 128
+#define rMENU_File 129
+#define rMENU_Edit 130
+
+/* STR# Error messages. */
+#define rSTRS_Errors 128
+
+/* WIND Windows */
+#define rWIND_Project 128
+
+#endif
diff --git a/macos/resources.r b/macos/resources.r
new file mode 100644
index 0000000..e96623d
--- /dev/null
+++ b/macos/resources.r
@@ -0,0 +1,147 @@
+#include "Controls.r"
+#include "Dialogs.r"
+#include "Finder.r"
+#include "MacTypes.r"
+#include "Processes.r"
+
+#include "resources.h"
+
+resource 'ALRT' (rAlrtError, purgeable) {
+	{ 40, 40, 141, 360 },
+	128, /* DITL ID */
+	{
+		OK, visible, sound1,
+		OK, visible, sound1,
+		OK, visible, sound1,
+		OK, visible, sound1,
+	},
+	centerMainScreen
+};
+
+/* See Macintosh Human Interface Guidelines (1992) p. 197 "Basic Dialog Box Layout" */
+/* Note that there are an extra 3px of margin for "free" on the dialog border. */
+resource 'DITL' (128, purgeable) {{
+	/* Quit button: 13px margin */
+	{ 71, 240, 91, 310 },
+	Button { enabled, "Quit" },
+	/* Error text: 13px margin, 3 lines @ 16px line height */
+	{ 10, 75, 58, 310 },
+	StaticText { enabled, "^0" },
+	/* Icon: 13px vertical, 23px horizontal */
+	{ 10, 20, 42, 52 },
+	Icon { enabled, 0 },
+}};
+
+resource 'DLOG' (rDLOG_OpenFolder, purgeable) {
+	/* Height += 31 */
+	{0, 0, 197, 344},
+	dBoxProc,
+	invisible,
+	noGoAway,
+	0,
+	129,
+	"",
+	noAutoCenter,
+};
+
+/* Open Folder, based on system -6042 */
+resource 'DITL' (129, purgeable) {{
+	/* Standard file (do not change, see IM: Files 3-18) */
+	{135, 252, 155, 332},
+	Button {enabled, "Open"},
+	{104, 252, 124, 332},
+	Button {enabled, "Cancel"},
+	{0, 0, 0, 0},
+	HelpItem {disabled, HMScanhdlg {-6042}},
+	{8, 235, 24, 337},
+	UserItem {enabled},
+	{32, 252, 52, 332},
+	Button {enabled, "Eject"},
+	{60, 252, 80, 332},
+	Button {enabled, "Desktop"},
+	{29, 12, 159, 230},
+	UserItem {enabled},
+	{6, 12, 25, 230},
+	UserItem {enabled},
+	{91, 251, 92, 333},
+	Picture {disabled, 11},
+	/* Additions */
+	{166, 252, 186, 332},
+	Button {enabled, "Choose"},
+	{166, 12, 186, 230},
+	Button {enabled, "Choose Current Directory"},
+}};
+
+#ifdef MPW
+resource 'SIZE' (-1) {
+	dontSaveScreen,
+	acceptSuspendResumeEvents,
+	enableOptionSwitch,
+	canBackground,
+	multiFinderAware,
+	backgroundAndForeground,
+	dontGetFrontClicks,
+	ignoreChildDiedEvents,
+	is32BitCompatible,
+	notHighLevelEventAware,
+	onlyLocalHLEvents,
+	notStationeryAware,
+	dontUseTextEditServices,
+	notDisplayManagerAware,
+	reserved,
+	reserved,
+	60 * 1024,
+	60 * 1024
+};
+#endif
+
+type kCreator as 'STR ';
+resource kCreator (0, purgeable) {
+	APPNAME " 0.1 ©2022 Dietrich Epp"
+};
+
+resource 'vers' (1) {
+	0x00, 0x01, development, 0x00,
+	verUS,
+	"0.1",
+	"0.1, Copyright ©2021 Dietrich Epp"
+};
+
+resource 'FREF' (128, purgeable) {'APPL', 0, ""};
+resource 'FREF' (129, purgeable) { kTypeProject, 1, ""};
+resource 'BNDL' (128, purgeable) {
+	kCreator,
+	0,
+	{
+		'ICN#', {0, 128, 1, 129},
+		'FREF', {0, 128, 1, 129},
+	},
+};
+
+resource 'STR#' (rSTRS_Errors) {{
+	"An unknown error occurred.",
+	"An internal error occurred.",
+	"Out of memory.",
+	"Could not save project “^1”.",
+}};
+
+resource 'WIND' (rWIND_Project, preload, purgeable) {
+	{32, 32, 256, 32 + 128*3},
+	documentProc,
+	invisible,
+	goAway,
+	0,
+	"Untitled Project",
+	staggerMainScreen,
+};
+
+resource 'CNTL' (rCNTL_ChooseFolder, preload, purgeable) {
+	{10, 10, 30, 90},
+	0,
+	visible,
+	1,
+	0,
+	pushButProc,
+	0,
+	"Choose…",
+};
diff --git a/macos/strutil.c b/macos/strutil.c
new file mode 100644
index 0000000..c01b66e
--- /dev/null
+++ b/macos/strutil.c
@@ -0,0 +1,135 @@
+#include "strutil.h"
+
+#include "error.h"
+
+#include <NumberFormatting.h>
+
+#include <stdarg.h>
+#include <string.h>
+
+static unsigned char *StrPtrAppend(unsigned char *ptr, unsigned char *end,
+                                   const unsigned char *str)
+{
+	unsigned i, len, rem;
+
+	len = str[0];
+	rem = end - ptr;
+	if (len > rem) {
+		len = rem;
+	}
+	str++;
+	for (i = 0; i < len; i++) {
+		*ptr++ = *str++;
+	}
+	return ptr;
+}
+
+static void StrAppendVFormat(unsigned char *dest, const unsigned char *msg,
+                             va_list ap)
+{
+	const unsigned char *mptr = msg + 1, *mend = mptr + msg[0];
+	unsigned char *ptr = dest + 1 + dest[0], *end = dest + 256;
+	unsigned char c, temp[16];
+	const unsigned char *pstrvalue;
+	int ivalue;
+
+next:
+	while (mptr < mend) {
+		c = *mptr++;
+		if (c == '%' && mptr < mend) {
+			switch (*mptr) {
+			case '%':
+				mptr++;
+				break;
+			case 'd':
+				mptr++;
+				ivalue = va_arg(ap, int);
+				NumToString(ivalue, temp);
+				ptr = StrPtrAppend(ptr, end, temp);
+				goto next;
+			case 'S':
+				mptr++;
+				pstrvalue = va_arg(ap, const unsigned char *);
+				ptr = StrPtrAppend(ptr, end, pstrvalue);
+				goto next;
+			}
+		}
+		if (ptr == end) {
+			break;
+		}
+		*ptr++ = c;
+	}
+
+	*dest = ptr - dest - 1;
+}
+
+void StrFormat(unsigned char *dest, const unsigned char *msg, ...)
+{
+	va_list ap;
+
+	dest[0] = 0;
+	va_start(ap, msg);
+	StrAppendVFormat(dest, msg, ap);
+	va_end(ap);
+}
+
+void StrAppendFormat(unsigned char *dest, const unsigned char *msg, ...)
+{
+	va_list ap;
+
+	va_start(ap, msg);
+	StrAppendVFormat(dest, msg, ap);
+	va_end(ap);
+}
+
+void StrCopy(unsigned char *dest, int dest_size, unsigned char *src)
+{
+	int len = src[0] + 1;
+	if (len > dest_size) {
+		EXIT_ASSERT(NULL);
+	}
+	BlockMoveData(src, dest, len);
+}
+
+struct StrRef {
+	const unsigned char *src;
+	int len;
+};
+
+void StrSubstitute(unsigned char *str, const unsigned char *param)
+{
+	int i, n, end;
+	struct StrRef refs[3];
+
+	n = str[0];
+	for (i = 0; i < n - 1; i++) {
+		if (str[i + 1] == '^' && str[i + 2] == '1') {
+			refs[0].src = str + 1;
+			refs[0].len = i;
+			refs[1].src = param + 1;
+			refs[1].len = param[0];
+			refs[2].src = str + i + 3;
+			refs[2].len = n - i - 2;
+			goto concat;
+		}
+	}
+	return;
+
+concat:
+	n = 0;
+	for (i = 0; i < 3; i++) {
+		n += refs[i].len;
+	}
+	str[0] = n > 255 ? 255 : n;
+	for (i = 3; i > 0;) {
+		i--;
+		end = n;
+		n -= refs[i].len;
+		if (end > 255) {
+			end = 255;
+		}
+		if (n < end) {
+			memmove(str + 1 + n, refs[i].src, end - n);
+		}
+	}
+}
diff --git a/macos/strutil.h b/macos/strutil.h
new file mode 100644
index 0000000..af492ec
--- /dev/null
+++ b/macos/strutil.h
@@ -0,0 +1,22 @@
+// strutil.h - String utilities.
+#ifndef MACOS_STRUTIL_H
+#define MACOS_STRUTIL_H
+
+// Write a formatted string.
+void StrFormat(unsigned char *dest, const unsigned char *msg, ...);
+
+// Append a formatted string to another string.
+//
+// %% - literal %
+// %d - int argument
+// %S - Pascal string argument
+void StrAppendFormat(unsigned char *dest, const unsigned char *msg, ...);
+
+// Copy a Pascal string. Aborts the program if the string does not fit in the
+// destination buffer.
+void StrCopy(unsigned char *dest, int dest_size, unsigned char *src);
+
+// Substitute ^1 in the string with param.
+void StrSubstitute(unsigned char *str, const unsigned char *param);
+
+#endif
diff --git a/macos/tempfile.c b/macos/tempfile.c
new file mode 100644
index 0000000..eeae512
--- /dev/null
+++ b/macos/tempfile.c
@@ -0,0 +1,50 @@
+#include "tempfile.h"
+
+#include <string.h>
+
+// Filename prefix for temporary files.
+static const char kTempPrefix[] = "SyncFiles.";
+
+// Hexadecimal digits.
+static const char kHexDigits[16] = {'0', '1', '2', '3', '4', '5', '6', '7',
+                                    '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
+
+static void U32ToHex(unsigned char *ptr, UInt32 x)
+{
+	int i;
+	for (i = 0; i < 8; i++) {
+		ptr[7 - i] = kHexDigits[x & 15];
+		x >>= 4;
+	}
+}
+
+OSErr MakeTempFile(short vRefNum, FSSpec *spec)
+{
+	short tempVRefNum;
+	long tempParID;
+	unsigned long time;
+	OSErr err;
+	int pos;
+	unsigned char name[32];
+
+	// This is inspired by IM: Files listing 1-10, page 1-25.
+
+	// Generate hopefully unique name.
+	GetDateTime(&time);
+	memcpy(name + 1, kTempPrefix, sizeof(kTempPrefix) - 1);
+	pos = sizeof(kTempPrefix);
+	U32ToHex(name + pos, time);
+	pos += 8;
+	name[0] = pos - 1;
+
+	err = FindFolder(vRefNum, kTemporaryFolderType, TRUE, &tempVRefNum,
+	                 &tempParID);
+	if (err != noErr) {
+		return err;
+	}
+	err = FSMakeFSSpec(tempVRefNum, tempParID, name, spec);
+	if (err != noErr && err != fnfErr) {
+		return err;
+	}
+	return noErr;
+}
diff --git a/macos/tempfile.h b/macos/tempfile.h
new file mode 100644
index 0000000..7741095
--- /dev/null
+++ b/macos/tempfile.h
@@ -0,0 +1,8 @@
+#ifndef MACOS_TEMPFILE_H
+#define MACOS_TEMPFILE_H
+
+// Create a FSSpec for a temporary file on the given volume. Does not actually
+// create the file.
+OSErr MakeTempFile(short vRefNum, FSSpec *spec);
+
+#endif