define a struct if a certain condition is met

Question

I want to typedef a struct in some way if the machine I am on is Little endian and in some other way if it is Big endian.I tried:

unsigned int i = 1; 
   char *c = (char*)&i; 
   if (*c)
   {
       /*Little endian"*/

       typedef struct
    {
    
        unsigned long A : A_BIT_SIZE;
        unsigned long R : R_BIT_SIZE;
        unsigned long E : E_BIT_SIZE;
        unsigned long funct : FUNCT_BIT_SIZE;
        unsigned long opsource : OPSOURCE_BIT_SIZE;
        unsigned long sourcetype : SOURCETYPE_BIT_SIZE;
        unsigned long opdest : OPDEST_BIT_SIZE;
        unsigned long desttype : DESTTYPE_BIT_SIZE;
        unsigned long opcode : OPCODE_BIT_SIZE;

    }command_byte;
   }   
   else
   {
          /*Big endian"*/
                 typedef struct
    {
        unsigned long opcode : OPCODE_BIT_SIZE;
        unsigned long desttype : DESTTYPE_BIT_SIZE;
        unsigned long opdest : OPDEST_BIT_SIZE;
        unsigned long sourcetype : SOURCETYPE_BIT_SIZE;
        unsigned long opsource : OPSOURCE_BIT_SIZE;
        unsigned long funct : FUNCT_BIT_SIZE;
        unsigned long E : E_BIT_SIZE;
        unsigned long R : R_BIT_SIZE;
        unsigned long A : A_BIT_SIZE;
    
    }command_byte;

    }

It compiles, but I am not sure whether this is the valid way to define structs inside an if statement. I am also not sure weather it will work when I actually use the struct. Will at work on header files too? what is the correct way to define a struct if condition is met?

Answer 1

Yes, you can use an if like that. But the typedef command_type can only be used within the innermost containing block that it was defined in.

Therefore there is no advantage over defining typedef ... command_type_big_endian and typedef ... command_type_little_endian, you cannot share the code either way.

You could use preprocessor macros to aid in avoiding repetition; then you could write the bulk of a serialization and deserialization macro only once and instantiate that for each command_type variation.