diff --git a/src/helper/log.c b/src/helper/log.c
index 0976f4e0f..afba0d172 100644
--- a/src/helper/log.c
+++ b/src/helper/log.c
@@ -526,7 +526,7 @@ void log_socket_error(const char *socket_desc)
 const char *find_nonprint_char(const char *buf, unsigned int buf_len)
 {
 	for (unsigned int i = 0; i < buf_len; i++) {
-		if (!isprint(buf[i]))
+		if (!isprint((unsigned char)buf[i]))
 			return buf + i;
 	}
 	return NULL;
diff --git a/src/jtag/adapter.c b/src/jtag/adapter.c
index 3f94ffec7..7bf049cd8 100644
--- a/src/jtag/adapter.c
+++ b/src/jtag/adapter.c
@@ -970,7 +970,7 @@ COMMAND_HANDLER(adapter_gpio_config_handler)
 	while (i < CMD_ARGC) {
 		LOG_DEBUG("Processing %s", CMD_ARGV[i]);
 
-		if (isdigit(*CMD_ARGV[i])) {
+		if (isdigit((unsigned char)*CMD_ARGV[i])) {
 			COMMAND_PARSE_NUMBER(uint, CMD_ARGV[i], gpio_config->gpio_num);
 			++i;
 			continue;
diff --git a/src/pld/efinix.c b/src/pld/efinix.c
index b6e5f9e47..eff62c62b 100644
--- a/src/pld/efinix.c
+++ b/src/pld/efinix.c
@@ -82,7 +82,8 @@ static int efinix_read_bit_file(struct raw_bit_file *bit_file, const char *filen
 			return ERROR_PLD_FILE_LOAD_FAILED;
 		}
 
-		if (!isxdigit(buffer[0]) || !isxdigit(buffer[1])) {
+		if (!isxdigit((unsigned char)buffer[0]) ||
+				!isxdigit((unsigned char)buffer[1])) {
 			fclose(input_file);
 			free(bit_file->data);
 			bit_file->data = NULL;
diff --git a/src/pld/gatemate.c b/src/pld/gatemate.c
index 479f6d183..11b6ae76e 100644
--- a/src/pld/gatemate.c
+++ b/src/pld/gatemate.c
@@ -57,7 +57,8 @@ static int gatemate_read_cfg_line(struct gatemate_bit_file *bit_file, const char
 		} else if (line_buffer[idx] == 0) {
 			break;
 		} else if (idx + 1 < nread) {
-			if (isxdigit(line_buffer[idx]) && isxdigit(line_buffer[idx + 1])) {
+			if (isxdigit((unsigned char)line_buffer[idx]) &&
+					isxdigit((unsigned char)line_buffer[idx + 1])) {
 				uint8_t byte;
 				unhexify(&byte, line_buffer + idx, 2);
 				int retval = gatemate_add_byte_to_bitfile(bit_file, byte);
diff --git a/src/server/telnet_server.c b/src/server/telnet_server.c
index ea7f38fb8..83d50c2d0 100644
--- a/src/server/telnet_server.c
+++ b/src/server/telnet_server.c
@@ -602,7 +602,8 @@ static void telnet_auto_complete(struct connection *connection)
 
 	/* user command position in the line, ignore leading spaces */
 	size_t usr_cmd_pos = seq_start;
-	while ((usr_cmd_pos < t_con->line_cursor) && isspace(t_con->line[usr_cmd_pos]))
+	while ((usr_cmd_pos < t_con->line_cursor) &&
+			isspace((unsigned char)t_con->line[usr_cmd_pos]))
 		usr_cmd_pos++;
 
 	/* check user command length */
@@ -616,9 +617,10 @@ static void telnet_auto_complete(struct connection *connection)
 	 * because info commands does not tolerate multiple spaces */
 	size_t optimized_spaces = 0;
 	char query[usr_cmd_len + 1];
+
 	for (size_t i = 0; i < usr_cmd_len; i++) {
-		if ((i < usr_cmd_len - 1) && isspace(t_con->line[usr_cmd_pos + i])
-				&& isspace(t_con->line[usr_cmd_pos + i + 1])) {
+		if ((i < usr_cmd_len - 1) && isspace((unsigned char)t_con->line[usr_cmd_pos + i])
+				&& isspace((unsigned char)t_con->line[usr_cmd_pos + i + 1])) {
 			optimized_spaces++;
 			continue;
 		}
diff --git a/src/target/riscv/riscv.c b/src/target/riscv/riscv.c
index 8054a1c9b..3dffce50e 100644
--- a/src/target/riscv/riscv.c
+++ b/src/target/riscv/riscv.c
@@ -4380,7 +4380,7 @@ static bool parse_csr_address(const char *reg_address_str, unsigned int *reg_add
 {
 	*reg_addr = -1;
 	/* skip initial spaces */
-	while (isspace(reg_address_str[0]))
+	while (isspace((unsigned char)reg_address_str[0]))
 		++reg_address_str;
 	/* try to detect if string starts with 0x or 0X */
 	bool is_hex_address = strncmp(reg_address_str, "0x", 2) == 0 ||