Thread: instances of globally declared structures unavailable from within main() function

Hello.

I was practicing working with structures and wrote out this program:

Code:

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

struct tree {
       int size, pos;
       };
       
int main()
{
    tree leaf;
    tree *ptr;
    malloc(sizeof *ptr);
    // asterisk indicates get value of
    leaf.size = 1024;
    leaf.pos = 1;
    ptr = &leaf;
    printf("%d\n", ptr->size, "\n", ptr->pos);
    free(ptr);
    getchar();
}

But the compiler complains about 'Undeclared first use in function main()", when I'm trying to create instances of the tree structure.

I've gone back over my notes, and can't find what's wrong, in fact the declaration for the leaf instance was taken right from the tutorial.

Am I missing something? I'd be grateful if someone could point me in the right direction for this.

Thanks.

Hussein.

you're freeing ptr, when ptr has no memory allocated to it. Just adding malloc(sizeof *ptr)
doesn't allocate memory to ptr. by the looks of your program though, neither are needed.
the malloc line and the free line can be taken out.

and I doubt that printf is going to do what your expecting it to

Thanks. That worked great.

BTW, that means there's also a mistype in the cprogramming.com tutorial on structures. They don't put the struct keyword in front of the instance.

Thanks again. I'm surprised I didn't think of that myself, lol.

Hussein.

Hi.

True, the printf doesn't print the pos variable. Also, why won't malloc allocate memory for ptr? I read that in these cases it would allocate memory to the largest item in the structure. But as there are two ints here, I would expect it to allocate memory for an int?

Hussein.

let me rephrase, malloc is allocating memory, but your not assigning the memory to anything.
malloc returns a pointer to the allocated memory

OK, also, why does the program wait for user input after printing the size variable (1024)? There are no input functions in the program.

sure there is, getchar()

OK I've gotten the printf to work. Still don't understand the malloc issue. The explanation I'm working from is from the cprogramming lesson on Structures.

Code:

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

struct tree {
       int size, pos;
       };
       
int main()
{
    struct tree leaf;
    struct tree *ptr;
    malloc(sizeof *ptr);
    // asterisk indicates get value of
    leaf.size = 1024;
    leaf.pos = 01;
    ptr = &leaf;
    printf("%d\n", ptr->size);
    printf("%d", ptr->pos);
    free(ptr);
    getchar();
}

malloc(N): allocates N bytes and returns a pointer to the allocated memory

ex.

Code:

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

struct tree {
       int size, pos;
};

int main()
{
    struct tree leaf;
    struct tree *ptr;

    ptr = malloc(sizeof(struct tree));
    ptr->size = 10;
    ptr->pos = 2;

    leaf.size = 1024;
    leaf.pos = 1;

    printf("LEAF: %d, %d\n", leaf.size, leaf.pos);
    printf("PTR: %d, %d\n", ptr->size, ptr->pos);

    free(ptr);
    return 0;
}

now you would have 2 tree structures, leaf and the one pointed to by ptr

Originally Posted by sl4nted

let me rephrase, malloc is allocating memory, but your not assigning the memory to anything.
malloc returns a pointer to the allocated memory

Originally Posted by hpteenagewizkid

OK I've gotten the printf to work. Still don't understand the malloc issue. The explanation I'm working from is from the cprogramming lesson on Structures.

read sl4nted's comment as

Code:

struct tree *ptr =  malloc(sizeof( struct tree));

Kurt

Originally Posted by Happy_Reaper

Correct, but he's right in saying there's a mistake. In the last example of the tutorial, they don't typedef either.

here is a simple example which is in teststruct.cpp file

Code:

#include<stdio.h>

struct test
{
    int data;
};

int main()
{
    test test1;
    
    test1.data = 10;
    
    printf("%d",test1.data);
    
    getchar();
    return 0;
}

this code should work fine with the .cpp extension with no error.

with .c extension this what u will get

Code:

10 teststruct.c `test' undeclared (first use in this function)

ssharish2005