“useless type qualifier” error

Question

I get the following error with the following code. I tried to figure out where the problem is over Google, but I didn't find anything helpful.

Compiling /home/tectu/projects/resources/chibios/ext/lcd/touchpad.c
In file included from /home/tectu/projects/resources/chibios/ext/lcd/touchpad.c:1:0:
/home/tectu/projects/resources/chibios/ext/lcd/touchpad.h:17:1: warning: useless type qualifier in empty declaration [enabled by default]

Here's the the code from line 12 to line 17 from touchpad.h :

volatile struct cal {
    float xm; 
    float ym; 
    float xn; 
    float yn; 
};

And here's how I use this struct inside touchpad.c :

static struct cal cal = { 
    1, 1, 0, 0  
};

Can anyone show me the light? :D

Answer 1

volatile as a qualifier can be applied to a particular instance of structure.
You are applying it to a type which is useless and the compiler correctly points it out.

Answer 2

volatile qualifies a variable, not a type.

Doing:

static volatile struct cal {
    float xm; 
    float ym; 
    float xn; 
    float yn; 
} cal;

would be legal, as would:

struct cal {
    float xm; 
    float ym; 
    float xn; 
    float yn; 
};

static volatile struct cal cal;

Answer 3

The volatile keyword makes sense with an object. Not a type definition.

Answer 4

You don't get an error, just a warning.

And that applies to how you declare your struct cal : it is not volatile by itself; the volatile only applies to a concrete variable definition.

So in static struct cal cal , your variable cal is just static , but not volatile .

In that sense, the volatile declaration is, as the warning says, useless.

Answer 5

易失性关键工作应该与实际变量一起使用，而不是类型定义。

Answer 6

You can't attach a volatile qualifier to a struct declaration.

But, contrary to the other answers, you can in fact use the volatile qualifier on types, but not on structs . If you use typedef , you can create a volatile type for a struct.

In the following example, Vol_struct is actually not volatile, as in the poster's question. But Vol_type will create volatile variables without further qualification:

/* -------------------------------------------------------------------- */
/* Declare some structs/types                                           */
/* -------------------------------------------------------------------- */

/* wrong: can't apply volatile qualifier to a struct
   gcc emits warning: useless type qualifier in empty declaration */
volatile struct Vol_struct
{
    int x;
};

/* effectively the same as Vol_struct */
struct Plain_struct
{
    int x;
};

/* you CAN apply volatile qualifier to a type using typedef */
typedef volatile struct
{
    int x;
} Vol_type;


/* -------------------------------------------------------------------- */
/* Declare some variables using the above types                         */
/* -------------------------------------------------------------------- */
struct Vol_struct g_vol_struct;                     /* NOT volatile */
struct Plain_struct g_plain_struct;                 /* not volatile */
volatile struct Plain_struct g_vol_plain_struct;    /* volatile */
Vol_type g_vol_type;                                /* volatile */