Yhyjyk

I'm trying to implement a struct person and I need to hide some fields or make them constant.
A trick for create private fields.

Header:

#pragma once

#define NAME_MAX_LEN 20

typedef struct _person 
 float wage;
 int groupid;
 Person;

const char const *getName (Person *p);
int getId (Person *p);

/// OTHER FUNCTIONS

Source

#include "person.h"


struct _person

 int id;

 float wage;
 int groupid;

 char name[NAME_MAX_LEN];
;

/// FUNCTIONS

GCC says that person.c:7:8: error: redefinition a 'struct _person' struct _person

I can write this in a header, but after it, I can't use fields of a struct.

typedef struct _person Person;

A struct cannot have multiple conflicting definitions. As such, you can't create a struct that hides some of the fields.

What you can do however it declare that the struct exists in the header without defining it. Then the caller is restricted to using only a pointer to the struct and using functions in your implementation to modify it.

For example, you could define your header as follows:

typedef struct _person Person;

Person *init(const char *name, int id, float wage, int groupid);

const char *getName (const Person *p);
int getId (const Person *p);
float getWage (const Person *p);
int getGroupid (const Person *p);

And your implementation would contain:

#include "person.h"

struct _person

 int id;

 float wage;
 int groupid;

 char name[NAME_MAX_LEN];
;

Person *init(const char *name, int id, float wage, int groupid)

 Person *p = malloc(sizeof *p);
 strcpy(p->name, name);
 p->id = id;
 p->wage= wage;
 p->groupid= groupid;
 return p;


...

C has no mechanism for hiding individual members of a structure type. However, by operating only in terms of pointers to such a type, and not providing a definition, you can make the whole type opaque. Users would then have to use the functions you provide to manipulate instances in any way. This is a thing that is sometimes done.

To some extent, you may be able to achieve something like what you describe with a hidden context. For example, consider this:

header.h

typedef struct _person 
 float wage;
 int groupid;
 Person;

implementation.c

struct _person_real 
 Person person; // must be first, and is a structure, not a pointer.
 int id;
 char name[NAME_MAX_LEN];
;

Now you can do this:

Person *create_person(char name[]) 
 struct _person_real *pr = malloc(sizeof(*pr));

 if (pr) 
 pr->person.wage = DEFAULT_WAGE;
 pr->person.groupid = DEFAULT_GROUPID;
 pr->id = generate_id();
 strncpy(pr->name, name, sizeof(pr->name));
 pr->name[sizeof(pr->name) - 1] = '';

 return &pr->person; // <-- NOTE WELL
 else 
 return NULL;
 

A pointer to the first member of a structure always points also to the whole structure, too, so if the client passes a pointer obtained from that function back to you, you can

struct _person_real *pr = (struct _person_real *) Person_pointer;

and work on the members from the larger context.

Be well aware, however, that such a scheme is risky. Nothing prevents a user from creating a Person without the larger context, and passing a pointer to it to a function that expects the context object to be present. There are other issues.

Overall, C APIs generally either take the opaque structure approach or just carefully document what clients are permitted to do with the data they have access to, or even just document how everything works, so that users can make their own choices. These, especially the latter, are well aligned with overall C approaches and idioms -- C does not hold your hand, or protect you from doing harm. It trusts you to know what you're doing, and to do only what you intend to do.

You can use a mixin style; e.g. write in the header:

struct person 
 float wage;
 int groupid;
;

struct person *person_new(void);
char const *getName (struct person const *p);
int getId (struct person const *p);

and in the source

struct person_impl 
 struct person p;
 char name[NAME_MAX_LEN];
 int id;


struct person *person_new(void)

 struct person_impl *p;

 p = malloc(sizeof *p);
 ...
 return &p->p;


chra const *getName(struct person const *p_)

 struct person_impl *p =
 container_of(p_, struct person_impl, p);

 return p->name;

See e.g. https://en.wikipedia.org/wiki/Offsetof for details of container_of().

What John Bollinger wrote is a neat way of utilising how structs and memory works, but it's also an easy way to get a segfault (imagine allocating an array of Person and then later passing the last element to a 'method' which accesses the id or it's name), or corrupt your data (in an array of Person the next Person is overwriting 'private' variables of the previous Person). You'd have to remember that you must create an array of pointers to Person instead of array of Person (sounds pretty obvious until you decide to optimise something and think that you can allocate and implement the struct more efficiently than the initialiser).

Don't get me wrong, it's a great way to solve the problem, but you've got to be careful when using it.
What I'd suggest (though using 4/8 bytes more memory per Person) is to create a struct Person which has a pointer to another struct which is only defined in the .c file and holds the private data. That way it'd be harder to make a mistake somewhere (and if it's a bigger project then trust me - you'll do it sooner or later).

.h file:

#pragma once

#define NAME_MAX_LEN 20

typedef struct _person 
 float wage;
 int groupid;

 __personPriv *const priv;
 Person;

void personInit(Person *p, const char *name);
Person* personNew(const char *name);

const char const *getName (Person *p);
int getId (Person *p);

.c file:

typedef struct 
 int id;
 char name[NAME_MAX_LEN];
 __personPriv;

const char const *getName (Person *p) 
 return p->priv->name;


int getId (Person *p) 
 return p->priv->id;


__personPriv* __personPrivNew(const char *name) 
 __personPriv *ret = memcpy(
 malloc(sizeof(*ret->priv)),
 &(__personPriv) 
 .id = generateId();
 ,
 sizeof(*ret->priv)
 );

 // if(strlen(name) >= NAME_MAX_LEN) 
 // raise an error or something?
 // return NULL;
 // 

 strncpy(ret->name, name, strlen(name));

 return ret;


void personInit(Person *p, const char *name) 
 if(p == NULL)
 return;

 p->priv = memcpy(
 malloc(sizeof(*p->priv)),
 &(__personPriv) 
 .id = generateId();
 ,
 sizeof(*p->priv)
 );

 ret->priv = __personPrivNew(name);
 if(ret->priv == NULL) 
 // raise an error or something
 


Person* personNew(const char *name) 
 Person *ret = malloc(sizeof(*ret));

 ret->priv = __personPrivNew(name);
 if(ret->priv == NULL) 
 free(ret);
 return NULL;
 
 return ret;

Side note: this version can be implemented so that private block is allocated right after/before the 'public' part of the struct to improve locality. Just allocate sizeof(Person) + sizeof(__personPriv) and initialise one part as Person and second one as __personPriv.