Print Page - C - help needed with jagged arrays (array of pointers to arrays)

Title: C - help needed with jagged arrays (array of pointers to arrays)
Post by: Norman on 10:39, 01 August 20

Good day everyone,

I've got a rather simple C newbie question. Working with CPCtelera, so SDCC is the compiler. What's the difference between these two programs:

Code Select


#include <stdio.h> 
#include <types.h> 

const u8 lvl1[3] = {1, 2, 3};
const u8 lvl2[2] = {4, 5};
u8* jagged[2];

int main() {
   jagged[0] = lvl1;
   jagged[1] = lvl2;
   for (int i = 0; i < 2; i++) {
      printf("%x\n", *jagged[i]);
   }
}

Outputs "1 4", as expected. If I print the pointer (jagged{i}) instead of the value, I get the correct memory adresses of lvl1 and lvl2 respectively.
However the following doesn't work:

Code Select

#include <stdio.h>
#include <types.h>

const u8 lvl1[3] = {1, 2, 3};
const u8 lvl2[2] = {4, 5};
u8* jagged[2] = {lvl1, lvl2};

int main() {
   for (int i = 0; i < 2; i++) {
      printf("%x\n", *jagged[i]);
   }
   while(1);
}

This outputs "1 1". If I print the pointer, it's just 0 for both arrays.
What's going wrong here?

Thanks for any help.
Cheers

Title: Re: C - help needed with jagged arrays (array of pointers to arrays)
Post by: Arnaud on 12:55, 01 August 20

Nothing is wrong in your C code, but cpctelera/SDCC don't initialize not constant global variable, because they take double space in memory.

Here the detailed explanation :
https://www.cpcwiki.eu/forum/programming/cpctelerasdcc-initialization-of-variables-and-arrays-in-headerfile/msg112905/#msg112905

Title: Re: C - help needed with jagged arrays (array of pointers to arrays)
Post by: Norman on 13:15, 01 August 20

Thanks for the quick response Arnaud, that clears up some things for me. It works now.

But what's the difference between u8* const arr[] and const u8* arr[]? I had already tried the latter, obviously.
Cheers

Title: Re: C - help needed with jagged arrays (array of pointers to arrays)
Post by: Singaja on 13:48, 01 August 20

Quote from: Norman on 13:15, 01 August 20u8* const arr[]

is a pointer to an array of constants.

Quote from: Norman on 13:15, 01 August 20 const u8* arr[]

Is a constant pointer to an array.

Title: Re: C - help needed with jagged arrays (array of pointers to arrays)
Post by: teopl on 14:08, 01 August 20

the last is a C related question, there are a lot of answers online.

this is what I remember: (quote from one of the online answers)

Read it backwards...

int* - pointer to int
int const * - pointer to const int
int * const - const pointer to int
int const * const - const pointer to const int

---

so:

u8* const arr[]

should be ARRAY of const pointers to u8

const u8* arr[]

should be ARRAY of pointers to const u8

but please double check...

Title: Re: C - help needed with jagged arrays (array of pointers to arrays)
Post by: reidrac on 14:38, 01 August 20

I would advice to check your main binary .map file for l__INITIALIZED, and its size MUST be 0 or you'll need initialised data and cpctelera apparently has removed that from the CRT0.

I do that too :)

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General Category => Programming => Topic started by: Norman on 10:39, 01 August 20