* macosx/tclMacOSXNotify.c (Tcl_InitNotifier, Tcl_WaitForEvent): create

	notifier thread lazily upon first call to Tcl_WaitForEvent() rather than
	in Tcl_InitNotifier(). Allows calling exeve() in processes where the
	event loop has not yet been run (Darwin's execve() fails in processes
	with more than one thread), in particular allows embedders to call
	fork() followed by execve(), previously the pthread_atfork() child
	handler's call to Tcl_InitNotifier() would immediately recreate the
	notifier thread in the child after a fork.

	* macosx/tclMacOSXFCmd.c (TclMacOSXCopyFileAttributes):    add support
	* macosx/tclMacOSXNotify.c (Tcl_InitNotifier):             for weakly
	* unix/tclUnixInit.c (Tcl_GetEncodingNameFromEnvironment): importing
	symbols not available on OSX 10.2 or 10.3, enables binaires built on
	later OSX versions to run on earlier ones.
	* macosx/Tcl.xcodeproj/project.pbxproj: enable weak-linking; turn on
	                                        extra warnings.
	* macosx/README: document how to enable weak-linking; cleanup.
	* unix/tclUnixPort.h: add support for weak-linking; conditionalize
	AvailabilityMacros.h inclusion; only disable realpath on 10.2 or earlier
	when threads are enabled.
	* unix/tclLoadDyld.c (TclpLoadMemoryGetBuffer): change runtime Darwin
	* unix/tclUnixInit.c (TclpInitPlatform):        release check to use
	                                                global initialized once.
	* unix/tclUnixFCmd.c (DoRenameFile, TclpObjNormalizePath): add runtime
	Darwin release check to determine if realpath is threadsafe.
	* unix/configure.in: add check on Darwin for compiler support of weak
	* unix/tcl.m4:       import and for AvailabilityMacros.h header;
	move Darwin specific checks & defines that are only relevant to the tcl
	build out of tcl.m4; restrict framework option to Darwin; cleanup
	quoting and help messages.
	* unix/configure: autoconf-2.59
	* unix/tclConfig.h.in: autoheader-2.59

	* unix/tclLoadDyld.c (TclpLoadMemory): fix signed-with-unsigned
	comparison and other warnings from gcc4 -Wextra.

4 files changed, 168 insertions, 56 deletions

diff --git a/macosx/README b/macosx/README
index 561c468..6c2c81a 100644
--- a/macosx/README
+++ b/macosx/README
@@ -1,7 +1,7 @@
 Tcl Mac OS X README 
 -----------------
 
-RCS: @(#) $Id: README,v 1.6 2005/11/27 02:33:49 das Exp $
+RCS: @(#) $Id: README,v 1.7 2006/07/20 06:18:37 das Exp $
 
 This is the README file for the Mac OS X/Darwin version of Tcl.
 
@@ -36,9 +36,12 @@ Mac OS X specific bugs should usually be assigned to 'das' or 'wolfsuit'.
 - At a minimum, Mac OS X 10.1 is required to run Tcl, but OS X 10.3 or higher is
 recommended (certain [file] operations behave incorrectly on earlier releases).
 
-- Tcl built on Mac OS X 10.x will not run on 10.y for y < x, on the other hand
-Tcl built on 10.y will run on 10.x for y < x (but without any of the fixes and
-optimizations that would be available in a binary built on 10.x).
+- Unless weak-linking is used, Tcl built on Mac OS X 10.x will not run on 10.y
+with y < x; on the other hand Tcl built on 10.y will always run on 10.x with
+y <= x (but without any of the fixes and optimizations that would be available
+in a binary built on 10.x).
+Weak-linking is available on OS X 10.2 or later, it additionally allows Tcl
+built on 10.x to run on any 10.y with x > y >= z (for a chosen z >= 2).
 
 - Tcl extensions can be installed in any of:
 	$HOME/Library/Tcl /Library/Tcl /Network/Library/Tcl /System/Library/Tcl
@@ -79,10 +82,9 @@ http://connect.apple.com (after you register for free ADC membership).
 (see below for details), but can also be built with the standard unix configure
 and make buildsystem in tcl/unix as on any other unix platform (indeed, the
 GNUmakefile is just a wrapper around the unix buildsystem).
-The Mac OS X specifc configure flags are --enable-framework and
+The Mac OS X specific configure flags are --enable-framework and
 --disable-corefoundation (which disables CF and notably reverts to the standard
-select based notifier, you will only need this if your require use of naked fork
-(i.e. not followed by execve) in an unthreaded core).
+select based notifier).
 
 - It is also possible to build with Apple's IDE via the projects in tcl/macosx,
 take care to only use the project matching your DevTools and OS version:
@@ -116,12 +118,22 @@ Notes about the native targets of the Xcode projects:
     export CFLAGS="-arch ppc -arch ppc64 -arch i386 \
 	-isysroot /Developer/SDKs/MacOSX10.4u.sdk -mmacosx-version-min=10.4"
 This requires Mac OS X 10.4 and Xcode 2.2 (_not_ Xcode 2.1) and will work on any
-of the architectures (on i386 DTKs, the -isysroot is not required). Note that it
-is not possible to configure correctly if the current architecture is not
+of the architectures (on intel Macs, the -isysroot is not required). Note that
+it is not possible to configure correctly if the current architecture is not
 present in CFLAGS (i.e. -arch `arch` must always be there). Universal builds of
 Tcl TEA extensions are also possible with CFLAGS set as above, they will be
 [load]able by universal as well as thin binaries of Tcl.
 
+- To enable weak-linking, set the MACOSX_DEPLOYMENT_TARGET environment variable
+to the minimal OS version (>= 10.2) the binaries should be able to run on, e.g:
+    export MACOSX_DEPLOYMENT_TARGET=10.2
+This requires Mac OS X 10.2 and gcc 3.1; if you have gcc 4 or later you can set
+CFLAGS instead:
+    export CFLAGS="-mmacosx-version-min=10.2"
+The Tcl.xcodeproj is setup to produce binaires that can run on 10.2 or later,
+except for the 'ReleaseUniversal'configuration, where they require 10.4.
+Support for weak-linking was added to the code for 8.4.14/8.5a5.
+
 Detailed Instructions for building with macosx/GNUmakefile
 ----------------------------------------------------------
 
@@ -152,8 +164,8 @@ instead by passing an INSTALL_ROOT argument to make:
 - The default Makefile targets will build _both_ debug and optimized versions of
 the Tcl framework with the standard convention of naming the debug library
 Tcl.framework/Tcl_debug.
-This allows you to dynamically link to the debug libraries at runtime by setting
-	setenv DYLD_IMAGE_SUFFIX _debug
+This allows switching to the debug libraries at runtime by setting
+	export DYLD_IMAGE_SUFFIX=_debug
 (c.f. man dyld for more details)
 
 If you only want to build and install the debug or optimized build, use the
diff --git a/macosx/Tcl.xcodeproj/project.pbxproj b/macosx/Tcl.xcodeproj/project.pbxproj
index 89c24e1..9c72230 100644
--- a/macosx/Tcl.xcodeproj/project.pbxproj
+++ b/macosx/Tcl.xcodeproj/project.pbxproj
@@ -1871,7 +1871,7 @@
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 			shellPath = /bin/bash;
-			shellScript = "## tcl configure shell script phase\n\ncd \"${TCL_SRCROOT}\"/macosx &&\nif [ configure.ac -nt configure -o ../unix/configure.in -nt configure -o ../unix/tcl.m4 -nt configure -o ../unix/aclocal.m4 -nt configure ]; then\n    echo \"Running autoconf & autoheader in tcl/macosx\"\n    rm -rf autom4te.cache\n    ${AUTOCONF:-/usr/bin/autoconf} && ${AUTOHEADER:-/usr/bin/autoheader}\n    rm -rf autom4te.cache\nfi\n\ncd \"${DERIVED_FILE_DIR}\" && mkdir -p tcl && cd tcl &&\nif [ \"${TCL_SRCROOT}\"/macosx/configure -nt config.status ]; then\n    \"${TCL_SRCROOT}\"/macosx/configure --cache-file=../config.cache --prefix=${PREFIX} --bindir=${BINDIR} --libdir=${LIBDIR} --mandir=${MANDIR} --includedir=${INCLUDEDIR} --disable-shared ${CONFIGURE_ARGS}\nelse\n    ./config.status\nfi\n";
+			shellScript = "## tcl configure shell script phase\n\ncd \"${TCL_SRCROOT}\"/macosx &&\nif [ configure.ac -nt configure -o ../unix/configure.in -nt configure -o ../unix/tcl.m4 -nt configure -o ../unix/aclocal.m4 -nt configure ]; then\n    echo \"Running autoconf & autoheader in tcl/macosx\"\n    rm -rf autom4te.cache\n    ${AUTOCONF:-/usr/bin/autoconf} && ${AUTOHEADER:-/usr/bin/autoheader}\n    rm -rf autom4te.cache\nfi\n\ncd \"${DERIVED_FILE_DIR}\" && mkdir -p tcl && cd tcl &&\nif [ \"${TCL_SRCROOT}\"/macosx/configure -nt config.status ]; then\n    echo \"Configuring tcl\"\n    \"${TCL_SRCROOT}\"/macosx/configure --cache-file=../config.cache --prefix=${PREFIX} --bindir=${BINDIR} --libdir=${LIBDIR} --mandir=${MANDIR} --includedir=${INCLUDEDIR} --disable-shared ${CONFIGURE_ARGS}\nelse\n    ./config.status\nfi\n";
 		};
 /* End PBXShellScriptBuildPhase section */
 
@@ -2084,7 +2084,6 @@
 				INCLUDEDIR = "${PREFIX}/include";
 				INSTALL_PATH = "${BINDIR}";
 				LIBDIR = "${PREFIX}/lib";
-				MACOSX_DEPLOYMENT_TARGET = 10.4;
 				MANDIR = "${PREFIX}/man";
 				OTHER_LDFLAGS = "-headerpad_max_install_names";
 				PER_ARCH_CFLAGS_ppc = "-mcpu=G3 -mtune=G4";
@@ -2100,6 +2099,9 @@
 				WARNING_CFLAGS = (
 					"-Wall",
 					"-Wno-implicit-int",
+					"-Wextra",
+					"-Wno-unused-parameter",
+					"-Wno-missing-field-initializers",
 				);
 				ZERO_LINK = NO;
 			};
@@ -2111,6 +2113,7 @@
 				CFLAGS = "-arch ppc -arch ppc64 -arch i386 -isysroot ${SDKROOT} -mmacosx-version-min=10.4";
 				CONFIGURE_ARGS = "--disable-symbols";
 				GCC_PREPROCESSOR_DEFINITIONS = NDEBUG;
+				MACOSX_DEPLOYMENT_TARGET = 10.4;
 				SDKROOT = /Developer/SDKs/MacOSX10.4u.sdk;
 			};
 			name = ReleaseUniversal;
@@ -2184,7 +2187,6 @@
 				INCLUDEDIR = "${PREFIX}/include";
 				INSTALL_PATH = "${BINDIR}";
 				LIBDIR = "${PREFIX}/lib";
-				MACOSX_DEPLOYMENT_TARGET = 10.4;
 				MANDIR = "${PREFIX}/man";
 				OTHER_LDFLAGS = "-headerpad_max_install_names";
 				PER_ARCH_CFLAGS_ppc = "-mcpu=G3 -mtune=G4";
@@ -2200,6 +2202,9 @@
 				WARNING_CFLAGS = (
 					"-Wall",
 					"-Wno-implicit-int",
+					"-Wextra",
+					"-Wno-unused-parameter",
+					"-Wno-missing-field-initializers",
 				);
 				ZERO_LINK = YES;
 			};
@@ -2231,7 +2236,6 @@
 				INCLUDEDIR = "${PREFIX}/include";
 				INSTALL_PATH = "${BINDIR}";
 				LIBDIR = "${PREFIX}/lib";
-				MACOSX_DEPLOYMENT_TARGET = 10.4;
 				MANDIR = "${PREFIX}/man";
 				OTHER_LDFLAGS = "-headerpad_max_install_names";
 				PER_ARCH_CFLAGS_ppc = "-mcpu=G3 -mtune=G4";
@@ -2247,6 +2251,9 @@
 				WARNING_CFLAGS = (
 					"-Wall",
 					"-Wno-implicit-int",
+					"-Wextra",
+					"-Wno-unused-parameter",
+					"-Wno-missing-field-initializers",
 				);
 				ZERO_LINK = NO;
 			};
@@ -2279,7 +2286,6 @@
 				INCLUDEDIR = "${PREFIX}/include";
 				INSTALL_PATH = "${BINDIR}";
 				LIBDIR = "${PREFIX}/lib";
-				MACOSX_DEPLOYMENT_TARGET = 10.4;
 				MANDIR = "${PREFIX}/man";
 				OTHER_LDFLAGS = "-headerpad_max_install_names";
 				PER_ARCH_CFLAGS_ppc = "-mcpu=G3 -mtune=G4";
@@ -2295,6 +2301,9 @@
 				WARNING_CFLAGS = (
 					"-Wall",
 					"-Wno-implicit-int",
+					"-Wextra",
+					"-Wno-unused-parameter",
+					"-Wno-missing-field-initializers",
 				);
 				ZERO_LINK = NO;
 			};
@@ -2305,6 +2314,7 @@
 			buildSettings = {
 				CONFIGURE_ARGS = "--enable-symbols";
 				GCC_PREPROCESSOR_DEFINITIONS = "__private_extern__=extern";
+				MACOSX_DEPLOYMENT_TARGET = 10.2;
 			};
 			name = Debug;
 		};
@@ -2313,6 +2323,7 @@
 			buildSettings = {
 				CONFIGURE_ARGS = "--disable-symbols";
 				GCC_PREPROCESSOR_DEFINITIONS = NDEBUG;
+				MACOSX_DEPLOYMENT_TARGET = 10.2;
 			};
 			name = Release;
 		};
@@ -2321,6 +2332,7 @@
 			buildSettings = {
 				CONFIGURE_ARGS = "--enable-symbols";
 				GCC_PREPROCESSOR_DEFINITIONS = "";
+				MACOSX_DEPLOYMENT_TARGET = 10.2;
 			};
 			name = DebugNoFixZL;
 		};
diff --git a/macosx/tclMacOSXFCmd.c b/macosx/tclMacOSXFCmd.c
index 80e9785..51fe222 100644
--- a/macosx/tclMacOSXFCmd.c
+++ b/macosx/tclMacOSXFCmd.c
@@ -10,7 +10,7 @@
  * See the file "license.terms" for information on usage and redistribution of
  * this file, and for a DISCLAIMER OF ALL WARRANTIES.
  *
- * RCS: @(#) $Id: tclMacOSXFCmd.c,v 1.8 2006/03/21 11:06:23 das Exp $
+ * RCS: @(#) $Id: tclMacOSXFCmd.c,v 1.9 2006/07/20 06:18:37 das Exp $
  */
 
 #include "tclInt.h"
@@ -21,7 +21,7 @@
 #include <libkern/OSByteOrder.h>
 #endif
 
-/* Darwin 8 copyfile API */
+/* Darwin 8 copyfile API. */
 #ifdef HAVE_COPYFILE
 #ifdef HAVE_COPYFILE_H
 #include <copyfile.h>
@@ -30,8 +30,14 @@ int copyfile(const char *from, const char *to, void *state, uint32_t flags);
 #define COPYFILE_ACL            (1<<0)
 #define COPYFILE_XATTR          (1<<2)
 #define COPYFILE_NOFOLLOW_SRC   (1<<18)
-#endif
-#endif
+#endif /* HAVE_COPYFILE_H */
+#if defined(HAVE_WEAK_IMPORT) && MAC_OS_X_VERSION_MIN_REQUIRED < 1040
+/* Support for weakly importing copyfile. */
+#define WEAK_IMPORT_COPYFILE
+extern int copyfile(const char *from, const char *to, void *state,
+                    uint32_t flags) WEAK_IMPORT_ATTRIBUTE;
+#endif /* HAVE_WEAK_IMPORT */
+#endif /* HAVE_COPYFILE */
 
 #include <libkern/OSByteOrder.h>
 
@@ -365,14 +371,22 @@ TclMacOSXCopyFileAttributes(
     CONST Tcl_StatBuf *statBufPtr)
 				/* Stat info for source file */
 {
-#if defined(HAVE_COPYFILE)
+#ifdef WEAK_IMPORT_COPYFILE
+    if (copyfile != NULL) {
+#endif
+#ifdef HAVE_COPYFILE
     if (copyfile(src, dst, NULL, COPYFILE_XATTR |
 	    (S_ISLNK(statBufPtr->st_mode) ? COPYFILE_NOFOLLOW_SRC :
 		                            COPYFILE_ACL)) < 0) {
 	return TCL_ERROR;
     }
     return TCL_OK;
-#elif defined(HAVE_GETATTRLIST)
+#endif /* HAVE_COPYFILE */
+#ifdef WEAK_IMPORT_COPYFILE
+    } else {
+#endif
+#if !defined(HAVE_COPYFILE) || defined(WEAK_IMPORT_COPYFILE)
+#ifdef HAVE_GETATTRLIST
     struct attrlist alist;
     fileinfobuf finfo;
     off_t *rsrcForkSize = (off_t*)(&finfo.data);
@@ -430,6 +444,10 @@ TclMacOSXCopyFileAttributes(
     return TCL_OK;
 #else
     return TCL_ERROR;
+#endif /* HAVE_GETATTRLIST */
+#endif /* !defined(HAVE_COPYFILE) || defined(WEAK_IMPORT_COPYFILE) */
+#ifdef WEAK_IMPORT_COPYFILE
+    }
 #endif
 }
 
diff --git a/macosx/tclMacOSXNotify.c b/macosx/tclMacOSXNotify.c
index a99e11b..6d892ea 100644
--- a/macosx/tclMacOSXNotify.c
+++ b/macosx/tclMacOSXNotify.c
@@ -13,7 +13,7 @@
  * See the file "license.terms" for information on usage and redistribution of
  * this file, and for a DISCLAIMER OF ALL WARRANTIES.
  *
- * RCS: @(#) $Id: tclMacOSXNotify.c,v 1.7 2006/05/27 05:22:58 das Exp $
+ * RCS: @(#) $Id: tclMacOSXNotify.c,v 1.8 2006/07/20 06:18:37 das Exp $
  */
 
 #include "tclInt.h"
@@ -163,6 +163,32 @@ static int receivePipe = -1; /* Output end of triggerPipe */
 
 #include <libkern/OSAtomic.h>
 
+#if defined(HAVE_WEAK_IMPORT) && MAC_OS_X_VERSION_MIN_REQUIRED < 1040
+/*
+ * Support for weakly importing spinlock API.
+ */
+#define WEAK_IMPORT_SPINLOCKLOCK
+extern void	OSSpinLockLock(OSSpinLock *lock) WEAK_IMPORT_ATTRIBUTE;
+extern void	OSSpinLockUnlock(OSSpinLock *lock) WEAK_IMPORT_ATTRIBUTE;
+extern void	_spin_lock(OSSpinLock *lock) WEAK_IMPORT_ATTRIBUTE;
+extern void	_spin_unlock(OSSpinLock *lock) WEAK_IMPORT_ATTRIBUTE;
+static void (* lockLock)(OSSpinLock *lock) = NULL;
+static void (* lockUnlock)(OSSpinLock *lock) = NULL;
+static pthread_once_t spinLockLockInitControl = PTHREAD_ONCE_INIT;
+static void SpinLockLockInit(void) {
+    lockLock   = OSSpinLockLock   != NULL ? OSSpinLockLock   : _spin_lock;
+    lockUnlock = OSSpinLockUnlock != NULL ? OSSpinLockUnlock : _spin_unlock;
+    if (lockLock == NULL || lockUnlock == NULL) {
+	Tcl_Panic("SpinLockLockInit: no spinlock API available.");
+    }
+}
+#define SpinLockLock(p) 	lockLock(p)
+#define SpinLockUnlock(p)	lockUnlock(p)
+#else
+#define SpinLockLock(p) 	OSSpinLockLock(p)
+#define SpinLockUnlock(p)	OSSpinLockUnlock(p)
+#endif /* HAVE_WEAK_IMPORT */
+
 #else
 /*
  * Otherwise, use commpage spinlock SPI directly.
@@ -171,8 +197,8 @@ static int receivePipe = -1; /* Output end of triggerPipe */
 typedef uint32_t OSSpinLock;
 extern void	_spin_lock(OSSpinLock *lock);
 extern void	_spin_unlock(OSSpinLock *lock);
-#define OSSpinLockLock(p)	_spin_lock(p)
-#define OSSpinLockUnlock(p)	_spin_unlock(p)
+#define SpinLockLock(p) 	_spin_lock(p)
+#define SpinLockUnlock(p)	_spin_unlock(p)
 
 #endif /* HAVE_LIBKERN_OSATOMIC_H && HAVE_OSSPINLOCKLOCK */
 
@@ -187,10 +213,10 @@ static OSSpinLock notifierLock = 0;
  * Macros abstracting notifier locking/unlocking
  */
 
-#define LOCK_NOTIFIER_INIT	OSSpinLockLock(&notifierInitLock)
-#define UNLOCK_NOTIFIER_INIT	OSSpinLockUnlock(&notifierInitLock)
-#define LOCK_NOTIFIER		OSSpinLockLock(&notifierLock)
-#define UNLOCK_NOTIFIER		OSSpinLockUnlock(&notifierLock)
+#define LOCK_NOTIFIER_INIT	SpinLockLock(&notifierInitLock)
+#define UNLOCK_NOTIFIER_INIT	SpinLockUnlock(&notifierInitLock)
+#define LOCK_NOTIFIER		SpinLockLock(&notifierLock)
+#define UNLOCK_NOTIFIER		SpinLockUnlock(&notifierLock)
 
 /*
  * The pollState bits
@@ -222,7 +248,13 @@ static int	atForkInit = 0;
 static void	AtForkPrepare(void);
 static void	AtForkParent(void);
 static void	AtForkChild(void);
-#endif
+#if defined(HAVE_WEAK_IMPORT) && MAC_OS_X_VERSION_MIN_REQUIRED < 1040
+/* Support for weakly importing pthread_atfork. */
+#define WEAK_IMPORT_PTHREAD_ATFORK
+extern int pthread_atfork(void (*prepare)(void), void (*parent)(void),
+                          void (*child)(void)) WEAK_IMPORT_ATTRIBUTE;
+#endif /* HAVE_WEAK_IMPORT */
+#endif /* HAVE_PTHREAD_ATFORK */
 
 /*
  *----------------------------------------------------------------------
@@ -247,8 +279,17 @@ Tcl_InitNotifier(void)
 
     tsdPtr->eventReady = 0;
 
+#ifdef WEAK_IMPORT_SPINLOCKLOCK
+    /*
+     * Initialize support for weakly imported spinlock API.
+     */
+    if (pthread_once(&spinLockLockInitControl, SpinLockLockInit)) {
+	Tcl_Panic("Tcl_InitNotifier: pthread_once failed.");
+    }
+#endif
+
     /*
-     * Initialize CFRunLoopSource and add it to CFRunLoop of this thread
+     * Initialize CFRunLoopSource and add it to CFRunLoop of this thread.
      */
 
     if (!tsdPtr->runLoop) {
@@ -267,28 +308,31 @@ Tcl_InitNotifier(void)
 	tsdPtr->runLoop = runLoop;
     }
 
-    /*
-     * Initialize trigger pipe and start the Notifier thread if necessary.
-     */
-
     LOCK_NOTIFIER_INIT;
 #ifdef HAVE_PTHREAD_ATFORK
     /*
-     * Install pthread_atfork handlers to reinstall the notifier thread in the
+     * Install pthread_atfork handlers to reinitialize the notifier in the
      * child of a fork.
      */
 
-    if (!atForkInit) {
+    if (
+#ifdef WEAK_IMPORT_PTHREAD_ATFORK
+	    pthread_atfork != NULL &&
+#endif
+	    !atForkInit) {
 	int result = pthread_atfork(AtForkPrepare, AtForkParent, AtForkChild);
 	if (result) { 
-	    Tcl_Panic("Tcl_InitNotifier: pthread_atfork failed");
+	    Tcl_Panic("Tcl_InitNotifier: pthread_atfork failed.");
 	}
 	atForkInit = 1;
     }
 #endif
     if (notifierCount == 0) {
-	int fds[2], status, result;
-	pthread_attr_t attr;
+	int fds[2], status;
+
+	/*
+	 * Initialize trigger pipe.
+	 */
 
 	if (pipe(fds) != 0) {
 	    Tcl_Panic("Tcl_InitNotifier: could not create trigger pipe.");
@@ -308,16 +352,13 @@ Tcl_InitNotifier(void)
 	receivePipe = fds[0];
 	triggerPipe = fds[1];
 
-	pthread_attr_init(&attr);
-	pthread_attr_setscope(&attr, PTHREAD_SCOPE_SYSTEM);
-	pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
-	pthread_attr_setstacksize(&attr, 60 * 1024);
-	result = pthread_create(&notifierThread, &attr,
-		(void * (*)(void *))NotifierThreadProc, NULL);
-	pthread_attr_destroy(&attr);
-	if (result) {
-	    Tcl_Panic("Tcl_InitNotifier: unable to start notifier thread.");
-	}
+	/*
+	 * Create notifier thread lazily in Tcl_WaitForEvent() to avoid
+	 * interfering with fork() followed immediately by execve()
+	 * (cannot execve() when more than one thread is present).
+	 */
+	
+	notifierThread = 0;
     }
     notifierCount++;
     UNLOCK_NOTIFIER_INIT;
@@ -378,9 +419,12 @@ Tcl_FinalizeNotifier(
 	write(triggerPipe, "q", 1);
 	close(triggerPipe);
 
-	result = pthread_join(notifierThread, NULL);
-	if (result) {
-	    Tcl_Panic("Tcl_FinalizeNotifier: unable to join notifier thread.");
+	if (notifierThread) {
+	    result = pthread_join(notifierThread, NULL);
+	    if (result) {
+		Tcl_Panic("Tcl_FinalizeNotifier: unable to join notifier thread.");
+	    }
+	    notifierThread = 0;
 	}
 
 	close(receivePipe);
@@ -784,6 +828,28 @@ Tcl_WaitForEvent(
     }
 
     /*
+     * Start notifier thread if necessary.
+     */
+
+    LOCK_NOTIFIER_INIT;
+    if (!notifierThread) {
+	int result;
+	pthread_attr_t attr;
+
+	pthread_attr_init(&attr);
+	pthread_attr_setscope(&attr, PTHREAD_SCOPE_SYSTEM);
+	pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
+	pthread_attr_setstacksize(&attr, 60 * 1024);
+	result = pthread_create(&notifierThread, &attr,
+		(void * (*)(void *))NotifierThreadProc, NULL);
+	pthread_attr_destroy(&attr);
+	if (result || !notifierThread) {
+	    Tcl_Panic("Tcl_WaitForEvent: unable to start notifier thread.");
+	}
+    }
+    UNLOCK_NOTIFIER_INIT;
+
+    /*
      * Place this thread on the list of interested threads, signal the
      * notifier thread, and wait for a response or a timeout.
      */
@@ -1158,9 +1224,13 @@ AtForkChild(void)
     }
     if (notifierCount > 0) {
 	notifierCount = 0;
-	/* Note that Tcl_FinalizeNotifier does not use its clientData
-	 * parameter, so discard return value of Tcl_InitNotifier here and
-	 * leave stale clientData in tclNotify.c's ThreadSpecificData.
+	/*
+	 * Assume that the return value of Tcl_InitNotifier() in the child
+	 * will be identical to the one stored as clientData in tclNotify.c's
+	 * ThreadSpecificData by the parent's TclInitNotifier(), so discard
+	 * the return value here. This assumption may require the fork() to
+	 * be executed in the main thread of the parent, otherwise
+	 * Tcl_AlertNotifier() may break in the child.
 	 */
 	Tcl_InitNotifier();
     }