Index: contrib/sendmail/mail.local/mail.local.c
===================================================================
--- contrib/sendmail/mail.local/mail.local.c
+++ contrib/sendmail/mail.local/mail.local.c
@@ -393,7 +393,7 @@
 	}
 
 	/* Non-LMTP from here on out */
-	if (*argv == '\0')
+	if (*argv == NULL)
 		usage();
 
 	/*
Index: contrib/tcsh/sh.func.c
===================================================================
--- contrib/tcsh/sh.func.c
+++ contrib/tcsh/sh.func.c
@@ -230,7 +230,7 @@
     globbed = v;
     cleanup_push(globbed, blk_cleanup);
 
-    while (*(fileptr = v++) != '\0') {
+    while (*(fileptr = v++) != NULL) {
 	res = filetest(ftest, &fileptr, 0);
 	cleanup_push(res, xfree);
 	xprintf("%S", res);
Index: contrib/tcsh/sh.glob.c
===================================================================
--- contrib/tcsh/sh.glob.c
+++ contrib/tcsh/sh.glob.c
@@ -307,7 +307,7 @@
     /*
      * Step 1: expand backquotes.
      */
-    while ((s = *v++) != '\0') {
+    while ((s = *v++) != NULL) {
 	if (Strchr(s, '`')) {
 	    int     i;
 	    Char **expanded;
@@ -405,7 +405,7 @@
 	for (t = vl; (p = *t++) != NULL; chars++)
 	    chars += Strlen(p);
 	str = xmalloc(chars * sizeof(Char));
-	for (t = vl, strp = str; (p = *t++) != '\0'; chars++) {
+	for (t = vl, strp = str; (p = *t++) != NULL; chars++) {
 	    while (*p)
 		 *strp++ = *p++ & TRIM;
 	    *strp++ = ' ';
@@ -583,7 +583,7 @@
 {
     Char *p;
 
-    while ((p = *t++) != '\0')
+    while ((p = *t++) != NULL)
 	while (*p)
 	    (*f) (*p++);
 }
@@ -593,7 +593,7 @@
 {
     Char *p;
 
-    while ((p = *t++) != '\0')
+    while ((p = *t++) != NULL)
 	while (*p) {
 #if INVALID_BYTE != 0
 	    if ((*p & INVALID_BYTE) != INVALID_BYTE)	/* *p < INVALID_BYTE */
@@ -610,7 +610,7 @@
     const Char *p;
 
     gflag = 0;
-    while ((p = *t++) != '\0') {
+    while ((p = *t++) != NULL) {
 	if (*p == '~' || *p == '=')
 	    gflag |= G_CSH;
 	else if (*p == '{' &&
Index: lib/libc/tests/nss/getgr_test.c
===================================================================
--- lib/libc/tests/nss/getgr_test.c
+++ lib/libc/tests/nss/getgr_test.c
@@ -153,7 +153,7 @@
 		if (strcmp(*c1, *c2) != 0)
 			goto errfin;
 
-	if (*c1 != '\0' || *c2 != '\0')
+	if (*c1 != NULL || *c2 != NULL)
 		goto errfin;
 
 	return 0;
@@ -182,7 +182,7 @@
 	buflen -= written;
 
 	if (grp->gr_mem != NULL) {
-		if (*(grp->gr_mem) != '\0') {
+		if (*(grp->gr_mem) != NULL) {
 			for (cp = grp->gr_mem; *cp; ++cp) {
 				written = snprintf(buffer, buflen, "%s%s",
 				    cp == grp->gr_mem ? "" : ",", *cp);
Index: lib/libc/tests/nss/getproto_test.c
===================================================================
--- lib/libc/tests/nss/getproto_test.c
+++ lib/libc/tests/nss/getproto_test.c
@@ -148,7 +148,7 @@
 		if (strcmp(*c1, *c2) != 0)
 			goto errfin;
 
-	if ((*c1 != '\0') || (*c2 != '\0'))
+	if ((*c1 != NULL) || (*c2 != NULL))
 		goto errfin;
 
 	return 0;
@@ -177,7 +177,7 @@
 	buflen -= written;
 
 	if (pe->p_aliases != NULL) {
-		if (*(pe->p_aliases) != '\0') {
+		if (*(pe->p_aliases) != NULL) {
 			for (cp = pe->p_aliases; *cp; ++cp) {
 				written = snprintf(buffer, buflen, " %s", *cp);
 				buffer += written;
Index: lib/libc/tests/nss/getrpc_test.c
===================================================================
--- lib/libc/tests/nss/getrpc_test.c
+++ lib/libc/tests/nss/getrpc_test.c
@@ -147,7 +147,7 @@
 		if (strcmp(*c1, *c2) != 0)
 			goto errfin;
 
-	if ((*c1 != '\0') || (*c2 != '\0'))
+	if ((*c1 != NULL) || (*c2 != NULL))
 		goto errfin;
 
 	return 0;
@@ -176,7 +176,7 @@
 	buflen -= written;
 
 	if (rpc->r_aliases != NULL) {
-		if (*(rpc->r_aliases) != '\0') {
+		if (*(rpc->r_aliases) != NULL) {
 			for (cp = rpc->r_aliases; *cp; ++cp) {
 				written = snprintf(buffer, buflen, " %s", *cp);
 				buffer += written;
Index: lib/libc/tests/nss/getserv_test.c
===================================================================
--- lib/libc/tests/nss/getserv_test.c
+++ lib/libc/tests/nss/getserv_test.c
@@ -153,7 +153,7 @@
 		if (strcmp(*c1, *c2) != 0)
 			goto errfin;
 
-	if ((*c1 != '\0') || (*c2 != '\0'))
+	if ((*c1 != NULL) || (*c2 != NULL))
 		goto errfin;
 
 	return 0;
@@ -182,7 +182,7 @@
 	buflen -= written;
 
 	if (serv->s_aliases != NULL) {
-		if (*(serv->s_aliases) != '\0') {
+		if (*(serv->s_aliases) != NULL) {
 			for (cp = serv->s_aliases; *cp; ++cp) {
 				written = snprintf(buffer, buflen, " %s", *cp);
 				buffer += written;
Index: usr.bin/tip/tip/acu.c
===================================================================
--- usr.bin/tip/tip/acu.c
+++ usr.bin/tip/tip/acu.c
@@ -190,7 +190,7 @@
 	acu_t *p;
 	extern acu_t acutable[];
 
-	for (p = acutable; p->acu_name != '\0'; p++)
+	for (p = acutable; p->acu_name != NULL; p++)
 		if (!strcmp(s, p->acu_name))
 			return (p);
 	return (NOACU);
Index: usr.sbin/fwcontrol/fwcontrol.c
===================================================================
--- usr.sbin/fwcontrol/fwcontrol.c
+++ usr.sbin/fwcontrol/fwcontrol.c
@@ -129,7 +129,7 @@
 	char *endptr;
 	int i, node;
 
-	if (nodestr == '\0')
+	if (nodestr == NULL || *nodestr == '\0')
 		return (-1);
 
 	/*