Pointer to multidimensional array

Question

I'm trying to address matrix of struct, but some goes wrong. This the code:

typedef struct {
    bool state;
    float val;
    char ch[11];
} st_t;

st_t matrix[3][5];


int main(int argc, char *argv[])
{
    int i, j;

    // Init matrix value matrix[i][j] = i.j
    ....

    // Init matrix pointer
    st_t (*pMatrix)[3][5];
    pMatrix = &matrix;

    // print address element
    fprintf(stderr, "\n\nsizeof st_t:%d\n\n", sizeof(st_t) );
    for( i = 0; i < 3; i++ )
    {
        for( j = 0; j < 5; j++ )
            fprintf(stderr, "matrix[%d][%d] ADDR:%p    pMatrix[%d][%d] ADDR:%p\n", i, j, &(matrix[i][j]), i, j, &pMatrix[i][j]);
        fprintf(stderr, "\n");
    }
    return 0;
}

This is the output of code:

sizeof st_t:16

matrix[0][0] ADDR:0x8049a00             pMatrix[0][0] ADDR:0x8049a00
matrix[0][1] ADDR:0x8049a10             pMatrix[0][1] ADDR:0x8049a50
matrix[0][2] ADDR:0x8049a20             pMatrix[0][2] ADDR:0x8049aa0
matrix[0][3] ADDR:0x8049a30             pMatrix[0][3] ADDR:0x8049af0
matrix[0][4] ADDR:0x8049a40             pMatrix[0][4] ADDR:0x8049b40

matrix[1][0] ADDR:0x8049a50             pMatrix[1][0] ADDR:0x8049af0

For example why does pMatrix[0][1] is different from address of matrix[0][1]?

Thanks in advance.

Eric Postpischil · Accepted Answer · 2013-05-17 13:33:34Z

2

You have declared pMatrix to be a pointer to a 3⨉5 matrix of st_t, that is, it points to an array of 3 arrays of 5 st_t objects. Given this, pMatrix[0] is an array of 3 arrays of 5 st_t objects. However, since it is an array, it is automatically converted to a pointer to the first element of the array. So it becomes a pointer to an array of 5 st_t objects.

Then pMatrix[0][0], pMatrix[0][1], pMatrix[0][2], and so on are successive arrays of 5 st_t objects, not successive st_t objects.

Most likely, what you want is:

// Declare pMatrix to be a pointer to an array of 5 st_t objects,
// and point it to the first row of matrix.
st_t (*pMatrix)[5] = matrix;

answered May 17, 2013 at 13:33

Eric Postpischil

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3 Comments

GMERiello Over a year ago

Thank you, pMatrix[0] is a POINTER to array of 5 element. pMatrix[0][0] is THE array and pMatrix[0][0][2] is the third element of the array. We can also write (*pMatrix)[0][2] for the same element.

Bernd Elkemann Over a year ago

+1 to the answer. @GMERiello dont understand what you mean by the comment but the answer seems correct. Is this what you wanted or not?

GMERiello Over a year ago

@eznme yes, the answer helped me, I want to access to a single element of the matrix through the pointer pMatrix. Now pMatrix[0][1] is not a st_t element but a complete row. pMatrix[0][1][2] is the third element of second row, but also (*pMatrix)[1][2], in other word (obvious) pMatrix[0] == (*pMatrix). I hope now is more clear.

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