util: add util_vasprintf() for Windows (v2)

We don't have vasprintf() on Windows so we need to implement it ourselves.

v2: compute actual length of output string, per Nicolai Hähnle.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>

src/util/u_string.h

@@ -110,6 +110,27 @@ util_sprintf(char *str, const char *format, ...)
    va_end(ap);
 }
 
+static inline int
+util_vasprintf(char **ret, const char *format, va_list ap)
+{
+   va_list ap_copy;
+
+   /* Compute length of output string first */
+   va_copy(ap_copy, ap);
+   int r = util_vsnprintf(NULL, 0, format, ap);
+   va_end(ap_copy);
+
+   if (r < 0)
+      return -1;
+
+   *ret = (char *) malloc(r + 1);
+   if (!ret)
+      return -1;
+
+   /* Print to buffer */
+   return util_vsnprintf(*ret, r + 1, format, ap);
+}
+
 static inline char *
 util_strchr(const char *s, char c)
 {
@@ -186,6 +207,7 @@ util_strstr(const char *haystack, const char *needle)
 #define util_vsnprintf vsnprintf
 #define util_snprintf snprintf
 #define util_vsprintf vsprintf
+#define util_vasprintf vasprintf
 #define util_sprintf sprintf
 #define util_strchr strchr
 #define util_strcmp strcmp