Sudoku solver in C

Here's a little C program I wrote a while ago. It solves Sudoku's.

It's very basic but I figured someone might want to see it.

Input files should be in this format:

9x9
__6__53__
_3_____52
_2_47_6__
_____9_6_
__5___7__
_8_3_____
__9_82_3_
87_____1_
__35__9__

The output will then look like this:

$ ./sudokusolver sudoku_medium.txt
Reading sudoku of size 9x9.
Initial sudoku:
__6__53__
_3_____52
_2_47_6__
_____9_6_
__5___7__
_8_3_____
__9_82_3_
87_____1_
__35__9__

The sudoku was succesfully solved.

Solved sudoku:
196825374
437691852
528473691
342759168
965218743
781364529
659182437
874936215
213547986