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Pointer for 2D array seems to be 3D in C


2d array, address, memory, pointer

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Old Unix and Linux 1 Week Ago
yifangt yifangt is offline VIP Member  
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Pointer for 2D array seems to be 3D in C

I am struggling with the pointer to 2D-array (cf: 2D array of pointers). Can anybody help me elaborate how the pointer x moves in the memory to access the individual of y[2][6], especially the high lighted lines?
I have talked to one of the curators of the forum, but I am still not quite clear.
Here is my code:


int main(void)
    int (*x)[2][6];                 //pointer for integers array in size of 2x6 (2 rows x 6 columns),
                                    //.i.e the array is always with size of 12?
//    int (*a[8])[5];                 //Line 9: a is a pointer array of size 8, each for integer array of size 5 

    int y[2][6] = {{11,12,13,14,15,16},
                   {21,22,23,24,25,26}};    //2D array of integers
    int *z;                      //pointer to integer
    int i;

    z = y[0];
    for(i = 0;i<6;i++)
        printf("%d ",z[i]);

    x = &y;    // More complicated situation for me!
    x = y;     // Warning: incompatible pointer type.
//    x[0][0] = y[0][0];     // won't work
        printf("   (x[0][0]): %p\n",  x[0][0]);
        printf("  *(x[0][0]): %d\n",*(x[0][0]));        //Q1a
        printf("  x[0][0][0]: %d\n",x[0][0][0]);        //Q1b
        printf("*(x[0][0]+1): %d\n",*(x[0][0]+1));      //Q1c
//        printf("*(x[0][0]+2): %d\n",*(x[0][0]+2));
//        printf("*(x[0][0]+3): %d\n",*(x[0][0]+3));
//        printf("*(x[0][0]+4): %d\n",*(x[0][0]+4));
        printf("*(x[0][0]+5): %d\n",*(x[0][0]+5));

        printf("    x[0][1]: %p\n",   x[0][1]);        //Q2a
        printf("    *x[0][1]: %d\n",*(x[0][1]));
        printf("  x[0][1][0]: %d\n",x[0][1][0]);       //Q2b 
        printf("*(x[0][1]+1): %d\n",*(x[0][1]+1));     //Q2c 
//        printf("*(x[0][1]+2): %d\n",*(x[0][1]+2));
//        printf("*(x[0][1]+3): %d\n",*(x[0][1]+3));
        printf("*(x[0][1]+4): %d\n",*(x[0][1]+4));
        printf("*(x[0][1][4]): %d\n",*(x[0][4]));
    printf("&y: %p\n", &y);
    printf(" y: %p\n",  y);
    printf(" x: %p\n",  x);
    printf("&x: %p\n", &x);
    return 0;

1) Although y and &y are the same, but x = y issues warning;
2) Q1a/Q2a is the part I think I understand which is the first element of each row of y.
3) but Q1b/c, and Q2b/c turns out to be 3-D to me.
Can anybody give me a diagram how pointer x moves in the memory for each member of y?
4) Line 9: int (*a[8])[5]; is related, and I put it here for future reference but skip it at this moment.

Thanks a lot!
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Old Unix and Linux 1 Week Ago
Corona688 Corona688 is offline Forum Staff  
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1) They probably differ in being constants or non-constants.

3) It's "3d" because you have an extra pointer in the way. Every level of pointers requires another dereferencing operation to access it. One level pointer requires one. Two level pointer requires two. Three level pointer requires three. 97 level pointer requires 97. That is the pattern.

The first dereferencing operation gets rid of is x itself; once you do so, you are essentially accessing y. The next level of pointers points to individual rows inside y. The last level gets you an individual cell in that row.

x; // pointer to Y
(*x); // Y, i.e. pointer to rows
(*x)[0]; // Pointer to row 0
(*x)[0][0] // Row 0 col 0.  No longer a pointer.

The diagram is x -> y -> row -> column.
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