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As we know members of struct are public by default, lets implement the private access specifier behavior into the struct.

In the below samplestruct example, If you provide a samplestruct* create_struct() method as an interface to client and a series of methods that operate on samplestruct*, hiding the actual definition of samplestruct from the client, then a similar effect is achieved.

// File "samplestruct_private.h"

struct samplestruct {
    int priv_int;
    char priv_char;
};

// File "samplestruct.h"

typedef struct samplestruct samplestruct;
samplestruct * create_struct(int a, char b);
int mangle_struct(samplestruct *);

// file "samplestruct.c"

#include <stdlib.h>
#include "samplestruct.h
#include "samplestruct_private.h"

samplestruct * create_struct(int a, char b) {
    samplestruct * s = (samplestruct *) calloc(1, sizeof(samplestruct));
    s->priv_int = a ;
    s->priv_char = b ;
}    

int mangle_struct(samplestruct *s) {
    return s->priv_int + s->priv_char;
}

Distribute samplestruct.c compiled into a library, along with samplestruct.h.

The functions declared in samplestruct.h form the public interface of a type, but the internal structure of that type is opaque.
In effect, the clients who call create_struct() can't access the private members of the samplestruct object.