Why is my program crashing when I debug? - c

Question

I've started to write a program that reads and stores team info and stores in a structure, reorders and prints results.
First i'm trying to read team names and stores them in a member in a structure, then read team scores and store them in another member.
However, as soon as i debug the file, it crashes. It doesn't even start correctly. I get this error
Unhandled exception at 0xFEFEFEFE in PA2.exe: 0xC0000005 Access
violation executing location 0xFEFEFEFE.
I've backtracked enough to figure out that it is somewhere in the while loop but can't figure out what's wrong with it.
I am using visual studio 2012 and i get no errors when i build it.
#include "stdafx.h"
#include "string.h"
#include <stdio.h>
struct team
{
char name[20];
int no_games;
int points;
int goals_scored;
int goals_let;
};
int main(void)
{
FILE *input2a;
FILE *input2b;
int error1;
int error2;
int i;
char team1_name[20];
char team2_name[20];
int team1_goals;
int team2_goals;
struct team teamlist[20];
error1 = fopen_s(&input2a, "C:\\Users\\New PC\\Desktop\\input2a.dat", "r");
i = 0;
while (i < 20)
{
i ++;
fscanf_s(input2a, "%s", teamlist[i].name);
}
error2 = fopen_s(&input2b, "C:\\Users\\New PC\\Desktop\\input2b.dat", "r");
while (fscanf_s(input2b, "%s %d %s %d", team1_name, &team1_goals, team2_name, &team2_goals) !=EOF)
{
if (team1_goals < 0)
{
printf("Team %s has negative goals - Invalid entry\n", team1_name);
}
else if (team2_goals < 0)
{
printf("Team %s has negative goals - Invalid entry\n", team2_name);
}
else
{
}
}
return 0;
}

Answer 1

From only looking at the while loop: you write to memory out of bounds.
struct team teamlist[20];
And in the while:
i = 0;
while (i < 20)
{
i ++;
fscanf_s(input2a, "%s", teamlist[i].name);
}
So you enter the while when i=19 and increase it to 20 which makes it out of the bounds for the array.

Answer 2

The problem is here (see my comment):
i = 0;
while (i < 20)
{
i ++; // was 19, become 20, but maximal allowed value is 19
fscanf_s(input2a, "%s", teamlist[i].name);
}
The name of this problem is ABW (array bounds write). It means, that your program writes outside array (it happens when i==19, so the condition of the loop i < 20 is satisfied, but right after it you increase it: i++). To fix it you need just to swap two lines of the body of this loop:
{
fscanf_s(input2a, "%s", teamlist[i].name);
i++;
}
And much better and simplier it would be to use for loop here:
for (i = 0; i < 20; i++)
fscanf_s(input2a, "%s", teamlist[i].name);
And the last fix: now there is still possible ABW error. The reason of it is simple: size of teamlist[i].name is 20. So, if input file contains long lines, your program will write outside this array. To fix it you can extend "%s this way (we have here "19" instead of "20" because last character is '\0'):
for (i = 0; i < 20; i++)
fscanf_s(input2a, "%19s", teamlist[i].name);
And one more thing about fscanf_s(): this function shall return the number of successfully matched and assigned input items. So in the second loop it is better to check, that it returns 4.
Update:
If nothing happens, let's open documentation:
Unlike scanf and wscanf, scanf_s and wscanf_s require the buffer size
to be specified for all input parameters of type c, C, s, S, or string
control sets that are enclosed in []. The buffer size in characters is
passed as an additional parameter immediately following the pointer to
the buffer or variable. For example, if you are reading a string, the
buffer size for that string is passed as follows:
char s[10];
scanf_s("%9s", s, _countof(s)); // buffer size is 10, width specification is 9
So in your case in means, that first loop should be corrected this way:
for (i = 0; i < 20; i++)
fscanf_s(input2a, "%19s", teamlist[i].name, _countof(teamlist[i].name));
And you need the same fix for next call of fscanf_s in the second loop, because there are two "%s"s in that call.