xstring.h: add xappendstr

Introduce a macro for handling common case of partial writes
to a character array.

* xstring.h (get_pos_diff_): New function.
(xappendstr): New macro.

diff --git a/xstring.h b/xstring.h
index 11f9f009fd76f0ce11fb02b9d90f0742805ba706..823fb1b17d9ed74e3b32f262a1199e25359bd2bd 100644 (file)
--- a/xstring.h
+++ b/xstring.h
@@ -64,4 +64,43 @@ xsnprintf_(char *str, size_t size, const char *func, const char *argstr,
        xsnprintf((str_), sizeof(str_) + MUST_BE_ARRAY(str_), (fmt_), \
                  __VA_ARGS__)
 
+static inline size_t
+get_pos_diff_(char *str, size_t size, char *pos, const char *func,
+          const char *call)
+{
+       if ((str + size) < str)
+               error_msg_and_die("%s: string size overflow (%p+%zu) in %s",
+                                 func, str, size, call);
+
+       if (pos > (str + size))
+               error_msg_and_die("%s: got position (%p) beyond string "
+                                 "(%p+%zu) in %s",
+                                 func, pos, str, size, call);
+
+       if (pos < str)
+               error_msg_and_die("%s: got position %p before string %p in %s",
+                                 func, pos, str, call);
+
+       return pos - str;
+}
+
+/**
+ * Helper function for constructing string in a character array by appending
+ * new formatted parts.  Returns new position.  In aligment with the rest of x*
+ * functions, fails on error or buffer overflow.  Obtains buffer size via
+ * sizeof(str_).
+ *
+ * @param str_  Character array buffer to print into.
+ * @param pos_  Current position.
+ * @param fmt_  Format string.
+ * @param ...   Format arguments.
+ * @return      New position.
+ */
+#define xappendstr(str_, pos_, fmt_, ...) \
+       (xsnprintf((pos_), sizeof(str_) + MUST_BE_ARRAY(str_) - \
+                  get_pos_diff_((str_), sizeof(str_), (pos_), __func__, \
+                                "xappendstr(" #str_ ", " #pos_ ", " #fmt_ ", " \
+                                #__VA_ARGS__ ")"), \
+                  (fmt_), ##__VA_ARGS__) + (pos_))
+
 #endif /* !STRACE_XSTRING_H */