Fix incorrect buffer size passed to function

The function expects a widechar count rather than a byte count.
This is a serious error that would lead to a buffer overflow
if not for the fact that the input char buffer passed to the
function uses the same size constant as the output wchar buffer

src/line_info.c

@@ -343,7 +343,10 @@ static int print_wrap(WINDOW *win, struct line_info *line, int max_x, int max_y)
     return 0;
 }
 
-/* Converts `msg` into a widechar string and puts the result in `buf`.
+/* Converts the multibyte string `msg` into a wide character string and puts
+ * the result in `buf`.
+ *
+ * `buf_size` is the number of wide characters that `buf` can hold.
  *
  * Returns the widechar width of the string.
  */
@@ -353,13 +356,13 @@ static uint16_t line_info_add_msg(wchar_t *buf, size_t buf_size, const char *msg
         return 0;
     }
 
-    const wint_t wc_msg_len = mbs_to_wcs_buf(buf, msg, buf_size);
+    const int wc_msg_len = mbs_to_wcs_buf(buf, msg, buf_size);
 
     if (wc_msg_len > 0 && wc_msg_len < buf_size) {
         buf[wc_msg_len] = L'\0';
         int width = wcswidth(buf, wc_msg_len);
 
-        if (width == -1) {  // the best we can do on failure is to fall back to strlen
+        if (width < 0 || width > UINT16_MAX) {  // the best we can do on failure is to fall back to strlen
             width = strlen(msg);
         }
 
@@ -473,7 +476,7 @@ int line_info_add(ToxWindow *self, bool show_timestamp, const char *name1, const
             break;
     }
 
-    const uint16_t msg_width = line_info_add_msg(new_line->msg, sizeof(new_line->msg), frmt_msg);
+    const uint16_t msg_width = line_info_add_msg(new_line->msg, sizeof(new_line->msg) / sizeof(wchar_t), frmt_msg);
     len += msg_width;
 
     if (show_timestamp) {
@@ -853,7 +856,7 @@ void line_info_set(ToxWindow *self, uint32_t id, char *msg)
 
     while (line) {
         if (line->id == id) {
-            const uint16_t new_width = line_info_add_msg(line->msg, sizeof(line->msg), msg);
+            const uint16_t new_width = line_info_add_msg(line->msg, sizeof(line->msg) / sizeof(wchar_t), msg);
             line->len = line->len - line->msg_width + new_width;
             line->msg_width = new_width;
             return;

src/misc_tools.c

@@ -267,7 +267,6 @@ int wstring_is_empty(const wchar_t *string)
     return string[0] == L'\0';
 }
 
-/* convert a multibyte string to a wide character string and puts in buf. */
 int mbs_to_wcs_buf(wchar_t *buf, const char *string, size_t n)
 {
     size_t len = mbstowcs(NULL, string, 0) + 1;
@@ -283,7 +282,6 @@ int mbs_to_wcs_buf(wchar_t *buf, const char *string, size_t n)
     return len;
 }
 
-/* converts wide character string into a multibyte string and puts in buf. */
 int wcs_to_mbs_buf(char *buf, const wchar_t *string, size_t n)
 {
     size_t len = wcstombs(NULL, string, 0) + 1;

src/misc_tools.h

@@ -115,12 +115,24 @@ int wstring_is_empty(const wchar_t *string);
 /* converts a multibyte string to a wide character string (must provide buffer) */
 int char_to_wcs_buf(wchar_t *buf, const char *string, size_t n);
 
-/* converts wide character string into a multibyte string and puts in buf. */
-int wcs_to_mbs_buf(char *buf, const wchar_t *string, size_t n);
-
-/* converts a multibyte string to a wide character string and puts in buf) */
+/* Converts a multibyte string to a wide character string and puts in `buf`.
+ *
+ * `buf` must have room for at least `n` wide characters (wchar_t's).
+ *
+ * Return number of wide characters written on success.
+ * Return -1 on failure.
+ */
 int mbs_to_wcs_buf(wchar_t *buf, const char *string, size_t n);
 
+/* Converts a wide character string into a multibyte string and puts in `buf`.
+ *
+ * `buf` must have room for at least `n` bytes.
+ *
+ * Return number of bytes written on success.
+ * Return -1 on failure.
+ */
+int wcs_to_mbs_buf(char *buf, const wchar_t *string, size_t n);
+
 /* Returns 1 if connection has timed out, 0 otherwise */
 int timed_out(time_t timestamp, time_t timeout);