C, passing 2 dimensional array - c

Question

I haven't used pure C in a few years now, but I can't seem to make this really basic use case work. Here is the simple use-case in simple C, the actual situation is wrapped in the HDF library, but I need to start with this, first.
#include <stdio.h>
void print_data(float **data, int I, int J)
{
for(int i=0;i<I;i++)
{
for(int j=0;j<J;j++)
printf("%02.2f\t", data[i][j]);
printf("\n");
}
}
void initialize_data(float **data, int I, int J)
{
for(int i=0;i<I;i++)
for(int j=0;j<J;j++)
data[i][j] = i * 6 + j + 1;
}
int main(int argc, char *argv[])
{
float data[4][6];
int I=4;
int J=6;
initialize_data((float **)data, 4,6);
print_data((float **)data, 4, 6);
return 0;
}
The above program will cause a failure and raise a EXC_BAD_ACCESS signal. GDB outputs:
Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_PROTECTION_FAILURE at address: 0x00007fff5fc0131a
0x0000000100000de6 in initialize_data (data=0x7fff5fbff348, I=4, J=6) at simple.c:16
16 data[i][j] = i * 6 + j + 1;
I know this is really stupid simple, but I'm at my wits' ends trying to figure out this simple thing. Could someone point me in the right direction for this?

Answer 1

void print_data(float **data, int I, int J)
expects an array of pointers to (the first element of arrays of) float.
But when you pass
float data[4][6];
you pass a pointer to float[6].
So in print_data, an access to
data[i]
reads sizeof(float*) bytes at an offset of i * sizeof(float*) bytes after what address data holds, and interprets these bytes as a float* that it then dereferences (after adding a suitable offset) in data[i][j].
So when you pass your 2D array, some float values are interpreted as pointers and then followed. That often leads to a segmentation fault.
You can either declare
void print_data(float (*data)[6], int I, int J)
and pass your 2D array, or you need to pass an array of pointers,
float *rows[4];
for(i = 0; i < 4; ++i) {
rows[i] = &data[i][0];
}
and pass rows. Or, the third possibility is to pass and expect a flat array
void initialize_data(float* data, int I, int J) {
for(i = 0; i < I; ++i) {
for(j = 0; j < J; ++j) {
data[i*J + j] = whatever;
}
}
}
and pass &data[0][0] from main.

Answer 2

A bi-dimensionnal array is not evaluated as a pointer to pointer, so you need to use an array of a pointer to array in your prototype:
void print_data(float data[4][6]);
void print_data(float (*data)[6]);