Dereferencing type-punned pointer will break strict-aliasing rules while compiling cannot be fixed to compile C code

Question

I try to convert C Code from Standard 11 to 14. I work with gcc 7.5.0 in Ubuntu 20.04. I have the following lines of code

json.h:

extern const struct _json_value json_value_none;

typedef struct _json_object_entry
{
    json_char * name;
    unsigned int name_length;
    
    struct _json_value * value;
    
} json_object_entry;

typedef struct _json_value
{
   struct _json_value * parent;
   json_type type;
   union
   {
      int boolean;
      json_int_t integer;
      double dbl;
      struct
      {
         unsigned int length;
         json_object_entry * values;
      } object;
   } u;

   union
   {
      struct _json_value * next_alloc;
      void * object_mem;

   } _reserved;
} json_value;

json.c

static int new_value (json_state * state,
                      json_value ** top, json_value ** root, json_value ** alloc,
                      json_type type)
{
   json_value * value;
   int values_size;

   if (!state->first_pass)
   {
      value = *top = *alloc;
      *alloc = (*alloc)->_reserved.next_alloc;

      if (!*root)
         *root = value;

      switch (value->type)
      {
         case json_array:

            ...
            break;

         case json_object:

            if (value->u.object.length == 0)
               break;

            values_size = sizeof (*value->u.object.values) * value->u.object.length;

            if (! (value->u.object.values = (json_object_entry *) json_alloc
                  (state, values_size + ((unsigned long) value->u.object.values), 0)) )
            {
               return 0;
            }

            value->_reserved.object_mem = (*(char **) &value->u.object.values) + values_size;

            ...
            break;
      
      };

      return 1;
   }
   
   ...
   return 1;
}

When I compile, I got dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing] for the following line

value->_reserved.object_mem = (*(char **) &value->u.object.values) + values_size

I googled and found this link who means that the problem is probably because object_mem is defined with a void *type, who is an undefined type. I tried to substitute it by int * type, without success. I'm stucked.

Any helps are welcome.

score 0 · Answer 1 · answered Jun 11 '22 at 20:08

0

Instead of casting the address of the pointer, you should convert its value and write:

value->_reserved.object_mem = (char *)value->u.object.values + values_size;

answered Jun 11 '22 at 20:08

chqrlie

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It works ! But I don't understand really what is the meaning ot fhe line of code that I gave in my question. It seem that it was too complicated code and then, it can be siimplified? – Sylvain Racine Jun 11 '22 at 20:22
Sorry, but I cannot upvote. – Sylvain Racine Jun 11 '22 at 20:23
@SylvainRacine: you can accept the answer by clicking on the grey checkmark below its score. You will be able to upvote once you get enough reputation. – chqrlie Jun 11 '22 at 20:52
I clicked on the grey checkmark. But I don't have enough reputation... Sorry – Sylvain Racine Jun 11 '22 at 21:44

Dereferencing type-punned pointer will break strict-aliasing rules while compiling cannot be fixed to compile C code

1 Answers1