Accessing C Struct member from a different file(the main) - c

Question

Checked couple of stack overflow questions/answers but none correspond to what I am trying to do. Here is it:
I have a c object file myobject.c containing struct type populated at run time (as initialised by main file having the main() function. Below is the skeletal structure of myobject.c:
typedef struct
{
uint16_t ID;
float tempo;
char unit[10];
unsigned long timestamp;
} prv_data_t;
static uint8_t prv_value(lwm2m_data_t* dataP,
prv_data_t* tempData)
{
uint8_t ret = COAP_205_CONTENT;
//TO DO here
.
.
.
return ret;
}
static uint8_t prv_read(..paramList)
{
//TO DO here
.
.
//then call prv_value here
result = prv_value((*tlvArrayP)+i, tempData);
return result;
}
object_t * get_object(){
//this func get called by main.c to initialize myobject
}
Skeletal structure of the main.cfile:
myFunc(mypar p) {
}
main(){
//initialize myobject
//.....
//access myobject struct member here, pass to myFunc call
myFunc(tempo)
}
The main.c initialises myobject.c. Now I want to access tempo a member of prv_data_tfrom myobject.cfor some computation. How do I achieve such a task without exposing prv_data_t in main.c?
EDIT: here's what I mean by main.c initialises myobject.c and all other objects, please:
/*
* Now the main function fill an array with each object,
* Those functions are located in their respective object file.
*/
objArray[0] = get_security_object();
if (NULL == objArray[0])
{
fprintf(stderr, "Failed to create security object\r\n");
return -1;
}
.
.
.
The main file actually contains the main()function.

Answer 1

You can avoid exposing your private data by doing:
Let main work with pointers to incomplete type struct prv_data_t
Implement getter-functions (and setter-functions) for members that you allow main to access
Something like this:
a.h
#include <stdio.h>
struct prv_data_t; // Incomplete type
struct prv_data_t * get_obj();
float get_tempo(struct prv_data_t * this);
a.c
#include <stdio.h>
#include <stdlib.h>
struct prv_data_t
{
int ID;
float tempo;
char unit[10];
unsigned long timestamp;
};
float get_tempo(struct prv_data_t * this)
{
return this->tempo;
}
struct prv_data_t * get_obj()
{
struct prv_data_t * p = malloc(sizeof *p);
p->tempo = 42.0;
return p;
}
main.c
#include <stdio.h>
#include "a.h"
int main()
{
struct prv_data_t * p = get_obj();
printf("%f\n", get_tempo(p));
// The line below can't compile because the type is incomplete
// printf("%f\n", p->tempo);
return 0;
}
So with this kind of code main only knows that there exists a struct prv_data_t but main knows nothing about the members of that struct.