passing element of dynamic multidimensional array to a function

LibFunction( info_ptr,(unsigned char **)data_ptr[index] + 1);

is wrong because data_ptr is an unsigned char **, so data_ptr[index] is an unsigned char *. Leave out the cast and correct the function you're calling, it should accept an unsigned char *.

Some corrections in your program, observed from the top few lines

Since,

unsigned char **data_ptr; // a pointer to a char pointer

get the sizeof(char*) and always avoid typecasting the pointer returned by malloc()

data_ptr = malloc(sizeof(unsigned char *) * no_of_rows);

And for doing the allocation for the rows,

data_ptr[index] = (unsigned char *)malloc(sizeof(unsigned char*)* rowsize));

To pass the address of where in a row you want the function to begin writing data, change the function signature as

LibFunction(..., unsigned char *)

It should be LibFunction(&data_ptr[row][start_here]), exactly the same as if it was just an unsigned char[ROWS][COLUMNS];.

In general, it is my experience that if you think you require casts in modern-day C, it is probable that you are muddled up with what you are trying to do. A nice read is a comment on a post by Linus Torvalds on /. on this kind of stuff.

You're not allocating room for no_of_rows pointers to pointers; there's an asterisk too many in there. Also, you really [shouldn't cast the return value of malloc(), in C][1].

Your first allocation should be:

data_ptr = malloc(no_of_rows * sizeof *data_ptr);

But I need to pass the address of where in a row I want the function to begin writing data

So let's start simple, to make an array the correct size, forget trying to get the sizeof a complex type, we can simply do this:

unsigned char **data_ptr;
data_ptr = malloc(sizeof(data_ptr) * no_of_rows); //Just sizeof your var

Now you've got the correct memory malloc'd next you can malloc the memory for the rest easily:

for(index = 0; index < no_of_rows; index++)
  data_ptr[index] = malloc(sizeof(unsigned char*) * rowsize);

Last point, now that we've got all that set up, you should initialize your array:

for(index = 0; index < no_of_rows; index++)
  for(index2 = 0; index2 < rowsize; index2++)
     data_ptr[index][index2] = 0;

As for your function, you want it to take a "portion" of an array, so we need it to take an array and a size (the length of the array to initialize):

void LibFunction(unsigned char data[], int size);

Then we're your ready to store some data it's as easy as:

LibFunction(&data_ptr[1][2], 3);  // store data in second row, 3rd column, store
                                  // three values.

You can do something like this:

unsigned char* ptr = &data[0][1];
LibFunction(info_ptr, &ptr);