Use char array as two dimensional array in C

Question

I have a struct that contains an unsigned char * for storing arbitrary data. At some point I want to use this data as if it were a two-dimensional array. This is how I do it:

#define DATA_SIZE 10
unsigned char *data = malloc(DATA_SIZE * DATA_SIZE * sizeof(unsigned char));

// this is not allowed
unsigned char (* matrix)[DATA_SIZE] = (unsigned char*[DATA_SIZE]) &data;

// this gives a warning and doesn't work at all
unsigned char (* matrix)[DATA_SIZE] = (unsigned char **) &data;

I want to cast a pointer to arbitrary data to a two-dimesional array, but of course I can't cast to array types. How would I need to go about this?

Thanks in advance.

Daniel Fischer · Accepted Answer · 2012-09-25 11:19:02Z

4

You have gotten the syntax of the cast wrong,

unsigned char (* matrix)[DATA_SIZE] = (unsigned char(*)[DATA_SIZE]) data;

works after you fixed your #define by removing the =.

#define DATA_SIZE 10

    unsigned char *data = malloc(DATA_SIZE * DATA_SIZE * sizeof(unsigned char));

    // this is the correct way to cast
    unsigned char (* matrix)[DATA_SIZE] = (unsigned char(*)[DATA_SIZE]) data;

edited Sep 25, 2012 at 11:19

answered Sep 25, 2012 at 11:06

Daniel Fischer

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1 Comment

pancake Over a year ago

Thank you very much, and the #define was incorrect of course, I changed it in my question.

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