util/qemu-option: Document the get_opt_value() function

Coverity noticed commit 950c4e6 introduced a dereference before
null check in get_opt_value (CID1391003):

  In get_opt_value: All paths that lead to this null pointer
  comparison already dereference the pointer earlier (CWE-476)

We fixed this in commit 6e3ad3f, but relaxed the check in commit
0c2f6e7 because "No callers of get_opt_value() pass in a NULL
for the 'value' parameter".

Since this function is publicly exposed, it risks new users to do
the same error again. Avoid that documenting the 'value' argument
must not be NULL.

Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Laurent Vivier <[email protected]>

Author: philmd

include/qemu/option.h

@@ -28,6 +28,19 @@
 
 #include "qemu/queue.h"
 
+/**
+ * get_opt_value
+ * @p: a pointer to the option name, delimited by commas
+ * @value: a non-NULL pointer that will received the delimited options
+ *
+ * The @value char pointer will be allocated and filled with
+ * the delimited options.
+ *
+ * Returns the position of the comma delimiter/zero byte after the
+ * option name in @p.
+ * The memory pointer in @value must be released with a call to g_free()
+ * when no longer required.
+ */
 const char *get_opt_value(const char *p, char **value);
 
 void parse_option_size(const char *name, const char *value,