CODEWITHSUNDEEP
cprintingstructscanfprintf
OhSnap
OhSnap

Reputation: 376

C fscanf and pointer

After trying to read/write binaries failed miserably I tried to do the same thing with fscanf and fprintf and it seemed to work fine, but again .. reading doesn't work at all and once again I can't really tell why:

struct + pointer:

typedef struct flug
{
    int         flugnummer;
    char        flugziel[50];
    enum TAG    flugtag;
    int         flugzeit_stunde;
    int         flugzeit_minute;
    int         gateway;
    char        status[10];
    struct flug *next;
}FLUG;

typedef FLUG *ELEM_ZGR;

global variables:

enum TAG {
Sonntag,
Montag,
Dienstag,
Mittwoch,
Donnerstag,
Freitag,
Samstag
};

static ELEM_ZGR first;
char const datei[] = "ddslist.txt";

write:

int fluege_sichern() {
ELEM_ZGR curr;
FILE *fp;

curr = first;

    if (fopen_s(&fp, datei,"a+") != 0)
    {
        printf("\nDatei %s nicht zum Anhaengen zu oeffnen",datei);
        PAUSE;
        exit(1);
    }

    while (curr != NULL) {
        fprintf(fp,"%d %s %d %d %d ",
            curr->flugnummer, curr->flugziel, curr->flugzeit_stunde, curr->flugzeit_minute, curr->gateway);
        fclose(fp);

        curr = curr->next;
    }

}

read:

void fluege_laden() {
ELEM_ZGR curr;
FILE *fp;

int i = 0;
curr = NULL;

    if (fopen_s(&fp, datei,"r") != 0)
    {
        printf("\nDatei %s nicht zum Lesen zu oeffnen",datei);
        PAUSE;
        exit(1);
    }

    printf("\n\nArtikelliste\nArtikelnummer  Artikelbezeichnung  Artikelpreis");

    while (fscanf_s(fp,"%d %s %d %d %d ",
            &curr->flugnummer, curr->flugziel, &curr->flugzeit_stunde, &curr->flugzeit_minute, &curr->gateway) != EOF)
    {
           printf("\n%d %s %d %d %d ",
            curr->flugnummer, curr->flugziel, curr->flugzeit_stunde, curr->flugzeit_minute, curr->gateway);
           if (++i%10==0)
               PAUSE;
    }

    fclose(fp);
}

Please help me out here. Even a little hint would be a huge help. [EDIT1]

void fluege_laden() {
ELEM_ZGR curr;
FILE *fp;

int i = 0;
curr = (ELEM_ZGR)malloc(sizeof(struct flug));

    if (fopen_s(&fp, datei,"r") != 0)
    {
        printf("\nDatei %s nicht zum Lesen zu oeffnen",datei);
        PAUSE;
        exit(1);
    }

    printf("\n\nArtikelliste\nArtikelnummer  Artikelbezeichnung  Artikelpreis");

    while (fscanf_s(fp,"%d %s %d %d %d ",
            &curr->flugnummer, curr->flugziel, &curr->flugzeit_stunde, &curr->flugzeit_minute, &curr->gateway) != EOF)
    {
           printf("\n%d %s %d %d %d ",
            curr->flugnummer, curr->flugziel, curr->flugzeit_stunde, curr->flugzeit_minute, curr->gateway);
           if (++i%10==0)
               PAUSE;
    }

    fclose(fp);
}

Upvotes: 0

Views: 262

Answers (2)

Mike
Mike

Reputation: 49373

In your read function curr is a typedef to a FLUG *, and you initialized it as NULL:

curr = NULL;

Then you try to access deference it in your fscanf:

while (fscanf_s(fp,"%d %s %d %d %d ", &curr->flugnummer,...

That's no good. If you have a pointer and you want to store something to it, you need to give it some memory:

curr = malloc(sizeof(FLUG));

Then you should be able to use it. Once you're done you'll need to call free(curr) to release that memory as well.

Upvotes: 0

unwind
unwind

Reputation: 399723

Look:

void fluege_laden() {
  ELEM_ZGR curr;
  FILE *fp;

  int i = 0;
  curr = NULL;  /* NULL! */

When loading, curr is NULL. You cannot write to a NULL pointer, that's undefined behavior. You need to allocate memory.

Also, consider not including the pointer in the typedef, it's hard to understand and remember that ELEM_ZGR is a pointer. Pointer semantics are important in C, it's best to make it clear to all readers of the code what's going on.

Upvotes: 1

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