Wsrtjtyk

I want to dynamically allocate an array of strings.
So arr should consist of twenty pointers to the first character of each string.

For simplicity, each string is just the same, stored in base.
The first for loop should now create my array, which seems to work just fine.

#include <stdio.h>

#include <stdlib.h>

#include <string.h>



int main()

{

  int n = 20;

  char** arr = malloc( sizeof(char*) + n );



  char* base = "abcdefghijklmnopqrstuvwxyz";



  for(int i = 0; i < n; i++)

  {

    *(arr + sizeof(char*) * i) = base;

  }



  for(int i = 0; i < n; i++)

  {

    printf("%sn", *(arr + sizeof(char*) * i));

  }

}

The seconds for loop creates a segmentation-fault during the second iteration.

Issues

1. char** arr = malloc( sizeof(char*) + n ); allocates (most likely) 24 bytes which can only store (most likely) six char *.




2. You don't need to try offset the address (*(arr + sizeof(char*) * i) = base;) by the base type. The offset is automatically adjusted by the sizeof the base type.

The following changes must be made:

#include <stdio.h>

#include <stdlib.h>

#include <string.h>



int main(void)

{

    int n = 20;

    char **arr = malloc(sizeof(char *) * n);



    char *base = "abcdefghijklmnopqrstuvwxyz";



    for(int i = 0; i < n; i++)

        arr[i] = base;



    for(int i = 0; i < n; i++)

        printf("%d: %sn", i, arr[i]);



    return 0;

}

The preceding uses array notation. You can also use pointer notation if you'd like. Change arr[i] to *(arr + i).

Output

$ gcc main.c -o main.exe; ./main.exe

0: abcdefghijklmnopqrstuvwxyz

1: abcdefghijklmnopqrstuvwxyz

2: abcdefghijklmnopqrstuvwxyz

3: abcdefghijklmnopqrstuvwxyz

4: abcdefghijklmnopqrstuvwxyz

5: abcdefghijklmnopqrstuvwxyz

6: abcdefghijklmnopqrstuvwxyz

7: abcdefghijklmnopqrstuvwxyz

8: abcdefghijklmnopqrstuvwxyz

9: abcdefghijklmnopqrstuvwxyz

10: abcdefghijklmnopqrstuvwxyz

11: abcdefghijklmnopqrstuvwxyz

12: abcdefghijklmnopqrstuvwxyz

13: abcdefghijklmnopqrstuvwxyz

14: abcdefghijklmnopqrstuvwxyz

15: abcdefghijklmnopqrstuvwxyz

16: abcdefghijklmnopqrstuvwxyz

17: abcdefghijklmnopqrstuvwxyz

18: abcdefghijklmnopqrstuvwxyz

19: abcdefghijklmnopqrstuvwxyz

As recommended by Swordfish you should use the const qualifier on arr and base:

const char **arr = malloc(sizeof(char *) * n);

const char * const base = "abcdefghijklmnopqrstuvwxyz";

In this way:

1. 
   arr cannot change (without a warning/error) pointed to chars.


2. Neither the address of base nor the pointed to chars can be changed (without a warning/error).

Thanks

To Tim Randall for catching a math error.