C :: Find The Maximum Value In A Column Of Text File

I have problem with finding the maximum value in a column. This is my code

Code:
#include <stdio.h>
#include <stdlib.h>
int main() {
FILE *fp;
fp = fopen ("C:
ocket1.txt", "r");
float time, altitude, velocity, acceleration, amax, bmax;

[Code] ....

The first loop will print out all of numbers in the velocity column. The second while loop is looking for the maximum value in that column. It is supposed to print out the max velocity in the end of the second loop. However, when I build and run the program, it's crashed. I run debugger and received "Segmentation fault" error. When I eliminate the second loop, it run just fine I use Code::Block for text editor.