confused by C syntax - c

Question

i'm new at c.. and still having trouble at the syntax, hope you can help me... cause i'm stuck at this code
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int main(void){
FILE *stream = NULL;
stream = fopen("studentinfo.txt", "a+");
/*some of initialization were used for testing purposes only*/
char arr[5];
char arr2[5];
int i;
char name[3];
char course[5];
printf("enter details: ");
scanf("%s", arr2);
while(!feof(stream)){
fgets(arr, 100, stream);//i am confused if the line capture was stored at arr[0]
if(strcmp(arr, arr2)==0){//i want to compare
printf("success");//testing
}
printf("%s", arr);//i wonder does fgets just overwrites the new line to arr[0]
}
fclose(stream);
getch();
}
thanks guys...

Answer 1

You're opening studentinfo.txt for appending, but then reading from it (and you don't check the open succeeded
you've allocated 5 characters for arr, but read up to 100 characters into it with the fgets. This will overflow and cause memory corruption
you've allocated 5 characters for arr2, but read an arbitary number of characters into it - this will overflow and cause memory corruption
Fgets reads characters into memory starting at arr. arr[0] is the first character. &arr[0] is the same as arr
What's the getch() at the end for?
Also, "a+" positions the stream at the end of the file, so you won't be able to read anything.

Answer 2

if you have an existing file... and your file has data on it. then you could check if the data you typed is existing on the file or not. i'm not sure if this is what you want.
example if you typed... love
and the file also contains the exact word... love (on one line)
then it will print "success".
if the data you typed is not existing on the file, it will be appended on the file (on the next line).
int main(void){
char arr[5];
char arr2[5];
int i;
int n=0;
FILE *stream = NULL;
FILE *append = NULL;
stream = fopen("studentinfo.txt", "rt");
append = fopen("studentinfo.txt", "a+");
printf("enter details: ");
scanf("%s", arr2);
while(!feof(stream)){
fgets(arr, 6, stream);
if(strcmp(arr, arr2)==0){
printf("success");
} else n=-1;
}
if (n==-1){
fprintf(append, "%s\n", arr2);
}
fclose(stream);
fclose(append);
system("pause");
}

Answer 3

I am not sure why you are opening the stream with a+ because you never actually write to it. Maybe you want to make sure the file exists even if 0 length? You should still check that the open succeeded though.
You are then reading 100 characters into an array of just 5 bytes so you will get a serious memory overwrite if the file really does contain that number.
The scanf is unsafe too of course as the user may enter too many characters (they are actually limited to 4 because there is a NULL terminator that gets read).
At the end you appear to be writing the last line randomly if the user did not enter a matching line from the file.