Write to struct

[john7]

I have a struct:

typedef struct command
{
    char *name;  //command name
    char mode;   //0-read, 1- read/write 
    int minval;  
    int maxval;  
    void (*fp) (int com_num);  //pointer to function
    void *vp;   //pointer to variable
}command;

command commands[] = {
    {"imax1", 1, 0, 10000, GetSetImax, &max_current1},
    {"imax2", 1, 0, 10000, GetSetImax, &max_current2},
};

I can read from variable:

UsartSendInt( (int)commands[com_num].vp );

But can not write to:

(int *)(commands[com_num].vp) =  10;

What I'm doing wrong?

[grumpy]

Your second example is trying to set a pointer to have the value 10. Language rules don't generally allow setting pointers to have numeric values. That's why the compiler is forbidding the assignment.

As to how to fix it ..... that depends on what you're really trying to achieve.

I assume max_current1 and max_current2 are of type int, and you want your assignment to change those variables to have the value 10. In that case, you need to dereference the pointer on the left hand side of the assignment

   *((int *)(commands[com_num].vp)) = 10;

This will compile, but only have the intended effect if commands[com_num].vp points at an actual int. If commands[com_num].vp doesn't point at a valid int, the assignment will have undefined behaviour.


Note that the first example is interpreting the value of a pointer as if it is an int. It is fairly unusual for that to have an intended effect - even though it does compile, since the (int) type conversion does bludgeon the compiler into submission.

[john7]

Thank you very much.