How do I check if an ID already exists in a file

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I have to write a code to check if an ID already exists in a file.

What I have tried:

This is the code i wrote :

printf("Please enter the ID of the student : ");
        scanf("%d", &newstudentID);
        printf("\n");
        input= fopen("stData.txt", "r");
        while((fread(buffer, sizeof(int), 6, input))!= EOF){
            strcmp(newstudentID, buffer) == diff;
            if(diff=0){
                printf("This student already exists.");
            }

When I run the code, it prompts me to enter the ID, when I do enter an ID the code simply crashes.

Posted 21-Mar-22 5:02am

Réda 2022

Updated 21-Mar-22 5:23am

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jeron1 21-Mar-22 11:22am

You should post the entire code if possible.

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k5054 · Accepted Answer · 2022-03-21T05:23:00

So we have

C

scanf(%d, &newstudentID)

so presumably newstudentID is of type int. Then a bit later we have

strcmp(newstudentID, buffer)

Here, we are passing an int to strcmp as the first argument, but strcmp() is expecting a char *. So strcmp() uses whatever the student ID is as an address into memory - perhaps 42, which is almost certainly not an address that you have read access to, and tries to use that as a starting point for a string. Bad Things happen at this point.
There's no reason you can't use fscanf() to read "stData.txt" e.g.

C

int testID;
while( fscanf(input, "%d", &testID) {
    if(testID == newstudentID) {
       printf("ID already in use\n");
    }
}