+ errno and return NULL.
+
+ When dynamic memory allocation occurs, the preallocated buffer is left
+ alone (with possibly modified contents). This makes it possible to use
+ a statically allocated or stack-allocated buffer, like this:
+
+ char buf[100];
+ size_t len = sizeof (buf);
+ char *output = vasnprintf (buf, &len, format, args);
+ if (output == NULL)
+ ... error handling ...;
+ else
+ {
+ ... use the output string ...;
+ if (output != buf)
+ free (output);
+ }
+ */