Thread: error with Array of structure

I am getting "initializer element is not constant" while compiling following program.

In my source I want to keep list of all objects. I have created same scenario in the following program. Can you please help me to understand what went wrong with the following program.

Code:

struct ex_attribute {
        void (*fn1)(int);
        void (*fn2)(int, int);
};


void one(int a) {  return; }
void two(int a,int b) { return; }
void three(int a) { return; }
void four(int a, int b) { return; }


int main()
{
        struct ex_attribute one1  = {
                .fn1 = one,
                .fn2 = two,
        };


        struct ex_attribute two2 = {
                .fn1 = three,
                .fn2 = four,
        };


        static struct ex_attribute total_attrs[] = {
                one1,
                two2,
        };


        return 0;
}

I also tried with array of structure pointers also as shown below but still I am getting the same error while compiling.

Code:

        static struct ex_attribute *total_attrs[] = {
                &one1,
                &two2,
        };

Can you please tell what is wrong with my program.

Originally Posted by vart

one1 and two2 are not defined

one1 and two2 are of type "struct ex_attribute" and I have initialized these structure variables.

Code:

      struct ex_attribute one1  = {
                .fn1 = one,
                .fn2 = two,
        };


        struct ex_attribute two2 = {
                .fn1 = three,
                .fn2 = four,
        };

yes it is working. If I remove static keyword also for total_attrs then it is not giving any error.
I think static keyword is related to lifetime and visibility, I am not getting clearly why static keyword is creating issue here.
Any more properties for static keyword in C.

Thanks for the information.

Is function names in C are constants?


I have defined a structure having function pointers, Created a static object for that and initialized function pointers. This program compiled without any issues.

Code:

struct attribute {
        const char              *name;
};


struct one_attribute {
        struct attribute atr;
        int (*f1)();
        int (*f2)();
};


int ex_f1()
{   return 0;  }


int ex_f2()
{   return 0;  }


static struct one_attribute ex_attr_cnt = {
        .atr = { .name = "cont"},
        .f1 = ex_f1,
        .f2 = ex_f2,
};


static struct attribute *names[] = {
        &ex_attr_cnt.atr,
};


int main()
{ return 0; }

Originally Posted by laserlight

Yes, the static keyword here specifies that total_attrs has static storage duration. If by "visibility" you mean "scope", then no, the static keyword does not affect scope: despite having a lifetime that is the entire execution of the program, total_attrs is a local variable because it was declared in the scope of the main function. As for why the static keyword is an "issue":

As per my understanding scope means a region in code where a variable can be accessed. If we declare a global variable as static then its scope is limited to that file. so using global variable as static will affect the variable scope to that file or translation unit.

I have initialized a static structure with function name as shown below. I think we can initialize a static structure with constants.
function names are constants in C?

Code:

struct fp {
        int (*fn)();
};

int f1()
{
        printf("f1 called \n");
        return 0;
}

static struct fp fps = {
        .fn = f1,
};

int main()
{
        fps.fn();
        return 0;
}

When I initialized structure as shown below then also it is working fine.

Code:

static struct fp fps = {
        .fn = &f1,
};

Is "f1" and "&f1" is same in C.

C compiler will traat function names as string constants?

Originally Posted by laserlight

No, function names are not string constants. Rather, a function (or more precisely, function designator) is converted to a pointer to the function.

If function name is not conatant then following program should throw error, because initializers should be constants for static objects. But it is compiling successfully.

Code:

struct ex {
        void (*fp)();
};
void f1()
{
        return ;
}
static struct ex temp = {
        .fp = f1,
};
int main()
{
        return 0;
}