Segmentation Fault w/ fwrite

Question

I've a problem with the fwrite function (I checked it using debugging on C::B). This is the code:

struct  studente
{   char Cognome_Nome[30];
    char Matricola [11];
    short Superati;
    float Media_Pesata;
    short Crediti;
};
typedef struct studente STUDENTE;

void main()
{   FILE *fp;
    STUDENTE Dati;

    if((fp = fopen("studente.dat","w+b")) == NULL)
        printf("Error\n");
    else
    {   fflush(stdin);
        printf("Inserire il cognome e nome: ");
        fgets(Dati.Cognome_Nome, 30, stdin);

        fflush(stdin);
        printf("\nInserire la matricola: ");
        fgets(Dati.Matricola, 11, stdin);

        fflush(stdin);
        printf("Inserire il numero di esami superati: ");
        scanf("%hd", &Dati.Superati);

        fflush(stdin);
        printf("Inserire la media pesata: ");
        scanf("%f", &Dati.Media_Pesata);

        fflush(stdin);
        printf("Inserire il numero di crediti: ");
        scanf("%hd", &Dati.Crediti);

        fwrite(&Dati, sizeof(STUDENTE), 1, fp);
    }
}

I receive an Segmentation Fault when fwrite is called. I can't understand where is the problem. I checked the fwrite prototype and I think it's all right.

Thank you in advance.

Plz Refer the following discussion http://stackoverflow.com/questions/2979209/using-fflushstdin — Jeyaram, Sep 11 '14 at 16:17
Maybe your system is old and does not understand the `'h'` in `"%h..."`? What OS? — chux - Reinstate Monica, Sep 11 '14 at 16:26
@ Jeyaram I've used fflush() just for trying. I use a _fflush() function that I found on the web. @chux I use Windows 8 but also with the last Ubuntu, the code doesn't work. — Federico Cuozzo, Sep 11 '14 at 16:34
You have done `#include .h>`? Otherwise odd things might happen. Also, it should be `int main`, but that's unlikely to cause your problem. — Tom Tanner, Sep 11 '14 at 16:35
1) If system does not understand the 'h', change to `int i; scanf("%d", &i); Dati.Superati = i;` and same for `Dati.Crediti`. 2) More detail on "code doesn't work" - do you mean it also seg faults.? 3) sorry I asked what OS - meant to ask what compiler? — chux - Reinstate Monica, Sep 11 '14 at 16:36
Yes, I have included and I use int main(), but, nothing to do. It doesn't work. — Federico Cuozzo, Sep 11 '14 at 16:37
instead of fwrite(&Dati, sizeof(STUDENTE), 1, fp), try fwrite(&Dati, 1, sizeof(STUDENTE), fp). that might work — Haris, Sep 11 '14 at 16:38
@ralph It _might_ help but `size_t fwrite(const void * restrict ptr, size_t size, size_t nmemb, FILE * restrict stream);` (`nmemb` elements whose size is specified by `size`) and OP appears to be calling it correctly. For style: I would recommend `fwrite(&Dati, sizeof Dati, 1, fp);`, but that is not the major issue here. — chux - Reinstate Monica, Sep 11 '14 at 16:40
Changing `"%hd"` to `"%d"` is not good unless the fields are changed also from `short` to `int`. — chux - Reinstate Monica, Sep 11 '14 at 16:50
have you checked the exact code you've posted on ubuntu? it works fine on mine — Ingo Leonhardt, Sep 11 '14 at 16:52

chux - Reinstate Monica · Answer 1 · 2014-09-11T17:05:02.823

Your compiler (e.g VC10 @alk) does not understand the 'h' in format specifiers which, which I believe, is a C99 addition. 3 solutions:

Change to a new compiler.

Read the short via a temporary variable.

int i;
scanf("%d", &i);
Dati.Superati = i;

Make the 2 fields int rather than short and use "%d"

struct  studente {
   ...
   int Superati;
   int Crediti;
} 
...
scanf("%d", &Dati.Superati);
...
scanf("%d", &Dati.Crediti);

Other minor suggestions:

// @ Tom Tanner
// void main()
int main(void)

// Avoid magic numbers
// fgets(Dati.Cognome_Nome, 30, stdin);
fgets(Dati.Cognome_Nome, sizeof Dati.Cognome_Nome, stdin);

// @ Jeyaram
// Do not use fflush(stdin). Better methods exist to handle stray input data
// fflush(stdin);

// Less error prone and easier to maintain code
// fwrite(&Dati, sizeof(STUDENTE), 1, fp);
fwrite(&Dati, sizeof Dati, 1, fp);

// Check I/O function results

Good call. That's what I was thinking. The `scanf` into the final field, Crediti, is stomping on `fp` which likely appears immediately after Crediti in the stack frame. — Colin D Bennett, Sep 11 '14 at 16:58

score 0 · Answer 2 · answered Sep 11 '14 at 16:46

0

Probably what is happening is this:

A call to fgets() or scanf() is storing data beyond the structure Dati. This stomps on the fp pointer variable, which is likely stored immediately after Dati on the stack frame. Then you call fwrite() which dereferences fp and boom you have undefined behavior and a segmentation fault.

I bet if you comment out all scanf and fgets functions that store into Dati and replace them with hard-coded assignments to Dati fields, you'll see that the crash is fixed.

answered Sep 11 '14 at 16:46

Colin D Bennett

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which `fgets()` or `scanf()` are you thinking of? They all look good to me – Ingo Leonhardt Sep 11 '14 at 16:49
Right, I think they look good. But I bet that `fp` is getting stomped by someone, and that's the only thing I see as a possibility. – Colin D Bennett Sep 11 '14 at 16:57

Segmentation Fault w/ fwrite

2 Answers2