Program crashes with fclose

Question

I am working with the following text file: https://mega.nz/#!EN4iRJxA!VtKFEl9dlQHWHIzcgfOfpXTt9_ill_YkgsLWL3nORLg And the following code:

    #include <stdio.h>
    #include <stdlib.h>

    typedef struct
    {
        int id;
        char lastname[25];
        char name[20];
        float average;
    }student;

    int main(){
        int i;
        student *pointer= (student*) malloc(3);

        FILE *file= fopen("Struct.txt", "r");
        fread(pointer, sizeof(student), 3, file);
        for(i= 0; i< 3; i++){
            printf("Id: %d\nName: %s\nLast name: %s\nAverage: %.2f\n",
                (pointer+ i) -> id,
                (pointer+ i) -> name,
                (pointer+ i) -> lastname,
                (pointer+ i) -> average);
        }
        fclose(file);
        system("pause");
        return 0;   
    }

When I delete the fclose function it works but I checked it and seems to be correct. What am I missing?

The file `struct.txt` from the download link is anything but a text file. Two thirds (119 out of 168) bytes are null bytes; text files do not contain any null bytes. — Jonathan Leffler, Nov 15 '15 at 05:12
I don't know why, a previously program saved it that way. I think it is correct as this program reads it correctly. — , Nov 15 '15 at 05:23

artm · Accepted Answer · 2015-11-15T05:36:13.797

5

It has nothing to do with your fclose, instead you need to fix two things:

You need to check the return error of fopen:

if( NULL == file )
{
    printf( "Cannot open\n" );
    exit( 1 );
}

Your malloc code doesn't look right, should be: student *pointer= malloc(3 * sizeof(student));

edited Nov 15 '15 at 05:36

answered Nov 15 '15 at 05:01

artm

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`student *pointer= (student*) malloc(3 * sizeof(student));` should be `student *pointer= malloc(3 * sizeof(student));` – Spikatrix Nov 15 '15 at 05:32
ok I see - http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc thanks – artm Nov 15 '15 at 05:35
Yes, I thought that too, but if I delete the cast my compiler complains and doesn't compile. – Nov 15 '15 at 18:40
what compiler do you use? It should work, see another duplicate Q here: http://stackoverflow.com/questions/33798608/c-pointer-memory-allocation – artm Nov 19 '15 at 09:01

score 3 · Answer 2 · answered Nov 15 '15 at 05:09

Your program isn't allocating memory the way you think it is. malloc(3) returns three bytes of memory. Most likely the file pointer is immediately after it in memory

[ | | ][*FILE]

When you read in the file, the data first goes in to the three bytes of memory you've allocated. But since it doesn't fit, it keeps going and overwrites the memory containing the pointer to the file too.

[student data student data]

Then calling fclose closes the file at dat or whatever was read in from the file. This is a programming error, sometimes called a "wild pointer dereference", and if the data read in doesn't point to something that can be closed like a file can (as is happening to you) the program will crash.

The fixes suggested by artm should resolve your problem, but I thought this explanation might help you understand why you saw the crash.

Yes it helped a lot. Thank you for your effort and time! – Nov 15 '15 at 05:25 — , Nov 15 '15 at 05:25

Program crashes with fclose

2 Answers2