diff --git a/src/port.c b/src/port.c
index fe36b33c..f8403de0 100644
--- a/src/port.c
+++ b/src/port.c
@@ -107,7 +107,7 @@ pic_get_output_string(pic_state *pic, struct pic_port *port)
 void
 pic_close_port(pic_state *pic, struct pic_port *port)
 {
-  if (xfclose(port->file) == EOF) {
+  if ((int)xfclose(port->file) == (int)EOF) {
     pic_error(pic, "close-port: failure");
   }
   port->status = PIC_PORT_CLOSE;
@@ -377,7 +377,7 @@ pic_port_get_output_bytevector(pic_state *pic)
 static pic_value
 pic_port_read_char(pic_state *pic)
 {
-  char c;
+  int c;
   struct pic_port *port = pic_stdin(pic);
 
   pic_get_args(pic, "|p", &port);
@@ -388,14 +388,14 @@ pic_port_read_char(pic_state *pic)
     return pic_eof_object();
   }
   else {
-    return pic_char_value(c);
+    return pic_char_value((char)c);
   }
 }
 
 static pic_value
 pic_port_peek_char(pic_state *pic)
 {
-  char c;
+  int c;
   struct pic_port *port = pic_stdin(pic);
 
   pic_get_args(pic, "|p", &port);
@@ -407,14 +407,14 @@ pic_port_peek_char(pic_state *pic)
   }
   else {
     xungetc(c, port->file);
-    return pic_char_value(c);
+    return pic_char_value((char)c);
   }
 }
 
 static pic_value
 pic_port_read_line(pic_state *pic)
 {
-  char c;
+  int c;
   struct pic_port *port = pic_stdin(pic), *buf;
   struct pic_string *str;
 
@@ -453,12 +453,13 @@ pic_port_read_string(pic_state *pic){
   struct pic_port *port = pic_stdin(pic), *buf;
   pic_str *str;
   int k, i;
-  char c;
+  int c;
 
   pic_get_args(pic, "i|p", &k,  &port);
 
   assert_port_profile(port, PIC_PORT_IN | PIC_PORT_TEXT, PIC_PORT_OPEN, "read-stritg");
 
+  c = EOF;
   buf = pic_open_output_string(pic);
   for(i = 0; i < k; ++i) {
     c = xfgetc(port->file);
@@ -481,7 +482,7 @@ pic_port_read_string(pic_state *pic){
 static pic_value
 pic_port_read_byte(pic_state *pic){
   struct pic_port *port = pic_stdin(pic);
-  char c;
+  int c;
   pic_get_args(pic, "|p", &port);
 
   assert_port_profile(port, PIC_PORT_IN | PIC_PORT_BINARY, PIC_PORT_OPEN, "read-u8");
@@ -495,14 +496,15 @@ pic_port_read_byte(pic_state *pic){
 static pic_value
 pic_port_peek_byte(pic_state *pic)
 {
-  char c;
+  int c;
   struct pic_port *port = pic_stdin(pic);
 
   pic_get_args(pic, "|p", &port);
 
   assert_port_profile(port, PIC_PORT_IN | PIC_PORT_BINARY, PIC_PORT_OPEN, "peek-u8");
 
-  if ((c = xfgetc(port->file)) == EOF) {
+  c = xfgetc(port->file);
+  if (c == EOF) {
     return pic_eof_object();
   }
   else {
diff --git a/src/read.c b/src/read.c
index 5ee93da4..464b13b4 100644
--- a/src/read.c
+++ b/src/read.c
@@ -13,10 +13,10 @@
 #include "picrin/blob.h"
 #include "picrin/port.h"
 
-typedef pic_value (*read_func_t)(pic_state *, struct pic_port *, char);
+typedef pic_value (*read_func_t)(pic_state *, struct pic_port *, int);
 
-static pic_value read(pic_state *pic, struct pic_port *port, char c);
-static pic_value read_nullable(pic_state *pic, struct pic_port *port, char c);
+static pic_value read(pic_state *pic, struct pic_port *port, int c);
+static pic_value read_nullable(pic_state *pic, struct pic_port *port, int c);
 
 static noreturn void
 read_error(pic_state *pic, const char *msg)
@@ -24,8 +24,8 @@ read_error(pic_state *pic, const char *msg)
   pic_throw(pic, PIC_ERROR_READ, msg, pic_nil_value());
 }
 
-static char
-skip(struct pic_port *port, char c)
+static int
+skip(struct pic_port *port, int c)
 {
   while (isspace(c)) {
     c = xfgetc(port->file);
@@ -33,16 +33,16 @@ skip(struct pic_port *port, char c)
   return c;
 }
 
-static char
+static int
 next(struct pic_port *port)
 {
   return xfgetc(port->file);
 }
 
-static char
+static int
 peek(struct pic_port *port)
 {
-  char c;
+  int c;
 
   xungetc((c = xfgetc(port->file)), port->file);
 
@@ -52,9 +52,9 @@ peek(struct pic_port *port)
 static bool
 expect(struct pic_port *port, const char *str)
 {
-  char c;
+  int c;
 
-  while ((c = *str++) != 0) {
+  while ((c = (int)*str++) != 0) {
     if (c != peek(port))
       return false;
     next(port);
@@ -64,9 +64,9 @@ expect(struct pic_port *port, const char *str)
 }
 
 static bool
-isdelim(char c)
+isdelim(int c)
 {
-  return c == EOF || strchr("();,|\" \t\n\r", c) != NULL; /* ignores "#", "'" */
+  return c == EOF || strchr("();,|\" \t\n\r", (char)c) != NULL; /* ignores "#", "'" */
 }
 
 static bool
@@ -82,7 +82,7 @@ strcaseeq(const char *s1, const char *s2)
 }
 
 static pic_value
-read_comment(pic_state *pic, struct pic_port *port, char c)
+read_comment(pic_state *pic, struct pic_port *port, int c)
 {
   UNUSED(pic);
 
@@ -94,9 +94,9 @@ read_comment(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read_block_comment(pic_state *pic, struct pic_port *port, char c)
+read_block_comment(pic_state *pic, struct pic_port *port, int c)
 {
-  char x, y;
+  int x, y;
   int i = 1;
 
   UNUSED(pic);
@@ -119,7 +119,7 @@ read_block_comment(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read_datum_comment(pic_state *pic, struct pic_port *port, char c)
+read_datum_comment(pic_state *pic, struct pic_port *port, int c)
 {
   UNUSED(c);
 
@@ -129,9 +129,9 @@ read_datum_comment(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read_directive(pic_state *pic, struct pic_port *port, char c)
+read_directive(pic_state *pic, struct pic_port *port, int c)
 {
-  switch (peek(port)) {
+  switch ((char)peek(port)) {
   case 'n':
     if (expect(port, "no-fold-case")) {
       /* :FIXME: set no-fold-case flag */
@@ -150,7 +150,7 @@ read_directive(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read_quote(pic_state *pic, struct pic_port *port, char c)
+read_quote(pic_state *pic, struct pic_port *port, int c)
 {
   UNUSED(c);
 
@@ -158,7 +158,7 @@ read_quote(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read_quasiquote(pic_state *pic, struct pic_port *port, char c)
+read_quasiquote(pic_state *pic, struct pic_port *port, int c)
 {
   UNUSED(c);
 
@@ -166,11 +166,11 @@ read_quasiquote(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read_comma(pic_state *pic, struct pic_port *port, char c)
+read_comma(pic_state *pic, struct pic_port *port, int c)
 {
   c = next(port);
 
-  if (c == '@') {
+  if ((char)c == '@') {
     return pic_list2(pic, pic_sym_value(pic->sUNQUOTE_SPLICING), read(pic, port, next(port)));
   } else {
     return pic_list2(pic, pic_sym_value(pic->sUNQUOTE), read(pic, port, c));
@@ -178,7 +178,7 @@ read_comma(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read_symbol(pic_state *pic, struct pic_port *port, char c)
+read_symbol(pic_state *pic, struct pic_port *port, int c)
 {
   size_t len;
   char *buf;
@@ -193,7 +193,7 @@ read_symbol(pic_state *pic, struct pic_port *port, char c)
     }
     len += 1;
     buf = pic_realloc(pic, buf, len + 1);
-    buf[len - 1] = c;
+    buf[len - 1] = (char)c;
   } while (! isdelim(peek(port)));
 
   buf[len] = '\0';
@@ -204,7 +204,7 @@ read_symbol(pic_state *pic, struct pic_port *port, char c)
 }
 
 static size_t
-read_uinteger(pic_state *pic, struct pic_port *port, char c, char buf[])
+read_uinteger(pic_state *pic, struct pic_port *port, int c, char buf[])
 {
   size_t i = 0;
 
@@ -212,9 +212,9 @@ read_uinteger(pic_state *pic, struct pic_port *port, char c, char buf[])
     read_error(pic, "expected one or more digits");
   }
 
-  buf[i++] = c;
+  buf[i++] = (char)c;
   while (isdigit(c = peek(port))) {
-    buf[i++] = next(port);
+    buf[i++] = (char)next(port);
   }
 
   buf[i] = '\0';
@@ -223,7 +223,7 @@ read_uinteger(pic_state *pic, struct pic_port *port, char c, char buf[])
 }
 
 static pic_value
-read_number(pic_state *pic, struct pic_port *port, char c)
+read_number(pic_state *pic, struct pic_port *port, int c)
 {
   char buf[256];
   size_t i;
@@ -231,10 +231,10 @@ read_number(pic_state *pic, struct pic_port *port, char c)
 
   i = read_uinteger(pic, port, c, buf);
 
-  switch (peek(port)) {
+  switch ((char)peek(port)) {
   case '.':
     do {
-      buf[i++] = next(port);
+      buf[i++] = (char)next(port);
     } while (isdigit(peek(port)));
     buf[i] = '\0';
     return pic_float_value(atof(buf));
@@ -265,7 +265,7 @@ negate(pic_value n)
 }
 
 static pic_value
-read_minus(pic_state *pic, struct pic_port *port, char c)
+read_minus(pic_state *pic, struct pic_port *port, int c)
 {
   pic_value sym;
 
@@ -285,7 +285,7 @@ read_minus(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read_plus(pic_state *pic, struct pic_port *port, char c)
+read_plus(pic_state *pic, struct pic_port *port, int c)
 {
   pic_value sym;
 
@@ -305,13 +305,13 @@ read_plus(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read_boolean(pic_state *pic, struct pic_port *port, char c)
+read_boolean(pic_state *pic, struct pic_port *port, int c)
 {
   UNUSED(pic);
   UNUSED(port);
 
   if (! isdelim(peek(port))) {
-    if (c == 't') {
+    if ((char)c == 't') {
       if (! expect(port, "rue")) {
         goto fail;
       }
@@ -322,7 +322,7 @@ read_boolean(pic_state *pic, struct pic_port *port, char c)
     }
   }
 
-  if (c == 't') {
+  if ((char)c == 't') {
     return pic_true_value();
   } else {
     return pic_false_value();
@@ -333,12 +333,12 @@ read_boolean(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read_char(pic_state *pic, struct pic_port *port, char c)
+read_char(pic_state *pic, struct pic_port *port, int c)
 {
   c = next(port);
 
   if (! isdelim(peek(port))) {
-    switch (c) {
+    switch ((char)c) {
     default: read_error(pic, "unexpected character after char literal");
     case 'a': c = '\a'; if (! expect(port, "lerm")) goto fail; break;
     case 'b': c = '\b'; if (! expect(port, "ackspace")) goto fail; break;
@@ -368,7 +368,7 @@ read_char(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read_string(pic_state *pic, struct pic_port *port, char c)
+read_string(pic_state *pic, struct pic_port *port, int c)
 {
   char *buf;
   size_t size, cnt;
@@ -390,7 +390,7 @@ read_string(pic_state *pic, struct pic_port *port, char c)
       case 'r': c = '\r'; break;
       }
     }
-    buf[cnt++] = c;
+    buf[cnt++] = (char)c;
     if (cnt >= size) {
       buf = pic_realloc(pic, buf, size *= 2);
     }
@@ -417,7 +417,7 @@ read_pipe(pic_state *pic, struct pic_port *port, char c)
   cnt = 0;
   while ((c = next(port)) != '|') {
     if (c == '\\') {
-      switch (c = next(port)) {
+      switch ((char)(c = next(port))) {
       case 'a': c = '\a'; break;
       case 'b': c = '\b'; break;
       case 't': c = '\t'; break;
@@ -433,7 +433,7 @@ read_pipe(pic_state *pic, struct pic_port *port, char c)
         break;
       }
     }
-    buf[cnt++] = c;
+    buf[cnt++] = (char)c;
     if (cnt >= size) {
       buf = pic_realloc(pic, buf, size *= 2);
     }
@@ -447,7 +447,7 @@ read_pipe(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read_unsigned_blob(pic_state *pic, struct pic_port *port, char c)
+read_unsigned_blob(pic_state *pic, struct pic_port *port, int c)
 {
   int nbits, n;
   size_t len, i;
@@ -493,7 +493,7 @@ read_unsigned_blob(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read_pair(pic_state *pic, struct pic_port *port, char c)
+read_pair(pic_state *pic, struct pic_port *port, int c)
 {
   char tOPEN = c, tCLOSE = (tOPEN == '(') ? ')' : ']';
   pic_value car, cdr;
@@ -530,7 +530,7 @@ read_pair(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read_vector(pic_state *pic, struct pic_port *port, char c)
+read_vector(pic_state *pic, struct pic_port *port, int c)
 {
   pic_value list;
 
@@ -543,9 +543,9 @@ static pic_value
 read_label_set(pic_state *pic, struct pic_port *port, int i)
 {
   pic_value val;
-  char c;
+  int c;
 
-  switch (c = skip(port, ' ')) {
+  switch ((char)(c = skip(port, ' '))) {
   case '(': case '[':
     {
       pic_value tmp;
@@ -612,7 +612,7 @@ read_label_ref(pic_state *pic, struct pic_port *port, int i)
 }
 
 static pic_value
-read_label(pic_state *pic, struct pic_port *port, char c)
+read_label(pic_state *pic, struct pic_port *port, int c)
 {
   int i;
 
@@ -631,11 +631,11 @@ read_label(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read_dispatch(pic_state *pic, struct pic_port *port, char c)
+read_dispatch(pic_state *pic, struct pic_port *port, int c)
 {
   c = next(port);
 
-  switch (c) {
+  switch ((char)c) {
   case '!':
     return read_directive(pic, port, c);
   case '|':
@@ -659,7 +659,7 @@ read_dispatch(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read_nullable(pic_state *pic, struct pic_port *port, char c)
+read_nullable(pic_state *pic, struct pic_port *port, int c)
 {
   c = skip(port, c);
 
@@ -667,7 +667,7 @@ read_nullable(pic_state *pic, struct pic_port *port, char c)
     read_error(pic, "unexpected EOF");
   }
 
-  switch (c) {
+  switch ((char)c) {
   case ';':
     return read_comment(pic, port, c);
   case '#':
@@ -697,7 +697,7 @@ read_nullable(pic_state *pic, struct pic_port *port, char c)
 }
 
 static pic_value
-read(pic_state *pic, struct pic_port *port, char c)
+read(pic_state *pic, struct pic_port *port, int c)
 {
   pic_value val;
 
@@ -716,7 +716,7 @@ pic_value
 pic_read(pic_state *pic, struct pic_port *port)
 {
   pic_value val;
-  char c = next(port);
+  int c = next(port);
 
  retry:
   c = skip(port, c);
diff --git a/src/string.c b/src/string.c
index 73dba061..ab679f50 100644
--- a/src/string.c
+++ b/src/string.c
@@ -62,13 +62,13 @@ pic_strlen(pic_str *str)
 char
 pic_str_ref(pic_state *pic, pic_str *str, size_t i)
 {
-  char c;
+  int c;
 
   c = xr_at(str->rope, i);
   if (c == -1) {
     pic_errorf(pic, "index out of range %d", i);
   }
-  return c;
+  return (char)c;
 }
 
 static xrope *
diff --git a/tools/main.c b/tools/main.c
index 9771df6e..e5129daf 100644
--- a/tools/main.c
+++ b/tools/main.c
@@ -71,7 +71,7 @@ repl(pic_state *pic)
 #if PIC_ENABLE_READLINE
   char *read_line;
 #else
-  char last_char;
+  int last_char;
   int char_index;
 #endif
 
@@ -111,7 +111,7 @@ repl(pic_state *pic)
 	goto eof;
       if (char_index == LINE_MAX_LENGTH)
 	goto overflow;
-      line[char_index++] = last_char;
+      line[char_index++] = (char)last_char;
     }
     line[char_index] = '\0';
 #endif