how can I assign a pointer to an array inside a struct?

Question

typedef struct
{
 uint8_t (*flags)[5];
} type_t;
void setstate(type_t* driver, uint8_t flag)
{
 driver->flags[flag] = 1;
}
void printall(type_t* driver)
{
 for (int a = 0; a < 5; a++)
 {
 printf("%d\n", driver->flags[a]);
 }
}
int main(int argc, char** argv) 
{
 static type_t driver[1];
 uint8_t (*states)[5] = { 0 };
 driver->flags = states;
 setstate(driver, 2);
 printall(driver);
}

I wanted to assign a local array pointer to an array pointer inside a struct but I cant seem to get this to work.. thanks.

• tadman: Why are you declaring a pointer like that? Length-"limited" pointers are pretty useless.
• Mike Robinson: What is the exact statement that you are attempting, and exactly where is it? Please don't ask us to "guess."
• sorh: all sorted, thanks guys.

Answer 1

your local variable states is also a pointer to an array. I think you meant to declare an actual array? And then assign a pointer to that array to the field in the structure? That's what the text said, anyway. The way your initializer is written, with 0 instead of NULL and with extra { } around it, I think you are thinking that you did declare an array (so what's with the * ?).

uint8_t states[8];

Now you normally don't declare a pointer to a whole array, but a pointer to the same type as the array element. So the structure field would be:

uint8_t* flags; // points to first of 8 consecutive values

then you can write

driver->flags= states;

and it will mean that.

Though driver being an array of 1, it is very strange to refer to it as a pointer like that. What's the point of making it an array, if there is only one element?

• sorh: thanks, yes that makes sense. the point of making it a fixed size array is to avoid using malloc since this will need to be misra compliant, also this is just a small example of the real implementation.

Answer 2

Seems work,

Maybe setstate need work.

#include<stdlib.h>
#include<stdio.h>
#include<stdint.h>
typedef struct _type
{
 uint8_t * flags[5];
 uint8_t * flag;
 uint8_t data[5];
} type_t;
void m_setstate(type_t* driver, uint8_t flag)
{
 *driver->flags[flag] = 1;
}
void printall(type_t* driver)
{
 for (int a = 0; a < 5; a++)
 {
 if( driver->flags[a] != NULL )
 {
 printf("INIT DONE %p FIRST VALUE %d \n", driver->flags[a], driver->flags[a][0]);
 }
 else
 {
 printf("NULL >> %p\n", driver->flags);
 }
 }
}
int main(int argc, char** argv) 
{
 static type_t driver = { 0 };
 uint8_t case0_states[5] = { 5, 0 };
 uint8_t case1_states[4] = { 4, 0 };
 uint8_t case2_states[3] = { 3, 0 };
 uint8_t case3_states[2] = { 2, 0 };
 uint8_t case4_states[1] = { 1 };
 driver.flag = &(case0_states[0]);
 driver.data[0] = 3;
 printf("DEBUG\n");
 printall(&driver);
 printf("DEBUG\n");
 driver.flags[0] = (uint8_t*) &(case0_states[0]);
 driver.flags[1] = (uint8_t*) &(case1_states[0]);
 driver.flags[2] = (uint8_t*) &(case2_states[0]);
 driver.flags[3] = (uint8_t*) &(case3_states[0]);
 driver.flags[4] = (uint8_t*) &(case4_states[0]);
 printall(&driver);
 m_setstate(&driver, 2);
 printall(&driver);
}

And result:

koala@K-desktop:~/Workspace/koala-home/15-Stack/01-20210310$ gcc -o hello.exe hello.c 
koala@K-desktop:~/Workspace/koala-home/15-Stack/01-20210310$ ./hello.exe 
DEBUG
NULL >> 0x559eeef4e040
NULL >> 0x559eeef4e040
NULL >> 0x559eeef4e040
NULL >> 0x559eeef4e040
NULL >> 0x559eeef4e040
DEBUG
INIT DONE 0x7ffeb3406143 FIRST VALUE 5 
INIT DONE 0x7ffeb340613f FIRST VALUE 4 
INIT DONE 0x7ffeb340613c FIRST VALUE 3 
INIT DONE 0x7ffeb340613a FIRST VALUE 2 
INIT DONE 0x7ffeb3406139 FIRST VALUE 1 
INIT DONE 0x7ffeb3406143 FIRST VALUE 5 
INIT DONE 0x7ffeb340613f FIRST VALUE 4 
INIT DONE 0x7ffeb340613c FIRST VALUE 1 
INIT DONE 0x7ffeb340613a FIRST VALUE 2 
INIT DONE 0x7ffeb3406139 FIRST VALUE 1 

Answer 3

The problem is this statement

driver->flags[flag] = 1;

You are assigning 1 to an address. Instead you should have

*(driver->flags[flag]) = 1;

But then, you need to have a valid memory for flags to point to. So you need to malloc() flags.

driver->flags = (uint8_t(*)[])malloc(5);

• sorh: I was trying to put a pointer to each array element in a 'mirror' array inside the struct, but as JDługosz points out, all I need is just a pointer of the same type to the first item of the array. Nothing wrong in having a one dimentional array, and avoid using malloc() altogether, memory is allocated the same way, being the difference one is defined at compile time, the other at runtime. thanks!
• JDługosz: I was trying to put a pointer to each array element in a 'mirror' array inside the struct oh, that would be (uint8_t*)flags[8]; that is an array of 8 pointers. You wrote a pointer to an array of 8 integers.