Why does it print that my list is empty?

Question

I have an add function and use a realloc to take in the data from a struct and store it. But when I print it out, it says that it's empty, which comes from my display function. How can I fix this?

void additem(food *head, int index) {
    head = (food*)realloc(head, sizeof(food)*(index + 1));
    if (head != NULL) {
        printf("Enter name: ");
        gets((head + index)->name);
        printf("Type unit of meauserement: ");
        gets((head + index)->unit);
        printf("Enter value: ");
        scanf("%d", &(head + index)->data);
    }
    else {
        printf("\nExiting!!");
        free(head);
    }
}

My main function looks like this:

food *foods = NULL;
int option, nofood = 0;
while (1) {
    system("CLS");
    printf("1 - add food\n2 - print shopping list\n3 - delete item\n4 - change amount\n5 - end\nWhat do you want to do? ");
    scanf("%d", &option);
    clear();
    switch (option) {
    case 1: additem(foods, nofood);
        break;

[___DO NOT___ use `gets()`, it is dangerous. use `fgets()` instead.](https://stackoverflow.com/a/41383540/2173917) — Sourav Ghosh, Oct 25 '17 at 12:35
[Please see this discussion on why not to cast the return value of malloc() and family in C..](https://stackoverflow.com/q/605845/2173917) — Sourav Ghosh, Oct 25 '17 at 12:35
You know what? Outside the function, `head` remains unchanged!! — Sourav Ghosh, Oct 25 '17 at 12:36
Also `noFood` never changes resulting in `index` within `additem` being 0 forever. — Gerhardh, Oct 25 '17 at 12:39
@SouravGhosh This doesn't help because this is exactly what was asked. Be more constructive. — Jean-Baptiste Yunès, Oct 25 '17 at 12:43
@Adrian Kindly show your research / debugging effort so far. Please read [Ask] page first. — Sourav Ghosh, Oct 25 '17 at 12:45
"which comes from my display function" Why don't you even show your display function? I assume it does not come from this function but from passing parameters by value instead of reference. — Gerhardh, Oct 25 '17 at 12:55
My display function looks like this: pastebin.com/WjEfCHgH @Gerhardh — Adrian, Oct 25 '17 at 13:05

Paul Ogilvie · Answer 1 · 2017-10-25T12:56:25.430

1

The updated value of head is never returned to the caller. Use:

void additem(food **head, int index) {

    *head = (food*)realloc(*head, sizeof(food)*(index + 1));

and call it like:

    additem(&foods, nofood);

When you pass food you pass the value of food. Although the function updates this value, the value is not communicated back to the caller. This is called "call-by-value". If you pass the address of food, and assign to the thing (variable) that head points to, you have changed the caller's value. This is called "call-by-reference" (though strictly speaking, passing any pointer is call-by-reference).

edited Oct 25 '17 at 12:56

answered Oct 25 '17 at 12:40

Paul Ogilvie

25,048
4
23
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I did that, and I wrote the get function like this: gets((*head + index)->name); etc. But it still doesn't work. – Adrian Oct 25 '17 at 12:55
That is because you have to increment `nofood` too. – Paul Ogilvie Oct 25 '17 at 12:57
Okay, but I can't get past the realloc function because a breakpoint gets triggered. – Adrian Oct 25 '17 at 13:03
Use a debugger to check what you do wrong. I can't see what you did exactly. And increment `nofood` _after_ calling `additem`, of course. – Paul Ogilvie Oct 25 '17 at 13:07
I made it work, and I added a noFood++; in main after calling additem ofcourse, but it still prints that the list is empty from my display function. – Adrian Oct 25 '17 at 13:27
Now it prints the list, and I can add several items, but it breaks on my print within my display function. – Adrian Oct 25 '17 at 13:47
Show your print function. – Paul Ogilvie Oct 25 '17 at 14:42

score 0 · Answer 2 · answered Oct 25 '17 at 12:43

0

Consider this simple program:

void foo(int bar)
{
  bar = bar + 1;
}

int main()
{
  int test = 4;
  foo(bar);
  printf("bar = %d\n", bar);
}

What is the output of this simple program? If you know the answer you know the answer to your problem.

answered Oct 25 '17 at 12:43

Jabberwocky

48,281
17
65
115

Why does it print that my list is empty?

2 Answers2