difficulty extracting an array element from a multi- dimensional array in a file using C

Question

Another beginner problem I'm afraid. the intention is to create a 3D array of strings, fill each string with an initial value that identifies it, (to aid with later manual filling of array elements) and save it to a file. Then to recover an individual element, pre-filled with its initial value for editing.

the problem is the program is returning nonsense when I when I try to extract an individual string from the array.

I dont know if the problem is with the storing or extracting the array or both.

The code snippet below seems long, but I have needed to include both the read and write sections.

#include <stdio.h>
#include <string.h>

#define ELEMENT_SIZE 16

struct data_cell
{
char my_data[2][2][2][ELEMENT_SIZE];
int element_num[3];
};

void main()
{
    /** for writing**/
            struct data_cell save;
            int x , y , z;
            char d[] = {'0'};
    /** for reading **/
            struct data_cell from_file;
            int dimensions[2];
            int a,b,c ;

            char path[] = "C:\\Users\\dave\\Documents\\data.txt";
            FILE *fp ;

      if ((fp = fopen(path,"w")) == NULL)
        {
        printf("cannot open file");
        exit(1);
        }

        for(x=0 ; x< 2 ; x++)
            for(y =0 ; y < 2 ; y++)
                for(z =0 ; z < 2 ; z++)
            {
            strcpy(save.my_data[x][y][z],"element   is at ");
            save.my_data[x][y][z][8] = d[0];
            d[0]+=1;
            save.element_num[0] = x;
            save.element_num[1] = y;
            save.element_num[2] = z;

            //fprintf(fp,"%s[%d][%d][%d] "  ,save.my_data[x][y][z] , save.element_num[0] , save.element_num[1] ,save.element_num[2] );
            printf("data_cell %s[%d][%d][%d]\n"  ,save.my_data[x][y][z] ,save.element_num[0] ,save.element_num[1] ,save.element_num[2] );

            }
     fwrite(&save, sizeof(save), 1 , fp);

     fclose(fp);


     /** reading **/
      if ((fp = fopen(path,"r")) == NULL)
        {
        printf("cannot open file");
        exit(1);
        }

        printf("enter array element to be read eg 1  2  1");  /** Range 0-2, press return between each**/

        scanf("%d %d %d" , &dimensions[0] , &dimensions[1] , &dimensions[2] );

        a = dimensions[0];
        b = dimensions[1];
        c = dimensions[2];

        fread(&from_file , sizeof(from_file) ,1 ,fp );

        printf("data_cell.my_data[%d][%d][%d] contains %s", a ,b ,c, from_file.my_data[a][b][c]);

        fclose(fp);
}

The array int dimensions[2]; have only dimensions[0] and dimensions[1]. dimensions[2] is out-of-range. — MikeCAT
– MikeCAT, Commented Oct 13, 2020 at 11:41
16-element array is not enough to store "element is at " because there are no room for terminating null-character. — MikeCAT
– MikeCAT, Commented Oct 13, 2020 at 11:42
Don't tweezer-assemble strings, use snprintf and make sure you have enough space in the buffer. — n. m. could be an AI
– n. m. could be an AI, Commented Oct 13, 2020 at 11:52

MikeCAT · Accepted Answer · 2020-10-13 12:14:41Z

1

Both storing and extracting have errors.

In storing, you are having strcpy() cause out-of-range access: "element is at " takes 17 bytes (16 characters + 1 terminating null-character), so it won't fit in 16-element char arrays.

In extracting, you are using dimensions[2] but the array dimensions have only 2 elements. Here another out-of-range access occure.

Also the message enter array element to be read eg 1 2 1 is misleading. Only 0 and 1 are allowed as the indice and 2 is not allowed.

answered Oct 13, 2020 at 12:14

MikeCAT

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