How to use typedef variables to functions as arguments

Question

typedef struct {
    int age, height, weight;
} Person;
//this is in global variables

...

Person *p = (Person *)malloc(sizeof(int) * 20); //this is local variable in main function

In this situation, I want to make functions that use typedef variables as arguments.

For example, if I make a function like void print_line() that print age, height, weight , what should I write in ()? At first I wrote void print_line(Person); But VS says it's an error.

Please help me.

`typedef` defines a "shortcut" to an existing type, it doesn't define a variable. `Person` is a type, not a variable — Alex Lop., Oct 03 '20 at 16:31
Thank you. I wrote `void print_line(Person *p) and it works. — ssrr00, Oct 03 '20 at 16:45
The `malloc()` line in your example looks dangerous. You should use `sizeof` to get the size of the `Person` type rather than trying to figure out how many `int` worth of space you need for one, eg. `malloc(sizeof(Person) * 5)` for 5 of them. — Dmitri, Oct 03 '20 at 16:56
Also see [c - Do I cast the result of malloc? - Stack Overflow](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — MikeCAT, Oct 03 '20 at 17:20
I don't get it. Is the size of Person decided with `N` and `sizeof(int)`? — ssrr00, Oct 03 '20 at 17:32

Support Ukraine · Answer 1 · 2020-10-03T17:48:23.253

2

How to use typedef variables to functions as arguments

Just like you use the build-in types.

That's the whole idea behind typedef - you create your own type and use it like any other type.

If you want to pass an int you do

void foo(int input) {...}

If you want to pass you own type you do

typedef ...... myType;

void foo(myType input) {...}

Off topic:

Your code:

Person *p=(Person*)malloc(sizeof(int)*20);

is wrong! Don't use sizeof(int) as your type Person doesn't have that size. Less important - but still - in C you don't cast malloc. The way to do it is:

    Person *p=malloc(20 * sizeof *p);  // Allocate memory for 20 Person

or

    Person *p=malloc(20 * sizeof(Person));   // Allocate memory for 20 Person

But the first is to be preferred.

edited Oct 03 '20 at 17:48

answered Oct 03 '20 at 17:33

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Thank you! Now I got it! You such a genius! – ssrr00 Oct 03 '20 at 17:41
1

@RiRiHui Genius... well - no I'm not - I've just spend quite some time programming C – Support Ukraine Oct 03 '20 at 17:47

How to use typedef variables to functions as arguments

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