Problem when assigning a pointer to an enum variable in C

Question

I am getting a warning of "assigment from incompatible pointer type". I don't understand why this warning is happening. I don't know what else to declare "the_go_status" variable to other than an integer. (Note: this is not all the code, but just a simplified version I posted to illustrate the problem.)

The warning occurs on the last line of the example I included below.

//In a header file  
enum error_type  
{  
    ERR_1 = 0,  
    ERR_2 = 1,  
    ERR_3 = 2,  
    ERR_4 = 4,  
};  


//In a header file  
struct error_struct  
{  
   int value;  
   enum error_type *status;  
};  



//In a C file  
int the_go_status;  

the_go_status = ERR_1;  

//Have the error_struct "status" point to the address of "the_go_status"  
error_struct.status = &the_go_status;    //WARNING HERE!

score 3 · Answer 1 · answered Dec 20 '10 at 19:41

3

Because status is a pointer to enum error_type, and the_go_status is a pointer to an int. They are pointers to different types.

answered Dec 20 '10 at 19:41

Jonathan Wood

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This is true, but what he wants to do seems, and I can't believe I'm finally seeing this as the best phrase to describe it, awfully smelly. – San Jacinto Dec 20 '10 at 19:45
codeySmurt: Yes, you are correct. &the_go_status is a pointer to an int. – Jonathan Wood Dec 20 '10 at 19:54
to be more specific &the_go_status is the address to what the pointer, if it had been declared, points. – st-h Dec 21 '10 at 02:15
So, for int * pointer; pointer = &anotherPointer you may set a pointer to the address of the other pointer and therefore to that value. – st-h Dec 21 '10 at 02:20

score 3 · Answer 2 · edited May 23 '17 at 12:04

This is because enum error_type * isn't compatible with int *, because they point to values of different types (and possibly even different sizes). You should declare the_go_status as:

enum error_type the_go_status;

Although simply casting the pointer (i.e. (enum error_type *)&the_go_status) will make the warning go away, it may result in bugs on some platforms. See Is the sizeof(enum) == sizeof(int), always?

score 2 · Answer 3 · answered Dec 20 '10 at 19:41

I'm not sure if this is related exactly to your warning or not, but be very careful assigning references to local variables to pointers within structs. If the_go_status is a local, as soon as your function returns, the reference to that local will become invalid. So, if your code (or someone else's code) uses your instance of error_struct outside the function declaring the_go_status, things will quickly break.

score 1 · Answer 4 · answered Dec 20 '10 at 19:50

1

you should declare a pointer if you want to use a pointer:

int * the_go_status

otherwise you declare a primitive, which is not placed on the heap, but on the stack. (please correct my when being wrong on that)

However, I don't get why you want to use a pointer at all. Just do something like this in your struct definition:

enum error_type status;

and change your last line to:

error_struct.status = the_go_status;

answered Dec 20 '10 at 19:50

st-h

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Just declaring a pointer doesn't do any good, you have to point it at something. – Chris Stratton Dec 20 '10 at 21:13
1

true. As for any other kind of "variable", you have to set it once, so it has a value. However, as this is one of the most basic things of programming, I assumed this being obvious – st-h Dec 21 '10 at 02:13

score 1 · Answer 5 · answered Jun 15 '20 at 04:14

1

//This might be the simplest 

#include<stdio.h>
typedef enum {err_1=0,err_2=1,err_3=2,err_4=4}error; 
typedef struct
{
    int val;
    error* status;

}errval;

int main() {

    error the_go_status=err_1;  
    errval val1;//just a variable name for the struct
    val1.status=&the_go_status;
    printf("%d",val1.status);
}

answered Jun 15 '20 at 04:14

Nishad Sandilya

25
6

this will give compilation error while printf(), – mastermind.pk Sep 20 '22 at 10:50

score 0 · Answer 6 · answered Dec 20 '10 at 19:42

0

The "the_go_status" should be typed "enum error_type". You could typedef the enum

answered Dec 20 '10 at 19:42

Mikkel Lund

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4

score 0 · Answer 7 · answered Sep 27 '22 at 06:08

#include <iostream>
   
using namespace std;
enum error_type  
{  
    ERR_1 = 0,  
    ERR_2 = 4,  
    ERR_3 = 78,  
    ERR_4 = 9
};  


//In a header file  
struct error_struct  
{  
   int value;  
   enum error_type *status;  
};  


    int main() {
        enum error_type the_go_status;
        the_go_status = ERR_2;
        
      
        
        cout<<the_go_status<<endl;
         
        error_struct p1;
       
        
        p1.value = 6;
        p1.status = (enum error_type *)&the_go_status;

        
        cout<<(*p1.status)<<endl;
        p1.status++;
      
        cout<<(*p1.status)<<endl;
  

}

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Problem when assigning a pointer to an enum variable in C

7 Answers7