Error invalid type argument of unary have int

I have a C Program:

#include <stdio.h>
int main(){
  int b = 10;             //assign the integer 10 to variable 'b'

  int *a;                 //declare a pointer to an integer 'a'

  a=(int *)&b;            //Get the memory location of variable 'b' cast it
                          //to an int pointer and assign it to pointer 'a'

  int *c;                 //declare a pointer to an integer 'c'

  c=(int *)&a;            //Get the memory location of variable 'a' which is
                          //a pointer to 'b'.  Cast that to an int pointer 
                          //and assign it to pointer 'c'.

  printf("%d",(**c));     //ERROR HAPPENS HERE.  

  return 0;
}    

Compiler produces an error:

error: invalid type argument of ‘unary *’ (have ‘int’)

Can someone explain what this error means?

Barebones C program to produce the above error:

#include <iostream>
using namespace std;
int main(){
    char *p;
    *p = 'c';

    cout << *p[0];  
    //error: invalid type argument of `unary *'
    //peeking too deeply into p, that's a paddlin.

    cout << **p;    
    //error: invalid type argument of `unary *'
    //peeking too deeply into p, you better believe that's a paddlin.
}

ELI5:

The master puts a shiny round stone inside a small box and gives it to a student. The master says: «Open the box and remove the stone». The student does so.

Then the master says: «Now open the stone and remove the stone». The student said: «I can’t open a stone».

The student was then enlightened.

answered Sep 30, 2013 at 19:18

asked Dec 18, 2013 at 11:27

You should write:

struct test_t* test;

Or use typedef if you want to avoid writing struct every time you declare a variable of that type:

typedef struct test_t {
    int var1[5];
    int var2[10];
    int var3[15];
} test_t;

test_t* test;

Side note: In C++ the struct name is placed in the regular namespace, therefore there is no need to write struct before declaring a variable of that type.

answered Dec 18, 2013 at 11:27

When you declare a structure variable, struct keyword should be there like

struct test_t* test;

If you don’t want to use struct keyword every time you declare a variable, simply use typedef.

answered Dec 18, 2013 at 11:36

Sep 18 ’10
#1

thanks a lot for the reply
i have a doubt
in the above program in the statement
int *a=5
does that symbolizes implicitly that a value is holding the memory location of 5?
i dint understand what is invalid about that……as am a newbie in c programming pointers concept
please help

The problem is that the pointer a does not point anywhere valid.

You say it compiles perfectly, do you get no compiler warnings? If not you probably don’t have you compiler warning level set high enough.

The reason I say this is that on line 4 you assign an int to an int *. This effectively makes a point at memory location 5, almost certainly not a valid location on your system.

p points to a

So when you try to print

p is valid, it is the address of a
*p is valid it is a, the address of an int
**p is not valid it is the thing that a points to, that is memory location 5, not the address of an allocate int and you get a segmentation fault (a memory address fault).

You can not take the address of a numerical constant like 5.

thanks a lot for the reply
i have a doubt
in the above program in the statement
int *a=5
does that symbolizes implicitly that a value is holding the memory location of 5?
i dint understand what is invalid about that……as am a newbie in c programming pointers concept
please help

int *a=5;

does explicitly set the value of a to (the memory location) 5. There is nothing wrong in that apart from being a little strange.

The problem comes when you try to dereference a with *a, you try to access the memory location contained in a, in this case 5 and that is not a valid memory location, or a least one you do not have access rights to.

Your code has a point to whatever int is stored at address 0x0005. The segmentation fault occurs because the operating system has not granted your program permission to access this address. In addition, you might get a run-time alignment fault if your program runs on one of the many processors that don’t allow ints to be at odd addresses.

Do you intend for a to point to an int whose value is 5? If so, then you want something like this: