diff --git a/lib/libc_r/uthread/pthread_private.h b/lib/libc_r/uthread/pthread_private.h
index 7d5bf89f3a3e..e4b377a00c6a 100644
--- a/lib/libc_r/uthread/pthread_private.h
+++ b/lib/libc_r/uthread/pthread_private.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1995 John Birrell <jb@cimlogic.com.au>.
+ * Copyright (c) 1995-1998 John Birrell <jb@cimlogic.com.au>.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -277,6 +277,13 @@ union pthread_wait_data {
  * Thread structure.
  */
 struct pthread {
+	/*
+	 * Magic value to help recognize a valid thread structure
+	 * from an invalid one:
+	 */
+#define	PTHREAD_MAGIC		((u_int32_t) 0xd09ba115)
+	u_int32_t		magic;
+
 	/*
 	 * Pointer to the next thread in the thread linked list.
 	 */
diff --git a/lib/libc_r/uthread/uthread_create.c b/lib/libc_r/uthread/uthread_create.c
index da00e4d44038..412f5580174c 100644
--- a/lib/libc_r/uthread/uthread_create.c
+++ b/lib/libc_r/uthread/uthread_create.c
@@ -60,6 +60,12 @@ _thread_create(pthread_t * thread, const pthread_attr_t * attr,
 		/* Insufficient memory to create a thread: */
 		ret = EAGAIN;
 	} else {
+		/*
+		 * Write a magic value to the thread structure to help
+		 * identify valid ones:
+		 */
+		new_thread->magic = PTHREAD_MAGIC;
+
 		/* Check if default thread attributes are required: */
 		if (attr == NULL || *attr == NULL) {
 			/* Use the default thread attributes: */
diff --git a/lib/libc_r/uthread/uthread_detach.c b/lib/libc_r/uthread/uthread_detach.c
index a9f24699e3c6..57c073ab3ab9 100644
--- a/lib/libc_r/uthread/uthread_detach.c
+++ b/lib/libc_r/uthread/uthread_detach.c
@@ -46,7 +46,7 @@ pthread_detach(pthread_t pthread)
 	_thread_kern_sig_block(&status);
 
 	/* Check for invalid calling parameters: */
-	if (pthread == NULL) {
+	if (pthread == NULL || pthread->magic != PTHREAD_MAGIC) {
 		/* Return an invalid argument error: */
 		rval = EINVAL;
 	}
diff --git a/lib/libc_r/uthread/uthread_join.c b/lib/libc_r/uthread/uthread_join.c
index 63d0d5880bec..9e86a0154b6b 100644
--- a/lib/libc_r/uthread/uthread_join.c
+++ b/lib/libc_r/uthread/uthread_join.c
@@ -42,6 +42,16 @@ pthread_join(pthread_t pthread, void **thread_return)
 	int             status;
 	pthread_t	pthread1;
 
+	/* Check if the caller has specified an invalid thread: */
+	if (pthread == NULL || pthread->magic != PTHREAD_MAGIC)
+		/* Invalid thread: */
+		return(EINVAL);
+
+	/* Check if the caller has specified itself: */
+	if (pthread == _thread_run)
+		/* Avoid a deadlock condition: */
+		return(EDEADLK);
+
 	/* Block signals: */
 	_thread_kern_sig_block(&status);
 
diff --git a/lib/libkse/thread/thr_create.c b/lib/libkse/thread/thr_create.c
index da00e4d44038..412f5580174c 100644
--- a/lib/libkse/thread/thr_create.c
+++ b/lib/libkse/thread/thr_create.c
@@ -60,6 +60,12 @@ _thread_create(pthread_t * thread, const pthread_attr_t * attr,
 		/* Insufficient memory to create a thread: */
 		ret = EAGAIN;
 	} else {
+		/*
+		 * Write a magic value to the thread structure to help
+		 * identify valid ones:
+		 */
+		new_thread->magic = PTHREAD_MAGIC;
+
 		/* Check if default thread attributes are required: */
 		if (attr == NULL || *attr == NULL) {
 			/* Use the default thread attributes: */
diff --git a/lib/libkse/thread/thr_detach.c b/lib/libkse/thread/thr_detach.c
index a9f24699e3c6..57c073ab3ab9 100644
--- a/lib/libkse/thread/thr_detach.c
+++ b/lib/libkse/thread/thr_detach.c
@@ -46,7 +46,7 @@ pthread_detach(pthread_t pthread)
 	_thread_kern_sig_block(&status);
 
 	/* Check for invalid calling parameters: */
-	if (pthread == NULL) {
+	if (pthread == NULL || pthread->magic != PTHREAD_MAGIC) {
 		/* Return an invalid argument error: */
 		rval = EINVAL;
 	}
diff --git a/lib/libkse/thread/thr_join.c b/lib/libkse/thread/thr_join.c
index 63d0d5880bec..9e86a0154b6b 100644
--- a/lib/libkse/thread/thr_join.c
+++ b/lib/libkse/thread/thr_join.c
@@ -42,6 +42,16 @@ pthread_join(pthread_t pthread, void **thread_return)
 	int             status;
 	pthread_t	pthread1;
 
+	/* Check if the caller has specified an invalid thread: */
+	if (pthread == NULL || pthread->magic != PTHREAD_MAGIC)
+		/* Invalid thread: */
+		return(EINVAL);
+
+	/* Check if the caller has specified itself: */
+	if (pthread == _thread_run)
+		/* Avoid a deadlock condition: */
+		return(EDEADLK);
+
 	/* Block signals: */
 	_thread_kern_sig_block(&status);
 
diff --git a/lib/libkse/thread/thr_private.h b/lib/libkse/thread/thr_private.h
index 7d5bf89f3a3e..e4b377a00c6a 100644
--- a/lib/libkse/thread/thr_private.h
+++ b/lib/libkse/thread/thr_private.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1995 John Birrell <jb@cimlogic.com.au>.
+ * Copyright (c) 1995-1998 John Birrell <jb@cimlogic.com.au>.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -277,6 +277,13 @@ union pthread_wait_data {
  * Thread structure.
  */
 struct pthread {
+	/*
+	 * Magic value to help recognize a valid thread structure
+	 * from an invalid one:
+	 */
+#define	PTHREAD_MAGIC		((u_int32_t) 0xd09ba115)
+	u_int32_t		magic;
+
 	/*
 	 * Pointer to the next thread in the thread linked list.
 	 */
diff --git a/lib/libpthread/thread/thr_create.c b/lib/libpthread/thread/thr_create.c
index da00e4d44038..412f5580174c 100644
--- a/lib/libpthread/thread/thr_create.c
+++ b/lib/libpthread/thread/thr_create.c
@@ -60,6 +60,12 @@ _thread_create(pthread_t * thread, const pthread_attr_t * attr,
 		/* Insufficient memory to create a thread: */
 		ret = EAGAIN;
 	} else {
+		/*
+		 * Write a magic value to the thread structure to help
+		 * identify valid ones:
+		 */
+		new_thread->magic = PTHREAD_MAGIC;
+
 		/* Check if default thread attributes are required: */
 		if (attr == NULL || *attr == NULL) {
 			/* Use the default thread attributes: */
diff --git a/lib/libpthread/thread/thr_detach.c b/lib/libpthread/thread/thr_detach.c
index a9f24699e3c6..57c073ab3ab9 100644
--- a/lib/libpthread/thread/thr_detach.c
+++ b/lib/libpthread/thread/thr_detach.c
@@ -46,7 +46,7 @@ pthread_detach(pthread_t pthread)
 	_thread_kern_sig_block(&status);
 
 	/* Check for invalid calling parameters: */
-	if (pthread == NULL) {
+	if (pthread == NULL || pthread->magic != PTHREAD_MAGIC) {
 		/* Return an invalid argument error: */
 		rval = EINVAL;
 	}
diff --git a/lib/libpthread/thread/thr_join.c b/lib/libpthread/thread/thr_join.c
index 63d0d5880bec..9e86a0154b6b 100644
--- a/lib/libpthread/thread/thr_join.c
+++ b/lib/libpthread/thread/thr_join.c
@@ -42,6 +42,16 @@ pthread_join(pthread_t pthread, void **thread_return)
 	int             status;
 	pthread_t	pthread1;
 
+	/* Check if the caller has specified an invalid thread: */
+	if (pthread == NULL || pthread->magic != PTHREAD_MAGIC)
+		/* Invalid thread: */
+		return(EINVAL);
+
+	/* Check if the caller has specified itself: */
+	if (pthread == _thread_run)
+		/* Avoid a deadlock condition: */
+		return(EDEADLK);
+
 	/* Block signals: */
 	_thread_kern_sig_block(&status);
 
diff --git a/lib/libpthread/thread/thr_private.h b/lib/libpthread/thread/thr_private.h
index 7d5bf89f3a3e..e4b377a00c6a 100644
--- a/lib/libpthread/thread/thr_private.h
+++ b/lib/libpthread/thread/thr_private.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1995 John Birrell <jb@cimlogic.com.au>.
+ * Copyright (c) 1995-1998 John Birrell <jb@cimlogic.com.au>.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -277,6 +277,13 @@ union pthread_wait_data {
  * Thread structure.
  */
 struct pthread {
+	/*
+	 * Magic value to help recognize a valid thread structure
+	 * from an invalid one:
+	 */
+#define	PTHREAD_MAGIC		((u_int32_t) 0xd09ba115)
+	u_int32_t		magic;
+
 	/*
 	 * Pointer to the next thread in the thread linked list.
 	 */