Writing 2D array into binary file in C (getting strange results) - c

Question

I'm using this code to write the file:
FILE *f = fopen("out/solution.raw", "wb");
int i, j;
//SIZE = 512
for(i = 0; i < SIZE; i++)
{
fwrite(matrix[i], 1, SIZE, f);
}
fclose(f);
The problem is that when I open the file, it 3 '0' between the numbers, here are 2 screenshots to help you understand what I mean:
This is what I should be getting:
And this is what I'm getting:
As you can see my code is correctly writing each number, but there are three 0 between each number and I have no idea why.
I've also tried this:
fwrite(matrix[i], sizeof(matrix[i][0]), SIZE, f);
But none seems to work, any help would be greatly appreciated.
my matrix is declared as a 2D array of ints, as I need to do some operations with those numbers:
matrix = (int**)malloc(SIZE * sizeof(int*));
for (i = 0; i < SIZE; i++)
{
matrix [i] = (int*)malloc(SIZE * sizeof(int*));
}
I've tried your solution but I can't assign an unsiged char to an int, so I've tried casting it and I get this warning:
cast from pointer to integer of different size.
unsigned char to_write;
for(i = 0; i < SIZE; i++)
{
to_write = (unsigned char)matrix[i];
fwrite(&to_write, 1, 1, f);
}
(code used)
After that this is what I'm getting:
And btw, my data is unsigned.

Answer 1

matrix[i] is a pointer on 32-bit integers. Even if you assign values that hold in 8 bits, that doesn't "compress" the data when you're writing to a binary stream (plus you probably don't write the 512 values but only 512/4 = 128 values)
Your machine is little endian, so you get the LSB first, then 3 zeros, for each value.
So change the type of matrix[i] from int32_t * to char * or unsigned char * depending on what you need, make sure your values are in 8-bit range, and you can use fwrite like you're planning to.
If you cannot change the data type of matrix, use a loop to convert the values
for(i = 0; i < SIZE; i++)
{
uint8_t to_write = matrix[i]; // assign/truncate for 0/255 range
fwrite(&to_write, 1, 1, f);
}
if your data is signed, you have to use int8_t. But in that case, you're not going to be able to write a0 as 160.
EDIT: your edit shows the real type of matrix. matrix[i] is a pointer on data, so you need to use a double loop to dump it, else you're copying the address of the array, not the value
for(i = 0; i < SIZE; i++)
{
const int *row = matrix[i];
for(j = 0; j < SIZE; j++)
{
uint8_t to_write = row[j]; // assign/truncate for 0/255 range
fwrite(&to_write, 1, 1, f);
}
}