Accessing values from array of struct pointers

Question

I am trying to access a pointer index value from an array of structure pointers:

typedef struct
{
  bool             status;           
  uint8_t          sensnr;            
  uint8_t          set_vallue;        
  uint8_t          actuval_value;  
} temp_t;
//array of struct
temp_t temp[5];

typedef struct
{           
  uint8_t          senspwrsource;          
  uint8_t          sensnr;           
  uint8_t          max_value;  
} sensor_t;

//array of structs
sensor_t sens[5];

typedef union {
   temp_t    temp;
   sensor_t  sensor;
} temp_sens_t;

typedef struct
{
  bool                status;                               
  struct temp_sens_t  *temp_value[3];                       
  struct temp_sens_t  *sensors[3];                        
  char                sensor_name[MAX_LEN]; 
} tempsensors_t;

//array of structures
tempsensors_t  sensors[5];

I am able to assign all values but I am not able to read values from struct pointer "temp_value[3]".

So far I have tried like this:

temp_t *tempinfo;
tempinfo = &sensors[1]->temp_value[0];

//trying to access values like this but not working.
teminfo.sensnr ;

How do I access values from an array of struct pointers with index [0 to 2]?

Answer 1

You have (in part):

typedef union {
   temp_t    temp;
   sensor_t  sensor;
} temp_sens_t;

typedef struct
{
  bool                status;                               
  struct temp_sens_t  *temp_value[3];                       
  struct temp_sens_t  *sensors[3];                        
  char                sensor_name[MAX_LEN]; 
} tempsensors_t;

That doesn't mean what you think it means, though. None of your defined structures or unions has a tag (as in struct tag { … }). You define a type named temp_sens_t, but you do not define a type struct temp_sens_t (and even if you did, it would be unrelated to the type known as temp_sens_t — structure tags are in a different namespace from 'ordinary identifiers' such as typedef names). That's "OK" — you're using an incomplete type. So the code compiles. But it's not OK because you can't access the members of an incomplete type.

Presumably, what you had in mind was:

typedef struct
{
  bool          status;                               
  temp_sens_t  *temp_value[3];                       
  temp_sens_t  *sensors[3];                        
  char          sensor_name[MAX_LEN]; 
} tempsensors_t;

Note the absence of the struct inside the structure definition.

With this revised definition, you should be able to do:

temp_t *tempinfo = &sensors[1]->temp_value[0]->temp;

tempinfo->sensnr = 1;

Working code:

#include <stdbool.h>
#include <stdint.h>

enum { MAX_LEN = 32 };

typedef struct
{
  bool             status;
  uint8_t          sensnr;
  uint8_t          set_vallue;
  uint8_t          actuval_value;
} temp_t;

typedef struct
{
  uint8_t          senspwrsource;
  uint8_t          sensnr;
  uint8_t          max_value;
} sensor_t;

typedef union {
   temp_t    temp;
   sensor_t  sensor;
} temp_sens_t;

typedef struct
{
  bool          status;
  temp_sens_t  *temp_value[3];
  temp_sens_t  *sensors[3];
  char          sensor_name[MAX_LEN];
} tempsensors_t;

tempsensors_t  sensors[5];

extern void access_it(int i);

void access_it(int i)
{
    temp_t *tinfo1 = &sensors[1].temp_value[0]->temp;
    tinfo1->sensnr = i;
    temp_sens_t *tinfo2 = sensors[1].temp_value[0];
    /* Either of these works - choose the correct part of the union */
    tinfo2->sensor.sensnr = i;
    tinfo2->temp.sensnr = i;
}

^{Notice: GCC 10.2.0 was consulted about the accuracy of this code.}

^{gcc -O3 -g -std=c11 -Wall -Wextra -Werror -Wmissing-prototypes -Wstrict-prototypes -fno-common -c as41.c}