probs with structs and functions

I've been asked to create a "file" system if you will that needs to incorporate nested structs. I have a problem passing my structs to my functions.

EX:
struct Disc
{
char arrayofalbums[30];
int numberofalbums;
int totalplayingtime;
} disc[10];

then later on I pass it into a function

file_function(disc);


I get an error saying how my var (disc) cannot be converted from a structure to char. Am I doing something wrong? I'm not trying to pass in the individual fields as arguments but the whole structure.

Can you post the functions prototype and source. (Dont forget the code tags please!)

Quote:

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Originally posted by Unregistered
I've been asked to create a "file" system if you will that needs to incorporate nested structs. I have a problem passing my structs to my functions.

EX:
struct Disc
{
char arrayofalbums[30];
int numberofalbums;
int totalplayingtime;
} disc[10];

then later on I pass it into a function

file_function(disc);


I get an error saying how my var (disc) cannot be converted from a structure to char. Am I doing something wrong? I'm not trying to pass in the individual fields as arguments but the whole structure.

---

Does your function look like this?

void file_function(char whatever);

Well this is what I have so far of my code:

//////////////////////////////////////////
#include <stdio.h>
#include <string.h>

void read_album(char *disc)
{


}



int main()
{


struct Disc
{
char arrayofalbums[30];
int numberofalbums;
int totalplayingtime;
} disc[10];


read_album(disc);


return 0;
}
//////////////////////////////////////////

I know it doesn't do anything right now, I'm just trying to figure out how to pass the structure into the function. If this isn't possible, is there some other alternative other than using global structures?

Well, it really depends on how it's all going to hang together, but if all you want to do is pass the struct to a function, examine this example prog:

Code:

---

#include <stdio.h>

struct mystruct
{
        int num1;
        int num2;
};

void ShowStruct(struct mystruct);

void ShowStruct(struct mystruct myS)
{
        printf("%d %d\n", myS.num1, myS.num2);
}

int main(void)
{
        struct mystruct s;

        s.num1 = 10;
        s.num2 = 20;
       
        ShowStruct(s);
               
        return(0);
}

---

This shows a struct being passed from main() to ShowStruct(). Does that help you understand better?

Yes, I understand your example and see how it works. However, this program uses a global structure. My problem requires that I use a local structure. Is there anything you can show me that will help me do this?

Try this

Code:

---

typedef struct
{
char              arrayofalbums[30];
int              numberofalbums;
int              totalplayingtime;
} DISC;

void read_album(DISC*  localdisc)
{

}
int main()
{
DISC        disc[10];

read_album(&disc[0]);
}

---

>My problem requires that I use a local structure.
You won't be able to pass it to another function then, not without some trickery.

You need to define the structure before the function is called so that you can have it as a parameter to that function. The only other way I can see doing it, is to use void pointers as parameters, and have the struct defined in both the functions. But what's the point?

I'm sure you need to do it the way I and novacain have showed you. Both bits of example code handle the passing differently, one passes the struct, the other passes a pointer to an array of structs. And both use globally defined structs.

Nested structs pass fine. The reason you are getting the error is because you are trying to pass the entire struct on the stack. You need to pass it by address.

Could you maybe show me an example of what you are talking about? I don't quite understand.

>>>My problem requires that I use a local structure.

A local INSTANCE (as the examples provide) or DECLARATION of the structure?

(an instance is the actual variable of type BLAH as opposed to the declaration where you define the elements in the struct BLAH.)

I think this is homework. So I think you are expected to show you can pass a struct (or large variable/array) around between functions. This is better/has to be done by adress. You can use void pointers and type casts and multiple struct declarations to get a local declaration (multiple struct declarations are not used in the real world as they lead to very bad errors).

>>Could you maybe show me an example of what you are talking about? I don't quite understand.

Look at the code I wrote. This passes the address of the instance of the struct.

int iInt=0;

int MyFunct(int *iIndex);//prototype

MyFunct(&iInt);//call to send address of int var

>>However, this program uses a global structure.
No. The struct is local to the main function.

It uses a local struct but is not passed by address which will lead to errors if the struct is large.