在指针数组中打印指针指向的值

Question

1. I am trying to print the values pointed by pointers stored inside ptr array. Expected output is 1,2,3,4 but the function prints the garbage.
2. Trying to understand how to access values within array and print them.

I know there's an easy way to just use double pointer to point to array and print but for learning sake trying to not use double pointer and just use pointer to whole array

#include <stdio.h>
#include <stdint.h>

uint8_t a = 1;
uint8_t b = 2;
uint8_t c = 3;
uint8_t d = 4;

uint8_t *ptr[] = {&a, &b, &c ,&d}; //Pointer is array of 4 pointer to uint8_t 


void foo(uint8_t* (*ptr)[4]) //Ptr is pointer to array of 4 pointer to uint8_t
{
    uint8_t * temp =(uint8_t*)ptr[0];
    for(uint8_t i=0;i<4;i++){
        
        printf("%d\n",*(temp+i)); //Does not print 1,2,3,4 as expected
    }
}

int main()
{
  
    foo(&ptr);
    return 0;
}

Answer 1

Your code seems unnecessarily complex but I guess you can do below.

#include <stdio.h>
#include <stdint.h>

uint8_t a = 1;
uint8_t c = 3;
uint8_t b = 2;
uint8_t d = 4;

uint8_t *ptr[] = {&a, &b, &c ,&d};  


void foo(uint8_t* (*ptr)[4])  
{
    uint8_t ** temp =(uint8_t**)*ptr; 
    for(uint8_t i=0;i<4;i++){
     
        printf("%d\n",**(temp+i)); 
    }
}

int main()
{
  
    foo(&ptr);
    return 0;
}
    

Answer 2

The type of ptr parameter of foo() function is uint8_t* (*) [4] , which is - pointer to an array of 4 uint8_t pointers.

In the foo() function, the problem is in this statement:

uint8_t* temp =(uint8_t*)ptr[0];

From definition of the subscript operator ¹⁾ , here,

ptr[0]  ->  *(ptr + 0) -> *(ptr) -> *ptr

the type of *ptr is uint8_t * [4] . So, you are initialising a uint8_t * type variable temp from uint8_t * [4] type variable ptr[0] . Hence, the initialisation of temp is from an incompatible type resulting in garbage value when accessing it. It is just that the cast is suppressing the compiler warning. Remove the cast and closely check the compiler warning.

Instead, you should initialise temp in for loop with every i ^th element of (*ptr) array, like this

uint8_t * temp = (*ptr)[i];   // type of (*ptr)[i] is uint8_t *

and dereferencing temp will give value at that location.

The type of (*ptr)[i] is uint8_t * , how? Think of it in this way -

the type of ptr is uint8_t * (*) [4] ,
the type of *ptr is uint8_t * [4] , and
the type of (*ptr)[i] is uint8_t * , where 0<=i<=3 , (which is same as type of temp variable).

Also, you should use the format specifier macros for printing fixed width integer types ²⁾ .

...but for learning sake trying to not use double pointer and just use pointer to whole array...

You don't need the temp pointer to print the values, you can directly print them using ptr pointer (parameter of foo() function).
You can do:

#include <stdio.h>
#include <inttypes.h>

uint8_t a = 1;
uint8_t b = 2;
uint8_t c = 3;
uint8_t d = 4;

uint8_t *ptr[] = {&a, &b, &c, &d}; //Pointer is array of 4 pointer to uint8_t 

void foo(uint8_t* (*ptr)[4]) {
    for (uint8_t i = 0; i < 4; i++) {
        /*
         * Using pointer to whole array to print the values
         */      
        printf("Using pointer to whole array : %" PRIu8 "\n", *(*ptr)[i]);

        /* 
         * Print values using temp pointer
         */
        uint8_t* temp = (*ptr)[i];
        /* 
         * temp is pointing to i'th element of (*ptr) array.
         * Dereference temp will give value at that location.
         */
        printf("Using temp pointer : %" PRIu8 "\n", *temp);

    }
}

int main (void) {
    foo (&ptr);
    return 0;
}

Output:

Using pointer to whole array : 1
Using temp pointer : 1
Using pointer to whole array : 2
Using temp pointer : 2
Using pointer to whole array : 3
Using temp pointer : 3
Using pointer to whole array : 4
Using temp pointer : 4

---

1). From C11 Standard#6.5.2.1:

The definition of the subscript operator [] is that E1[E2] is identical to (*((E1)+(E2))) .

2). If you are using fixed width integer types ( uint8_t , uint16_t , int8_t , int16_t , uint32_t .... etc.) then you should use their respective conversion specifiers macros specified by language standards to print them. The fprintf() family function macro for unsigned decimal integer value is PRIuN where N represents the width ( 8 , 16 , 32 etc..) of type. Check macros for format specifiers .
Also, for these macros, you need to include inttypes.h header file in your program and no need to include stdint.h header file because inttypes.h header file includes stdint.h .