diff --git a/gpsd.c b/gpsd.c
index 30ebc07..b591ce6 100644
--- a/gpsd.c
+++ b/gpsd.c
@@ -605,7 +605,7 @@ static ssize_t throttled_write(struct subscriber_t *sub, char *buf,
     ssize_t status;
 
     if (context.errout.debug >= LOG_CLIENT) {
-	if (isprint(buf[0]))
+	if (isprint((unsigned char) buf[0]))
 	    gpsd_report(&context.errout, LOG_CLIENT,
 			"=> client(%d): %s\n", sub_index(sub), buf);
 	else {
@@ -782,7 +782,7 @@ bool gpsd_add_device(const char *device_name, bool flag_nowait)
     static char stash[BUFSIZ];
 
     /*@ -temptrans -mayaliasunique @*/
-    for (q = p; isprint(*p) && !isspace(*p) && /address@hidden@*/ (p - q < BUFSIZ - 1);
+    for (q = p; isprint((unsigned char) *p) && !isspace((unsigned char) *p) && /address@hidden@*/ (p - q < BUFSIZ - 1);
 	 p++)
 	continue;
     (void)memcpy(stash, q, (size_t) (p - q));
@@ -1002,12 +1002,12 @@ static void set_serial(struct gps_device_t *device,
     int wordsize = 8;
 
     if (strchr("78", *modestring) != NULL) {
-	while (isspace(*modestring))
+	while (isspace((unsigned char) *modestring))
 	    modestring++;
 	wordsize = (int)(*modestring++ - '0');
 	if (strchr("NOE", *modestring) != NULL) {
 	    parity = *modestring++;
-	    while (isspace(*modestring))
+	    while (isspace((unsigned char) *modestring))
 		modestring++;
 	    if (strchr("12", *modestring) != NULL)
 		stopbits = (unsigned int)(*modestring - '0');
@@ -1077,7 +1077,7 @@ static void rstrip(char *str)
 {
     char *strend;
     strend = str + strlen(str) - 1;
-    while (isspace(*strend)) {
+    while (isspace((unsigned char) *strend)) {
 	*strend = '\0';
 	--strend;
     }
@@ -1360,7 +1360,7 @@ static void handle_request(struct subscriber_t *sub,
     } else {
 	const char *errend;
 	errend = buf + strlen(buf) - 1;
-	while (isspace(*errend) && errend > buf)
+	while (isspace((unsigned char) *errend) && errend > buf)
 	    --errend;
 	(void)snprintf(reply, replylen,
 		       "{\"class\":\"ERROR\",\"message\":\"Unrecognized request '%.*s'\"}\r\n",
@@ -1662,7 +1662,7 @@ static int handle_gpsd_request(struct subscriber_t *sub, const char *buf)
     if (buf[0] == '?') {
 	const char *end;
 	for (end = buf; *buf != '\0'; buf = end)
-	    if (isspace(*buf))
+	    if (isspace((unsigned char) *buf))
 		end = buf + 1;
 	    else
 		handle_request(sub, buf, &end,
diff --git a/gpsmon.c b/gpsmon.c
index 909f271..a5ee0b2 100644
--- a/gpsmon.c
+++ b/gpsmon.c
@@ -170,7 +170,7 @@ static void visibilize(/address@hidden@*/char *buf2, size_t len2, const char *buf)
 
     buf2[0] = '\0';
     for (sp = buf; *sp != '\0' && strlen(buf2)+4 < len2; sp++)
-	if (isprint(*sp) || (sp[0] == '\n' && sp[1] == '\0')
+	if (isprint((unsigned char) *sp) || (sp[0] == '\n' && sp[1] == '\0')
 	  || (sp[0] == '\r' && sp[2] == '\0'))
 	    (void)snprintf(buf2 + strlen(buf2), 2, "%c", *sp);
 	else
@@ -186,12 +186,12 @@ static void cond_hexdump(/address@hidden@*/char *buf2, size_t len2,
     size_t i;
     bool printable = true;
     for (i = 0; i < len; i++)
-	if (!isprint(buf[i]) && !isspace(buf[i]))
+	if (!isprint((unsigned char) buf[i]) && !isspace((unsigned char) buf[i]))
 	    printable = false;
     if (printable) {
 	size_t j;
 	for (i = j = 0; i < len && j < len2 - 1; i++)
-	    if (isprint(buf[i])) {
+	    if (isprint((unsigned char) buf[i])) {
 		buf2[j++] = buf[i];
 		buf2[j] = '\0';
 	    }
@@ -750,8 +750,8 @@ static bool do_command(const char *line)
     unsigned char buf[BUFLEN];
     const char *arg;
 
-    if (isspace(line[1])) {
-	for (arg = line + 2; *arg != '\0' && isspace(*arg); arg++)
+    if (isspace((unsigned char) line[1])) {
+	for (arg = line + 2; *arg != '\0' && isspace((unsigned char) *arg); arg++)
 	    arg++;
 	arg++;
     } else
diff --git a/gpsutils.c b/gpsutils.c
index 9b0761b..dfcaa3f 100644
--- a/gpsutils.c
+++ b/gpsutils.c
@@ -87,7 +87,7 @@ double safe_atof(const char *string)
      */
 
     p = string;
-    while (isspace(*p)) {
+    while (isspace((unsigned char) *p)) {
 	p += 1;
     }
     if (*p == '-') {
@@ -185,7 +185,7 @@ double safe_atof(const char *string)
 	    }
 	    expSign = false;
 	}
-	while (isdigit(*p)) {
+	while (isdigit((unsigned char) *p)) {
 	    exp = exp * 10 + (*p - '0');
 	    p += 1;
 	}
diff --git a/hex.c b/hex.c
index ac1705f..635bd53 100644
--- a/hex.c
+++ b/hex.c
@@ -17,7 +17,7 @@ const char /*@ observer @*/ *gpsd_packetdump(char *scbuf, size_t scbuflen,
 
     assert(binbuf != NULL);
     for (cp = binbuf; cp < binbuf + binbuflen; cp++)
-	if (!isprint(*cp) && !isspace(*cp))
+	if (!isprint((unsigned char) *cp) && !isspace((unsigned char) *cp))
 	    printable = false;
     if (printable)
 	return binbuf;
diff --git a/json.c b/json.c
index 1f6877f..8efc422 100644
--- a/json.c
+++ b/json.c
@@ -235,7 +235,7 @@ static int json_internal_read_object(const char *cp,
 			  statenames[state], *cp, cp));
 	switch (state) {
 	case init:
-	    if (isspace(*cp))
+	    if (isspace((unsigned char) *cp))
 		continue;
 	    else if (*cp == '{')
 		state = await_attr;
@@ -246,7 +246,7 @@ static int json_internal_read_object(const char *cp,
 	    }
 	    break;
 	case await_attr:
-	    if (isspace(*cp))
+	    if (isspace((unsigned char) *cp))
 		continue;
 	    else if (*cp == '"') {
 		state = in_attr;
@@ -294,7 +294,7 @@ static int json_internal_read_object(const char *cp,
 		*pattr++ = *cp;
 	    break;
 	case await_value:
-	    if (isspace(*cp) || *cp == ':')
+	    if (isspace((unsigned char) *cp) || *cp == ':')
 		continue;
 	    else if (*cp == '[') {
 		if (cursor->type != t_array) {
@@ -372,7 +372,7 @@ static int json_internal_read_object(const char *cp,
 	case in_val_token:
 	    if (pval == NULL)
 		return JSON_ERR_NULLPTR;
-	    if (isspace(*cp) || *cp == ',' || *cp == '}') {
+	    if (isspace((unsigned char) *cp) || *cp == ',' || *cp == '}') {
 		*pval = '\0';
 		json_debug_trace((1, "Collected token value %s.\n", valbuf));
 		state = post_val;
@@ -400,7 +400,7 @@ static int json_internal_read_object(const char *cp,
 		if ((strcmp(valbuf, "true")==0 || strcmp(valbuf, "false")==0)
 			&& seeking == t_boolean)
 		    break;
-		if (isdigit(valbuf[0])) {
+		if (isdigit((unsigned char) valbuf[0])) {
 		    bool decimal = strchr(valbuf, '.') != NULL;
 		    if (decimal && seeking == t_real)
 			break;
@@ -501,7 +501,7 @@ static int json_internal_read_object(const char *cp,
 		}
 	    /address@hidden@*/
 	case post_array:
-	    if (isspace(*cp))
+	    if (isspace((unsigned char) *cp))
 		continue;
 	    else if (*cp == ',')
 		state = await_attr;
@@ -518,7 +518,7 @@ static int json_internal_read_object(const char *cp,
 
   good_parse:
     /* in case there's another object following, consune trailing WS */
-    while (isspace(*cp))
+    while (isspace((unsigned char) *cp))
 	++cp;
     if (end != NULL)
 	*end = cp;
@@ -539,7 +539,7 @@ int json_read_array(const char *cp, const struct json_array_t *arr,
 
     json_debug_trace((1, "Entered json_read_array()\n"));
 
-    while (isspace(*cp))
+    while (isspace((unsigned char) *cp))
 	cp++;
     if (*cp != '[') {
 	json_debug_trace((1, "Didn't find expected array start\n"));
@@ -551,7 +551,7 @@ int json_read_array(const char *cp, const struct json_array_t *arr,
     arrcount = 0;
 
     /* Check for empty array */
-    while (isspace(*cp))
+    while (isspace((unsigned char) *cp))
 	cp++;
     if (*cp == ']')
 	goto breakout;
@@ -560,7 +560,7 @@ int json_read_array(const char *cp, const struct json_array_t *arr,
 	json_debug_trace((1, "Looking at %s\n", cp));
 	switch (arr->element_type) {
 	case t_string:
-	    if (isspace(*cp))
+	    if (isspace((unsigned char) *cp))
 		cp++;
 	    if (*cp != '"')
 		return JSON_ERR_BADSTRING;
@@ -605,7 +605,7 @@ int json_read_array(const char *cp, const struct json_array_t *arr,
 	    return JSON_ERR_SUBTYPE;
 	}
 	arrcount++;
-	if (isspace(*cp))
+	if (isspace((unsigned char) *cp))
 	    cp++;
 	if (*cp == ']') {
 	    json_debug_trace((1, "End of array found.\n"));
diff --git a/libgpsd_core.c b/libgpsd_core.c
index 3e2a0f2..bec2ddc 100644
--- a/libgpsd_core.c
+++ b/libgpsd_core.c
@@ -79,7 +79,7 @@ static void visibilize(/address@hidden@*/char *buf2, size_t len, const char *buf)
 
     buf2[0] = '\0';
     for (sp = buf; *sp != '\0' && strlen(buf2)+4 < len; sp++)
-	if (isprint(*sp) || (sp[0] == '\n' && sp[1] == '\0')
+	if (isprint((unsigned char) *sp) || (sp[0] == '\n' && sp[1] == '\0')
 	  || (sp[0] == '\r' && sp[2] == '\0'))
 	    (void)snprintf(buf2 + strlen(buf2), 2, "%c", *sp);
 	else
diff --git a/packet.c b/packet.c
index 9bdfae9..d5220fd 100644
--- a/packet.c
+++ b/packet.c
@@ -1500,7 +1500,7 @@ void packet_parse(struct gps_lexer_t *lexer)
 		 * Back up past any whitespace.  Need to do this because
 		 * at least one GPS (the Firefly 1a) emits \r\r\n
 		 */
-		for (end = (char *)lexer->inbufptr - 1; isspace(*end); end--)
+		for (end = (char *)lexer->inbufptr - 1; isspace((unsigned char) *end); end--)
 		    continue;
 		while (strchr("0123456789ABCDEF", *end))
 		    --end;
@@ -1509,8 +1509,8 @@ void packet_parse(struct gps_lexer_t *lexer)
 		    for (n = 1; (char *)lexer->inbuffer + n < end; n++)
 			crc ^= lexer->inbuffer[n];
 		    (void)snprintf(csum, sizeof(csum), "%02X", crc);
-		    checksum_ok = (csum[0] == toupper(end[1])
-				   && csum[1] == toupper(end[2]));
+		    checksum_ok = (csum[0] == toupper((unsigned char) end[1])
+				   && csum[1] == toupper((unsigned char) end[2]));
 		}
 		if (!checksum_ok) {
 		    gpsd_report(&lexer->errout, LOG_WARN,