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]?

What is sensor in tempinfo = &sensor[1]->temp_value[0];? Please show a minimal reproducible example. — Jabberwocky
– Jabberwocky, Commented Jul 1, 2021 at 13:30
@verendra please don't describe what you have tried but edit yopur question and show what you have tried as a minimal reproducible example — Jabberwocky
– Jabberwocky, Commented Jul 1, 2021 at 13:36
regarding: I am able to assign all values but I am not able to read values from struct pointer "temp_value[3]" in C, indexes are 0 based, so accessing temp_value[3]; is trying to access beyond the end of the array — user3629249
– user3629249, Commented Jul 2, 2021 at 14:26

Jonathan Leffler · Accepted Answer · 2021-07-01 21:10:06Z

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}

I am trying to loop through struct values inside array of struct and array of struct. for example one values from temp_value struct need to be checked all elements in struct and array of structs. any suggestions??

HannesH · Accepted Answer · 2021-07-01 13:56:23Z

-1

just like in a normal array

temp_t * tempinfo = sensors[1].temp_value[0];

since you are not storing pointers to your structs, but the structs themselves

inside the struct you are storing pointers as Andrew Henle pointed out.

to get their value you can do

tempinfo->sensor;

the dot is for accessing members of a struct while the arrow is for accessing members of a pointer to a struct. You seem to have them swapped.

edited Jul 1, 2021 at 13:56

answered Jul 1, 2021 at 13:40

HannesH

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1 Comment

Andrew Henle Over a year ago

struct temp_sens_t *temp_value[3]; is most definitely an array of pointers to struct temp_sens_t and not "structs themselves". Given the posted code, those pointers are never initialized and dereferencing them is undefined behavior.

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