diff --git a/c/argcount.h b/c/argcount.h
index aebb8cb..7dad18c 100644
--- a/c/argcount.h
+++ b/c/argcount.h
@@ -1,4 +1,4 @@
-static void argcount(char *fname, uint32_t nargs, uint32_t c)
+static void argcount(const char *fname, uint32_t nargs, uint32_t c)
 {
     if (__unlikely(nargs != c))
         lerrorf(ArgError, "%s: too %s arguments", fname,
diff --git a/c/error_dmc.h b/c/error_dmc.h
index 9510883..4254407 100644
--- a/c/error_dmc.h
+++ b/c/error_dmc.h
@@ -1,7 +1,7 @@
 extern void DivideByZeroError(void);
 #pragma noreturn(DivideByZeroError)
 
-extern void lerrorf(value_t e, char *format, ...);
+extern void lerrorf(value_t e, const char *format, ...);
 #pragma noreturn(lerrorf)
 
 extern void lerror(value_t e, const char *msg);
@@ -10,8 +10,8 @@ extern void lerror(value_t e, const char *msg);
 extern void fl_raise(value_t e);
 #pragma noreturn(fl_raise)
 
-extern void type_error(char *fname, char *expected, value_t got);
+extern void type_error(const char *fname, const char *expected, value_t got);
 #pragma noreturn(type_error)
 
-extern void bounds_error(char *fname, value_t arr, value_t ind);
+extern void bounds_error(const char *fname, value_t arr, value_t ind);
 #pragma noreturn(bounds_error)
diff --git a/c/error_gnuc.h b/c/error_gnuc.h
index 5cf3287..c5d9898 100644
--- a/c/error_gnuc.h
+++ b/c/error_gnuc.h
@@ -1,13 +1,14 @@
 void DivideByZeroError(void) __attribute__((__noreturn__));
 
-void lerrorf(value_t e, char *format, ...) __attribute__((__noreturn__));
+void lerrorf(value_t e, const char *format, ...)
+__attribute__((__noreturn__));
 
 void lerror(value_t e, const char *msg) __attribute__((__noreturn__));
 
 void fl_raise(value_t e) __attribute__((__noreturn__));
 
-void type_error(char *fname, char *expected, value_t got)
+void type_error(const char *fname, const char *expected, value_t got)
 __attribute__((__noreturn__));
 
-void bounds_error(char *fname, value_t arr, value_t ind)
+void bounds_error(const char *fname, value_t arr, value_t ind)
 __attribute__((__noreturn__));
diff --git a/c/error_msc.h b/c/error_msc.h
index 289f9c3..e535d27 100644
--- a/c/error_msc.h
+++ b/c/error_msc.h
@@ -1,13 +1,14 @@
 __declspec(noreturn) extern void DivideByZeroError(void);
 
-__declspec(noreturn) extern void lerrorf(value_t e, char *format, ...);
+__declspec(noreturn) extern void lerrorf(value_t e, const char *format, ...);
 
 __declspec(noreturn) extern void lerror(value_t e, const char *msg);
 
 __declspec(noreturn) extern void fl_raise(value_t e);
 
-__declspec(noreturn) extern void type_error(char *fname, char *expected,
+__declspec(noreturn) extern void type_error(const char *fname,
+                                            const char *expected,
                                             value_t got);
 
-__declspec(noreturn) extern void bounds_error(char *fname, value_t arr,
+__declspec(noreturn) extern void bounds_error(const char *fname, value_t arr,
                                               value_t ind);
diff --git a/c/error_watcomc.h b/c/error_watcomc.h
index cf04235..f281e74 100644
--- a/c/error_watcomc.h
+++ b/c/error_watcomc.h
@@ -2,7 +2,7 @@
 extern void DivideByZeroError(void);
 
 #pragma aux lerrorf aborts;
-extern void lerrorf(value_t e, char *format, ...);
+extern void lerrorf(value_t e, const char *format, ...);
 
 #pragma aux lerror aborts;
 extern void lerror(value_t e, const char *msg);
@@ -11,7 +11,7 @@ extern void lerror(value_t e, const char *msg);
 extern void fl_raise(value_t e);
 
 #pragma aux type_error aborts;
-extern void type_error(char *fname, char *expected, value_t got);
+extern void type_error(const char *fname, const char *expected, value_t got);
 
 #pragma aux bounds_error aborts;
-extern void bounds_error(char *fname, value_t arr, value_t ind);
+extern void bounds_error(const char *fname, value_t arr, value_t ind);
diff --git a/c/flisp.c b/c/flisp.c
index 92c218b..52dd3a6 100644
--- a/c/flisp.c
+++ b/c/flisp.c
@@ -208,7 +208,7 @@ void fl_raise(value_t e)
     longjmp(thisctx->buf, 1);
 }
 
-static value_t make_error_msg(char *format, va_list args)
+static value_t make_error_msg(const char *format, va_list args)
 {
     char msgbuf[512];
 
@@ -216,7 +216,7 @@ static value_t make_error_msg(char *format, va_list args)
     return string_from_cstr(msgbuf);
 }
 
-void lerrorf(value_t e, char *format, ...)
+void lerrorf(value_t e, const char *format, ...)
 {
     va_list args;
     value_t msg;
@@ -239,12 +239,12 @@ void lerror(value_t e, const char *msg)
     fl_raise(fl_list2(e, m));
 }
 
-void type_error(char *fname, char *expected, value_t got)
+void type_error(const char *fname, const char *expected, value_t got)
 {
     fl_raise(fl_listn(4, TypeError, symbol(fname), symbol(expected), got));
 }
 
-void bounds_error(char *fname, value_t arr, value_t ind)
+void bounds_error(const char *fname, value_t arr, value_t ind)
 {
     fl_raise(fl_listn(4, BoundsError, symbol(fname), arr, ind));
 }
diff --git a/c/flisp.h b/c/flisp.h
index 722cf59..220cb3d 100644
--- a/c/flisp.h
+++ b/c/flisp.h
@@ -335,7 +335,7 @@ int fl_isstring(value_t v);
 int fl_isnumber(value_t v);
 int fl_isgensym(value_t v);
 int fl_isiostream(value_t v);
-struct ios *fl_toiostream(value_t v, char *fname);
+struct ios *fl_toiostream(value_t v, const char *fname);
 value_t cvalue_compare(value_t a, value_t b);
 int numeric_compare(value_t a, value_t b, int eq, int eqnans, char *fname);
 
diff --git a/c/iostream.c b/c/iostream.c
index 72c9c69..43f3192 100644
--- a/c/iostream.c
+++ b/c/iostream.c
@@ -81,14 +81,14 @@ value_t fl_eof_objectp(value_t *args, uint32_t nargs)
     return (FL_EOF == args[0]) ? FL_T : FL_F;
 }
 
-static struct ios *toiostream(value_t v, char *fname)
+static struct ios *toiostream(value_t v, const char *fname)
 {
     if (!fl_isiostream(v))
         type_error(fname, "iostream", v);
     return value2c(struct ios *, v);
 }
 
-struct ios *fl_toiostream(value_t v, char *fname)
+struct ios *fl_toiostream(value_t v, const char *fname)
 {
     return toiostream(v, fname);
 }