typedef struct as pointer?

Question

I have a question in regards to typedef struct's that I believe I know the answer to but would like some clarification.

Lets say I have:

typedef struct Scanner{
    //information
}myScanner;

Would that have the same meaning as:

typedef struct Scanner *myScanner; 
struct Scanner {
    //information
};

That is not the same. The second is a pointer. typedefing pointers is discouraged! — Paul Ogilvie, Oct 24 '18 at 13:15
Note [Is it a good idea to typedef pointers](https://stackoverflow.com/questions/750178/is-it-a-good-idea-to-typedef-pointers) — TL;DR "No", except perhaps function pointers. — Jonathan Leffler, Oct 24 '18 at 13:52

score 0 · Answer 1 · answered Oct 24 '18 at 13:26

It will be easy to see the difference if you use the same format for both typedef definitions:

typedef struct Scanner myScanner;

versus

typedef struct Scanner *myScanner;

Defining a type-alias using typedef is very similar do declaring variables. For typedef the asterisk means "define as pointer", just like it does for variables.

score 0 · Answer 2 · answered Oct 24 '18 at 13:32

The type definitions are usually in a header file where no vars are created. In your main(), you are supposed to use this type to create vars. In our example, the newly minted type is 'Scanner'.

#include <stdio.h>
#include <stdlib.h>

struct _scanner
{       // example
    int a;
    double b;
    char c;
};
typedef struct _scanner Scanner;


int main()
{
    Scanner s;
    s.a = 10;
    s.b = 3.14;
    s.c = 'N';
    // -------------------
    // If you want to have a
    //  pointer p to scanner, then

    Scanner* p = &s;

    p->a = 20;
    p->b = 2.72;
    p->c = 'Y';

    printf("%d, %.2lf, %c\n",
           s.a, s.b, s.c);

    return EXIT_SUCCESS;
}

Output:

20, 2.72, Y

Once the variable is created, you can make a pointer to it, as a pointer must be able to point to something. The output shows that values have been successfully manipulated.

Note that you should not, in general, create function, variable, tag or macro names that start with an underscore. Part of [C11 §7.1.3 Reserved identifiers](https://port70.net/~nsz/c/c11/n1570.html#7.1.3) says: — _All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use._ — _All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces._ See also [What does double underscore (`__const`) mean in C?](https://stackoverflow.com/a/1449301) — Jonathan Leffler, Oct 24 '18 at 13:54

score 0 · Answer 3 · edited Oct 24 '18 at 13:58

typedef struct Scanner *myScanner; — here you are declaring type pointer to a struct Scanner.

typedef struct Scanner myScanner; — is a declaration of a new type.

What this means is, in the first case, you define a pointer to struct Scanner, for arithmetic calculations etc, the type of element that is pointed to. The size of myScanner will equal size of void* (pointer size on your os).

For the second case, you are defining a new type of element; it's size is
sizeof(struct Scanner) == size of(myScanner).

typedef struct as pointer?

3 Answers3