diff --git a/usr.bin/m4/eval.c b/usr.bin/m4/eval.c
index 0e77c6b6653d..59fde31138b4 100644
--- a/usr.bin/m4/eval.c
+++ b/usr.bin/m4/eval.c
@@ -378,8 +378,8 @@ expand(argv, argc)
 register char *argv[];
 register int argc;
 {
-	register char *t;
-	register char *p;
+	register unsigned char *t;
+	register unsigned char *p;
 	register int n;
 	register int argno;
 
@@ -688,7 +688,7 @@ dosub(argv, argc)
 register char *argv[];
 register int argc;
 {
-	register char *ap, *fc, *k;
+	register unsigned char *ap, *fc, *k;
 	register int nc;
 
 	if (argc < 5)
diff --git a/usr.bin/m4/extern.h b/usr.bin/m4/extern.h
index b54a9b975a9c..a8df3ebbab83 100644
--- a/usr.bin/m4/extern.h
+++ b/usr.bin/m4/extern.h
@@ -65,7 +65,7 @@ void	map __P((char *, char *, char *, char *));
 void	onintr __P((int));
 void	oops __P((const char *, ...));
 void	pbnum __P((int));
-void	pbstr __P((char *));
+void    pbstr __P((unsigned char *));
 void	putback __P((int));
 void	remhash __P((char *, int));
 void	usage __P((void));
@@ -79,13 +79,13 @@ extern int fp; 			/* m4 call frame pointer */
 extern int ilevel;		/* input file stack pointer */
 extern int oindex;		/* diversion index. */
 extern int sp;			/* current m4 stack pointer */
-extern char *bp;		/* first available character */
-extern char buf[];		/* push-back buffer */
-extern char *bufbase;		/* buffer base for this ilevel */
-extern char *bbase[];		/* buffer base per ilevel */
+extern unsigned char *bp;       /* first available character */
+extern unsigned char buf[];     /* push-back buffer */
+extern unsigned char *bufbase;  /* buffer base for this ilevel */
+extern unsigned char *bbase[];  /* buffer base per ilevel */
 extern char ecommt;		/* end character for comment */
 extern char *endest;		/* end of string space */
-extern char *endpbb;		/* end of push-back buffer */
+extern unsigned char *endpbb;   /* end of push-back buffer */
 extern char *ep;		/* first free char in strspace */
 extern char lquote;		/* left quote character (`) */
 extern char *m4temp;		/* filename for diversions */
diff --git a/usr.bin/m4/main.c b/usr.bin/m4/main.c
index 221b865a9b64..d16bbf15655c 100644
--- a/usr.bin/m4/main.c
+++ b/usr.bin/m4/main.c
@@ -63,11 +63,11 @@ static char sccsid[] = "@(#)main.c	8.1 (Berkeley) 6/6/93";
 #include "pathnames.h"
 
 ndptr hashtab[HASHSIZE];	/* hash table for macros etc.  */
-char buf[BUFSIZE];		/* push-back buffer	       */
-char *bufbase = buf;		/* the base for current ilevel */
-char *bbase[MAXINP];		/* the base for each ilevel    */
-char *bp = buf; 		/* first available character   */
-char *endpbb = buf+BUFSIZE;	/* end of push-back buffer     */
+unsigned char buf[BUFSIZE];              /* push-back buffer            */
+unsigned char *bufbase = buf;            /* the base for current ilevel */
+unsigned char *bbase[MAXINP];            /* the base for each ilevel    */
+unsigned char *bp = buf;                 /* first available character   */
+unsigned char *endpbb = buf+BUFSIZE;     /* end of push-back buffer     */
 stae mstack[STACKMAX+1]; 	/* stack of m4 machine         */
 char strspace[STRSPMAX+1];	/* string space for evaluation */
 char *ep = strspace;		/* first free char in strspace */
@@ -248,7 +248,7 @@ macro() {
 	register int  nlpar;
 
 	cycle {
-		if ((t = gpbc()) == '_' || isalpha(t)) {
+		if ((t = gpbc()) == '_' || (t != EOF && isalpha(t))) {
 			putback(t);
 			if ((p = inspect(s = token)) == nil) {
 				if (sp < 0)
@@ -327,7 +327,7 @@ macro() {
 		case LPAREN:
 			if (PARLEV > 0)
 				chrsave(t);
-			while (isspace(l = gpbc()))
+			while ((l = gpbc()) != EOF && isspace(l))
 				;		/* skip blank, tab, nl.. */
 			putback(l);
 			PARLEV++;
@@ -356,7 +356,7 @@ macro() {
 		case COMMA:
 			if (PARLEV == 1) {
 				chrsave(EOS);		/* new argument   */
-				while (isspace(l = gpbc()))
+				while ((l = gpbc()) != EOF && isspace(l))
 					;
 				putback(l);
 				pushs(ep);
@@ -380,13 +380,13 @@ ndptr
 inspect(tp) 
 register char *tp;
 {
-	register char c;
+	register int c;
 	register char *name = tp;
 	register char *etp = tp+MAXTOK;
 	register ndptr p;
 	register unsigned long h = 0;
 
-	while ((isalnum(c = gpbc()) || c == '_') && tp < etp)
+	while ((c = gpbc()) != EOF && (isalnum(c) || c == '_') && tp < etp)
 		h = (h << 5) + h + (*tp++ = c);
 	putback(c);
 	if (tp == etp)
diff --git a/usr.bin/m4/mdef.h b/usr.bin/m4/mdef.h
index cc256b012b3e..239993a44ade 100644
--- a/usr.bin/m4/mdef.h
+++ b/usr.bin/m4/mdef.h
@@ -142,7 +142,7 @@ typedef union {			/* stack structure */
  *      pushf() - push a call frame entry onto stack
  *      pushs() - push a string pointer onto stack
  */
-#define gpbc() 	 (bp > bufbase) ? *--bp : getc(infile[ilevel])
+#define gpbc()   (bp > bufbase) ? (*--bp ? *bp : EOF) : getc(infile[ilevel])
 #define pushf(x) if (sp < STACKMAX) mstack[++sp].sfra = (x)
 #define pushs(x) if (sp < STACKMAX) mstack[++sp].sstr = (x)
 
diff --git a/usr.bin/m4/misc.c b/usr.bin/m4/misc.c
index 2ed115bac4c6..916edbac41e4 100644
--- a/usr.bin/m4/misc.c
+++ b/usr.bin/m4/misc.c
@@ -73,8 +73,10 @@ char *s2;
  */
 void
 putback(c)
-char c;
+int c;
 {
+	if (c == EOF)
+		c = 0;
 	if (bp < endpbb)
 		*bp++ = c;
 	else
@@ -88,10 +90,10 @@ char c;
  */
 void
 pbstr(s)
-register char *s;
+register unsigned char *s;
 {
-	register char *es;
-	register char *zp;
+	register unsigned char *es;
+	register unsigned char *zp;
 
 	es = s;
 	zp = bp;