Is it possible to create 'empty' members within a struct in C?

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I'm tryin to create a few array-like structures for handeling MODBUS data & adressing.

I'm doing it within a structure so it's simpler later on to set certain values instead of referencing a datasheet to find out which address is coupled to the data.

Some addresses are left blank and are reserved for future use (I know this isn't specified in the MODBUS standard, the data shouldn't have to be exactly MODBUS specific).

I've created a struct which compiles on the software for the embedded device this should run on. However I wasn't quite sure it would run the way I imagined it, so I ran some test code on 2 different compiler. 1 being an online c-compiler which runs the way I excepected it to with just one warning at the reserved byte. The 2nd one is ran within a visual studio c++ console application, which won't run cause it generates the error:

Error C2208 'unsigned short': no members defined using this type

This is the complete code which should run:

#include <stdio.h>
#include <stdint.h>
#define U16 uint16_t
#define UI  unsigned int
struct t {
    U16 t1;
    union {
        struct {
            UI b_f1 : 2;
            UI b_f2 : 2;
            UI b_f3 : 2;
            UI b_f4 : 2;
            UI b_f5 : 2;
            UI b_f6 : 2;
            UI b_f7 : 2;
            UI b_f8 : 2;
        U16 c;
    U16; //Reserved space for later use <- this gives me the compiler error
    U16 t3;

int main()
    test.t3 = 12;
    test.t2.f.b_f1 = 0; //0
    test.t2.f.b_f2 = 2; //4 (8)
    test.t2.f.b_f3 = 0; //16
    test.t2.f.b_f4 = 0; //64
    test.t2.f.b_f5 = 0; //256
    test.t2.f.b_f6 = 0; //1024
    test.t2.f.b_f7 = 2; //4096 (8192)
    test.t2.f.b_f8 = 0; //16384
    //tot: 8200    
    test.t1 = 3000;
    int h; //test variable
    for (int i = 0; i < (sizeof(test) / sizeof(U16)); i++) {
        h = *(&test.t1 + (i*sizeof(U16));

        printf("%d", h); //here I can now send every MODBUS address & data

    return 0;

Setting the variables is not neccesarily needed, it's just for testing purposes

I excpected it will just generate a structure of 4 elements, one being a 0 pointer, that way the addressing is correct in a way that address 3 is not used and therefore 0 while address 4(t3) will contain more data.

Is there a way to make this work I planned to do, or should I just create some variables in between called Reserved1..Reserverd2..etc ?

A "declaration" like

U16; //Reserved space for later use

is simply not valid.

Just use some dummy name to solve your problem, like e.g.

U16 _reserved_; //Reserved space for later use

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No you can't. A type on its own is not a valid declaration within a structure definition.

What you could do is use __LINE__ to help guarantee that the name you assign to the variable is unique:

#define _concat(a,b) a##b
#define concat(a,b) _concat(a,b)

and write

U16 concat(t, __LINE__);

which is unique insofar that the struct name t is unique.

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Is there a way to make this work I planned to do?


or should I just create some variables in between called Reserved1..Reserverd2..etc


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  • Just what I was scared of, is it possible to use such a variable multiple times?
  • @user3005497 Then you have to use different names (e.g. _reserved_1_, _reserved_2_, etc.) . Or, if the data is consecutive, use a byte array instead.
  • A type followed by a semicolon is normally a valid declaration, but am not sure if it is valid inside a struct definition.
  • @machine_1: I'm not enough of a C expert to make that call, so I've cheated and amended the answer.
  • Indeed, in C11 it is valid yet gcc emits a warning but it is invalid in C++11.
  • If doing dirty, do it right, so it survives living in global scope, concat __FILE__ as well. ;)
  • Ah dxxm. A pity #define . _ does not work as well.