diff --git a/src/include/ngspice/cm.h b/src/include/ngspice/cm.h
index f1bd493c2..25db045ee 100644
--- a/src/include/ngspice/cm.h
+++ b/src/include/ngspice/cm.h
@@ -57,4 +57,27 @@ NON-STANDARD FEATURES
 #endif
 
 
+/*
+ * type safe variants of the <ctype.h> functions for char arguments
+ */
+
+#if !defined(isalpha_c)
+
+inline static int char_to_int(char c) { return (unsigned char) c; }
+
+#define isalpha_c(x) isalpha(char_to_int(x))
+#define islower_c(x) islower(char_to_int(x))
+#define isdigit_c(x) isdigit(char_to_int(x))
+#define isalnum_c(x) isalnum(char_to_int(x))
+#define isprint_c(x) isprint(char_to_int(x))
+#define isblank_c(x) isblank(char_to_int(x))
+#define isspace_c(x) isspace(char_to_int(x))
+#define isupper_c(x) isupper(char_to_int(x))
+
+#define tolower_c(x) ((char) tolower(char_to_int(x)))
+#define toupper_c(x) ((char) toupper(char_to_int(x)))
+
+#endif
+
+
 #endif
diff --git a/src/include/ngspice/ngspice.h b/src/include/ngspice/ngspice.h
index 3e9465614..5c3a90fae 100644
--- a/src/include/ngspice/ngspice.h
+++ b/src/include/ngspice/ngspice.h
@@ -334,4 +334,27 @@ ATTRIBUTE_NORETURN void controlled_exit(int status);
 #endif
 
 
+/*
+ * type safe variants of the <ctype.h> functions for char arguments
+ */
+
+#if !defined(isalpha_c)
+
+inline static int char_to_int(char c) { return (unsigned char) c; }
+
+#define isalpha_c(x) isalpha(char_to_int(x))
+#define islower_c(x) islower(char_to_int(x))
+#define isdigit_c(x) isdigit(char_to_int(x))
+#define isalnum_c(x) isalnum(char_to_int(x))
+#define isprint_c(x) isprint(char_to_int(x))
+#define isblank_c(x) isblank(char_to_int(x))
+#define isspace_c(x) isspace(char_to_int(x))
+#define isupper_c(x) isupper(char_to_int(x))
+
+#define tolower_c(x) ((char) tolower(char_to_int(x)))
+#define toupper_c(x) ((char) toupper(char_to_int(x)))
+
+#endif
+
+
 #endif
diff --git a/src/xspice/cmpp/cmpp.h b/src/xspice/cmpp/cmpp.h
index cf8f1bda5..00bbaa207 100644
--- a/src/xspice/cmpp/cmpp.h
+++ b/src/xspice/cmpp/cmpp.h
@@ -300,3 +300,26 @@ Status_t write_ifs_c_file(const char *filename, Ifs_Table_t *ifs_table);
 
 
 FILE *fopen_cmpp(const char **path_p, const char *mode);
+
+
+/*
+ * type safe variants of the <ctype.h> functions for char arguments
+ */
+
+#if !defined(isalpha_c)
+
+inline static int char_to_int(char c) { return (unsigned char) c; }
+
+#define isalpha_c(x) isalpha(char_to_int(x))
+#define islower_c(x) islower(char_to_int(x))
+#define isdigit_c(x) isdigit(char_to_int(x))
+#define isalnum_c(x) isalnum(char_to_int(x))
+#define isprint_c(x) isprint(char_to_int(x))
+#define isblank_c(x) isblank(char_to_int(x))
+#define isspace_c(x) isspace(char_to_int(x))
+#define isupper_c(x) isupper(char_to_int(x))
+
+#define tolower_c(x) ((char) tolower(char_to_int(x)))
+#define toupper_c(x) ((char) toupper(char_to_int(x)))
+
+#endif