(Dynamic Output): Document the return value of asprintf. Also make the asprintf/snprintf examples a little better (check for some error returns).

manual/stdio.texi

@@ -1613,6 +1613,9 @@ make_message (char *name, char *value)
   int nchars;
 @end group
 @group
+  if (buffer == NULL)
+    return NULL;
+
  /* @r{Try to print in the allocated space.} */
   nchars = snprintf (buffer, size, "value of %s is %s",
                      name, value);
@@ -1624,6 +1627,7 @@ make_message (char *name, char *value)
          how much space is needed.} */
       buffer = (char *) xrealloc (buffer, nchars + 1);
 
+      if (buffer != NULL)
         /* @r{Try again.} */
         snprintf (buffer, size, "value of %s is %s",
                   name, value);
@@ -1659,6 +1663,10 @@ buffer you allocate in advance.  The @var{ptr} argument should be the
 address of a @code{char *} object, and @code{asprintf} stores a pointer
 to the newly allocated string at that location.
 
+The return value is the number of characters allocated for the buffer, or
+less than zero if an error occured. Usually this means that the buffer
+could not be allocated.
+
 Here is how to use @code{asprintf} to get the same result as the
 @code{snprintf} example, but more easily:
 
@@ -1669,7 +1677,8 @@ char *
 make_message (char *name, char *value)
 @{
   char *result;
-  asprintf (&result, "value of %s is %s", name, value);
+  if (asprintf (&result, "value of %s is %s", name, value) < 0)
+    return NULL;
   return result;
 @}
 @end smallexample