printf struct like an array C

Question

I have a structure and wish to print all member variable like an array.

The struct was like

#pragma pack(push)     /* push current alignment to stack */ 
#pragma pack(1)     /* set alignment to 1 boundary */ 
struct testStruct { 
    int a = 6; 
    int b[5] = {1,2,3,4,5}; 
} testStruct1 ;
#pragma pack(pop)     /* restore original alignment from stack */

I am trying stuff like

for(int i = 0; i < 6; i++)
{
    printf("%d, ", testStruct1 + i);
}

This cannot compile. I'm not willing to declare a new array to memcpy all member in it.

I wish to see

6, 1, 2, 3, 4, 5,

Is there any way to do so??? Thanks

Write your own printX() or write a loop running over the array. printf() does not know how to print arrays. — the busybee
– the busybee, Commented Sep 4, 2020 at 10:30
testStruct1 + i is incorrect since a) testStruct1 isn't even a pointer to a struct and b) Pointers to structs would also not work that way . — A P Jo
– A P Jo, Commented Sep 4, 2020 at 10:39

Lundin · Accepted Answer · 2020-09-04 11:31:01Z

2

It sounds like you wish to access the same variables through different types and names. C allows you to do that through "union type punning", like this:

#include <stdio.h>

typedef union
{
  struct  // standard C anonymous struct
  {
    int a; 
    int b[5];
  };
  int array [6];
} testArray;

int main (void)
{
  testArray test = { .a = 6, .b={1,2,3,4,5} };
  
  for(int i=0; i<6; i++)
  {
    printf("%d ", test.array[i]);
  }
  
  return 0;
}

Output:

6 1 2 3 4 5

Packing isn't necessary in this case since it's all aligned int variables.

answered Sep 4, 2020 at 11:31

Lundin

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2 Comments

Olly Over a year ago

Hi, the code testArray test = { .a = 6, .b={1,2,3,4,5} }; only works on c, how can I do it in c++

Lundin Over a year ago

@Olly Initialize it as you would any struct or array? In C++ you can also do it from the constructor initializer list. However you can't do type punning in C++, it is undefined behavior there. Supposedly because C++ hates being useful for hardware-related programming...

Paul Ogilvie · Accepted Answer · 2020-09-04 10:37:05Z

0

If you want to use pragamas that is fine, but they have no influence on your question.

I believe the syntax you want is:

struct testStruct { 
    int a; 
    int b[5]; 
} testStruct1 = {6,1,2,3,4,5};

and you can print it like:

printf("%d, %d, %d, %d, %d, %d\n",
    testStruct1.a,
    testStruct1.b[0];
    testStruct1.b[1],
    testStruct1.b[2],
    testStruct1.b[3],
    testStruct1.b[4]);

And please remove all your redundant empty lines.

answered Sep 4, 2020 at 10:37

Paul Ogilvie

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3 Comments

A P Jo Over a year ago

Or OP may want to use a loop, if this code is just an example and he actually wants to use it on a seriously-sized array.

Paul Ogilvie Over a year ago

@APJo, of course it is just an example. A loop would be quite simple to implement. Maybe he can think hard and do it himself?

A P Jo Over a year ago

Absolutely, I just posted this so he would think of it. Alternatively he could use struct pointers as long as he knows what he's doing

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