Malloc and Realloc doesn't work (C11 - CLion)

Question

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

typedef struct Ogrenciler {
    int no;
    char adi[50];
    char soyadi[50];
    double vize;
    double final;
    double notu;
} Ogr;

int ogrenciSayisi = 0;

void KayitEkle(Ogr *ogrenci) {
    int simdikiOgr = ogrenciSayisi;
    if (ogrenciSayisi == 0) {
        ogrenciSayisi++;
        ogrenci = (Ogr *) malloc(ogrenciSayisi*sizeof(Ogr));
    } else {
        ogrenciSayisi++;
        ogrenci = (Ogr *) realloc(ogrenci, ogrenciSayisi * sizeof(Ogr));
    }
    printf("No:");
    scanf("%d", &ogrenci[simdikiOgr].no);
    printf("Adi:");
    scanf("%s", ogrenci[simdikiOgr].adi);
    printf("Soyadi:");
    scanf("%s", ogrenci[simdikiOgr].soyadi);
    printf("Vize:");
    scanf("%lf", &ogrenci[simdikiOgr].vize);
    printf("Final:");
    scanf("%lf", &ogrenci[simdikiOgr].final);
    ogrenci[simdikiOgr].notu = (ogrenci[simdikiOgr].vize * 0.4) + (ogrenci[simdikiOgr].final * 0.6);
    printf("Notu: %lf", ogrenci[simdikiOgr].notu);
    printf("\n\n");
    printf("Adi: %s\nNo: %d\nVize: %lf\nFinal: %lfNotu: %lf\n",
           ogrenci[simdikiOgr].adi, ogrenci[simdikiOgr].no, ogrenci[simdikiOgr].vize, ogrenci[simdikiOgr].final,
           ogrenci[simdikiOgr].notu);
}

int main() {
    int c;
    while (c != 5) {
        printf("\n1-\tYeni Kayit Ekle\n2-\tKayit Sil\n3-\tKayitlari Listele\n4-\tOrtalama Hesapla\n5-\tCikis\n");
        scanf(" %d", &c);
        Ogr *ogrenci;
        switch (c) {
            case 1:
                KayitEkle(ogrenci);
                break;
            case 2:
                KayitSil(ogrenci);
                break;
            case 3:
                KayitListele(ogrenci);
                break;
            case 4:
                OrtHesapla(ogrenci);
                break;
            case 5:
                printf("Cikiliyor");
                break;
            default:
                printf("Gecerli bir girdi yapiniz\n");
                break;
        }
    }

    return 0;
}

As u can see, I use malloc() and realloc() for my typedef struct and I'm able to enter only one entry. When I tried adding a new entry (switch case: 1) it doesn't work and crashes after this section:

printf("No:");
scanf("%d", &ogrenci[simdikiOgr].no);

At first, I tried I used calloc(ogrenciSayisi*10*sizeof(Ogr)) but it was created only one space. After that, in the debugger (CLion's) after realloc section, ogrenci pointer becomes a null pointer.

Edit: I'm not trying to return a value. As I know (int a) equals (int a[ ]) so KayitEkle(ogrenci) and void KayitEkle (Ogr ogrenci) seems legit to me. And my ogrenci should be empty in the first place so (Ogr *ogrenci=NULL) is correct as you said right?

Edit2: In malloc section 10 is a mistake. I fixed it. I was trying something and I forgot to delete it.

Unrelated to your problem, but you use c without initialization. That means its value is indeterminate and will seem almost random. Don't do that. — Some programmer dude
As for your problem, arguments to functions are passed by value. That means they are copied, and modifying the copy will not change the original. Do some research about emulating pass by reference in C. Oh, and don't forget to initialize the variable ogrenci too! — Some programmer dude
Possible duplicate of How to return a pointer as a function parameter — Ken Y-N
What @Someprogrammerdude said, was that your ogrenci variable is changed only locally when you realloc. main has its own copy which is unchanged. That is why you have problems. You need to understand how variables and pointers work or you will have this problem many times. Read stackoverflow.com/questions/5286453/… — Prof. Falken
Also worth noting, your realloc will actually reallocate to a smaller location, which does not appear to be what you intended to do. You have a 10x multiplier in the malloc for some reason. Additionally, you should always check the return value from malloc and realloc to confirm that they have actually returned memory to you. — David Hoelzer

KamilCuk KamilCuk · Accepted Answer · 2018-05-07T09:56:37

You pass ogrenci pointer by value to KayitEkle(), you modify it's value inside it, yet not return it's modified value to main(). You need to pass ogrenci value using a pointer (ie. KayitEkle(&ogrenci)) or return the new value to the called (ie. ogrenci = KayitEkle(ogrenci)). Example below is with the latter. ogrenci pointer is inside the while loop, so it will be reinitialized every time the loop runs, probably you meant to put it outside of the loop so it's value is preserved. Local variables have undefined (read: any) value without initialization, so you need to explicitly initialize ogrenci to NULL, if you need. See Initialization. You don't need to check for ogrenciSayisi == 0 when ogrenci == NULL, because realloc(NULL, ...) is equal to malloc(...). See realloc.

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

typedef struct Ogrenciler {
    int no;
    char adi[50];
    char soyadi[50];
    double vize;
    double final;
    double notu;
} Ogr;

int ogrenciSayisi = 0;

// or void KayitEkle(Ogr **ogrenci) and then use *ogrenci
Ogr *KayitEkle(Ogr *ogrenci) {
    int simdikiOgr = ogrenciSayisi;

    ogrenciSayisi++;
    ogrenci = realloc(ogrenci, ogrenciSayisi*sizeof(Ogr));

    printf("No:");
    scanf("%d", &ogrenci[simdikiOgr].no);
    printf("Adi:");
    scanf("%s", ogrenci[simdikiOgr].adi);
    printf("Soyadi:");
    scanf("%s", ogrenci[simdikiOgr].soyadi);
    printf("Vize:");
    scanf("%lf", &ogrenci[simdikiOgr].vize);
    printf("Final:");
    scanf("%lf", &ogrenci[simdikiOgr].final);
    ogrenci[simdikiOgr].notu = (ogrenci[simdikiOgr].vize * 0.4) + (ogrenci[simdikiOgr].final * 0.6);
    printf("Notu: %lf", ogrenci[simdikiOgr].notu);
    printf("\n\n");
    printf("Adi: %s\nNo: %d\nVize: %lf\nFinal: %lfNotu: %lf\n",
           ogrenci[simdikiOgr].adi, ogrenci[simdikiOgr].no, ogrenci[simdikiOgr].vize, ogrenci[simdikiOgr].final,
           ogrenci[simdikiOgr].notu);
    return ogrenci;
}

int main() {
    int c = 0;
    Ogr *ogrenci = NULL;
    while (c != 5) {
        printf("\n1-\tYeni Kayit Ekle\n2-\tKayit Sil\n3-\tKayitlari Listele\n4-\tOrtalama Hesapla\n5-\tCikis\n");
        scanf(" %d", &c);
        switch (c) {
            case 1:
                ogrenci = KayitEkle(ogrenci);
                break;
            case 2:
                ogrenci = KayitSil(ogrenci);
                break;
            case 3:
                ogrenci = KayitListele(ogrenci);
                break;
            case 4:
                ogrenci = OrtHesapla(ogrenci);
                break;
            case 5:
                printf("Cikiliyor");
                break;
            default:
                printf("Gecerli bir girdi yapiniz\n");
                break;
        }
    }

    // it's nice to free things
    free(ogrenci);

    return 0;
}

Malloc and Realloc doesn't work (C11 - CLion)

2 Answers