Good Coding Practice With C structs?

Question

In C, you can define structures to hold an assortment of variables;

typedef struct {
    float       sp;
    float       K;                 // interactive form - for display only
    float       Ti;                //  values are based in seconds
    float       Td;
} pid_data_t;

But lets say that K, Ti, and Td should never be set publicly, and should only be used for storing the values after they have been manipulated. So, I want these values not to be updated by;

pid_data_t = pid_data;
pid_data.K = 10;         // no good! changing K should be done via a function

I want them to be set via a function;

int8_t pid_set_pid_params(float new_K_dash, float new_Ti_dash, 
    float new_Td_dash)
{
    …                             // perform lots of things
    pid_data->K  = new_K_dash;
    pid_data->Ti = new_Ti_dash;
    pid_data->Td = new_Td_dash;
}

Any thoughts on this? I know C++ uses like a get/set property, but was wondering what people might do on C.

Answer 1

The canonical way to do this is by using a combination of opaque pointers and public structs, along with allocators, getters and setters for the private elements. About along these lines:

foo.h

typedef struct Foo {
    /* public elements */
} Foo;

Foo *new_Foo(void);
void Foo_something_opaque(Foo* foo);

foo.c

#include "foo.h"

typedef struct Private_Foo_ {
    struct Foo foo;
    /* private elements */
} Private_Foo_;

Foo *new_Foo(void)
{
    Private_Foo_ *foo = malloc(sizeof(Private_Foo_));
    /* initialize private and public elements */
    return (Foo*) foo;
}

void Foo_something_opaque(Foo *foo)
{
    Private_Foo_ *priv_foo = (Private_Foo_*) foo;
    /* do something */
}

This woks, because C guarantees, that the address of a struct variable always is equal to the address of the very first struct element. We can use this to have a Private_Foo_ struct, containing a public Foo at the beginning, giving out pointers to the whole thing, with the compilation units not having access to the Private_Foo_ struct defintion just seeing some memory without any context.

It should be noted that C++ works quite similar behind the curtains.

Update

As KereekSB pointed out, this will break if used in a array.

I say: Then don't make Foo f[], however tempting, but make an arrays of pointers to Foo: Foo *f[].

If one really insists on using it in arrays do the following:

foo_private.h

typedef struct Private_Foo_ {
    /* private elements */
} Private_Foo_;

static size_t Private_Foo_sizeof(void) { return sizeof(Private_Foo_); }

foo_private.h is written in a way, that it can be compiled into an object file. Use some helper program to link it and use the result of Private_Foo_sizeof() to generate the actual, plattform dependent foo.h from some foo.h.in file.

foo.h

#include

#define FOO_SIZEOF_PRIVATE_ELEMENTS <generated by preconfigure step>

typedef struct Foo_ {
    /* public elements */
    char reserved[FOO_SIZEOF_PRIVATE_ELEMENTS];
} Foo;

Foo *new_Foo(void);
void Foo_something_opaque(Foo* foo);

foo.c

#include "foo.h"
#include "foo_private.h"

Foo *new_Foo(void)
{
    Foo *foo = malloc(sizeof(Foo));
    /* initialize private and public elements */
    return (Foo*) foo;
}

void Foo_something_opaque(Foo *foo)
{
    Private_Foo_ *priv_foo = (Private_Foo_*) foo.reserved;
    /* do something */
}

IMHO this is really messy. Now I'm a fan of smart containers (unfortunately there's no standard container library for C). Anyway: In such a container is creates through a function like

Array *array_alloc(size_t sizeofElement, unsigned int elements);
void *array_at(Array *array, unsigned int index);
/* and all the other functions expected of arrays */

See the libowfaw for an example of such an implementation. Now for the type Foo it was trivial to provide a function

Array *Foo_array(unsigned int count);

Answer 2

Object orientation is a way of thinking and modelling, data encapsulation where struct data should not be modified directly by the user can be implemented this way:

my_library.h

#ifndef __MY_LIBRARY__
#define __MY_LIBRARY__
typedef void MiObject;

MyObject* newMyObject();

void destroyMyObject(MyObject*)

int setMyObjectProperty1(MyObject*,someDataType1*);

/*Return a pointer to the data/object,  classic pass by value */
someDataType1* getMyObjectProperty2Style1(MyObject*);

int setMyObjectProperty2(MyObject*,someDataType2*);

/* The data/object is passed through reference */
int getMyObjectProperty2Style2(MyObject*,someDataType2**);

    /* Some more functions here */
#endif

my_library.c

struct _MyHiddenDataType{
    int a;
    char* b;
    ..
    ..
};

MyObject* newMyObject(){
struct _MyHiddenData* newData = (struct _MyHiddenData*)malloc(sizeof(struct _MyHiddenData);
//check null, etc
//initialize data, etc
return (MyObject*)newData;
}

int setMyObjectProperty1(MyObject* object,someDataType1* somedata){
    struct _MyHiddenData* data = (struct _MyHiddenData*)object;
    //check for nulls, and process somedata
    data->somePropery=somedata;
}

someDataType1* getMyObjectProperty2Style1(MyObject*){
    struct _MyHiddenData* data = (struct _MyHiddenData*)object;
    //check for nulls, and process somedata
    return data->someProperty;
}
/* Similar code for the rest */

And this way you have encapsulated the struct properties as if they were private. On the same manner static functions inside my_libray.c would behave as private functions. Get a good look at C and you'll see, that your imagination is the limit to what you can do.

Answer 3

Your public interface should only offer an opaque pointer (maybe DATA*, or data_handle), as well as handler functions create_data(), set_data_value(), read_data_value(), free_data(), etc., which operate on the opaque pointer.

Much like FILE*.

Just don't give your clients the internal header files :-)

// library.h

typedef struct data_t * data_handle;

data_handle create_data();
void        free_data(data_handle);

Private implementation (don't ship):

#include "library.h"

struct data_t
{
  /* ... */
};

data_handle create_data() { return malloc(sizeof(struct data_t)); }
void        free_data(data_handle h) { free(h); }
/* etc. etc. */

Answer 4

in C, by convention....

for OO C like this...

I'd have a pid_data_create(&data) // initializes your struct

and pid_data_set_proportional_gain(&data, 0.1);

etc...

so basically achieving a C++ ish class... prefix all functions with the "class" / "struct" name and always pass the struct * as the first parameter.

also, it should store function pointers for polymorphisim, and you shouldn't call those function pointers directly, again, have a function that takes your struct as a parameter, and then the can make the function pointer call (can check for nulls, fake inheritance/virtual functions, and other stuff)