Realloc returning NULL

Question

int main() {
    struct lottery *array;      

    array = (struct lottery *)malloc(3000 * sizeof(struct lottery));       
    int opt, counter;

    menu1();
    scanf("%d", &opt);
    if (opt == 1)
        Load(array, &counter);
    else
        exit("0");
    menu2();
    counter--;
    scanf("%d", &opt);
    while (opt != 7) {
        switch (opt) {
        case 1:
            Save(array);
            break;
        case 2:
            Enterd(array, &counter);
            printf("%d\n", counter);
            break;
        }
        menu2();
        scanf("%d", &opt);
    }
    return 0;
}

void Enterd(struct lottery *a, int *count) {
     struct lottery *b;
     int x;

     (*count)++;
     x = *count;

    printf("Your new data will have an ID of %d\n",x);
    a[x].aa = x;

    b = (struct lottery *)realloc(a, x * sizeof(struct lottery));
    if (b == NULL) {
        printf("Memory could not be allocated for your new input.Program will now exit...\n");
        exit("0");
    }

    a = b;

    printf("What is the date of your new draw?\n");
    scanf("%d/%d/%d", &a[x].date1.day, &a[x].date1.month, &a[x].date1.year);
    printf("Now please insert the 5 non-joker numbers\n");
    scanf("%d%d%d%d%d", &a[x].n1, &a[x].n2, &a[x].n3, &a[x].n4, &a[x].n5);
    printf("What is the 'Joker' number of this draw?\n");
    scanf("%d", &a[x].joker);
    printf("Your input is now complete.");
}

I am writing a protect about some lottery files. I have this problem in my function which is adding more data to the lottery array. Whenever x contains 1989, my realloc call returns NULL. I set x to be 1985 and i could add 4 more inputs to the array, but whenever x is 1989, it still returns NULL. My question is: is there something wrong with the code or I am still running out of memory?

We need to see a bit more code; could you paste up a `main` showing how this function is called? — Bathsheba, Jan 20 '17 at 14:22
`exit` takes an integer not a string - replace `exit("0")` with `exit(0)`; — Govind Parmar, Jan 20 '17 at 14:24
Run your code under `valgrind`. You may run into undefined behavior because of something else in your code. — Jongware, Jan 20 '17 at 14:25
... And compile it with all the warnings on. When you have achieved no warnings - post that code — Ed Heal, Jan 20 '17 at 14:29
Thanks for the edit. (I was punting on the idea that you were not calling `malloc` initially.) I think now you need to submit the shortest example you can that demonstrates this behaviour. IMHO this is due to some undefined behaviour in your code somewhere. — Bathsheba, Jan 20 '17 at 14:35
I editted the code 1 more time.Now thats all my program,there is a file inculded but i think it doesnt matter because my Load function seems to be working perfectly — Edward, Jan 20 '17 at 14:36
@Edward - A person did format the code for you. Please edit that code as it was readable. I have rollback to that version — Ed Heal, Jan 20 '17 at 14:38
Can you please explain what do you mean by formating the code?It appears proper in my computer i think — Edward, Jan 20 '17 at 14:39
Indentation - No extra blank lines. Spacing - makes it readable. Compare your version with the above — Ed Heal, Jan 20 '17 at 14:44
You need to update your question with properly formatted code first. — dbush, Jan 20 '17 at 14:50
Your `x` becomes a 0 due to `counter` becomes -1 due to you missed the initialization/ — 0andriy, Jan 20 '17 at 15:46

score 5 · Answer 1 · answered Jan 20 '17 at 14:58

5

If realloc returns null, firstly print out the amount of memory you are asking to allocate. If it is a negative number or a huge amount, there's the problem. If it is a sensible amount, and you have a halfway decent machine, it's most unlikely you are out of memory. So the malloc() system must have been corrupted in some way. Either you are passing an invalid pointer, or you have written past the end of a block, maybe in a totally unrelated part of the program.

answered Jan 20 '17 at 14:58

Malcolm McLean

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I asked the question because i have 8GB or ram so i though it was unlike to get a memory full. – Edward Jan 20 '17 at 15:00
Modern machines "over commit" memory anyway, so even if the machine can't honour the request it claims to honour it, then starts shutting down apps if the memory is actually used. – Malcolm McLean Jan 20 '17 at 15:06

score 0 · Answer 2 · answered Jan 20 '17 at 14:50

0

Two significant errors:

C array indexing starts at zero, so after you realloc to x * sizeof(thing), only elements zero to x-1 are valid. Accessing element x will cause chaos.

Second, a = b modifies the local copy of a, but not the value array that you wanted it to...

answered Jan 20 '17 at 14:50

Roddy

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1st:So you are saying i sould change x to x-1? – Edward Jan 20 '17 at 14:57
And about the 2nd,it worked for me in the Load function,it properly changed my array in main too. – Edward Jan 20 '17 at 14:58
@Edward 1st, Maybe. Your code's too convoluted to easily tell. – Roddy Jan 20 '17 at 15:14
@Edward 2nd: If realloc moved the memory, `array` gets left pointing at the old data. One fix is to make `enterd` return a pointer to the new data. Then use `array = enterd(array, &counter)` – Roddy Jan 20 '17 at 15:17

score 0 · Answer 3 · answered Jan 20 '17 at 15:44

realloc can change the base address, but array pointer is passed by value, so a local reallocation is not visible in the main and generates some trouble.

You also reallocate to 0-sized array, probably not what you want, please use x+1 as the number of records in the reallocation. More, you access index x before reallocation, which is undefined behavior as before reallocation size is x-1, so move the line a[x].aa = x after reallocation.

Also please initialize your variables (like counter).

Realloc returning NULL

3 Answers3

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