assign int to a struct member

shogun1234 · 07-30-2007, 02:11 PM

I am newbie to the c programming and am confused with the concepts.
I create a struct c_list. Then I want to assign an int to the struct member id with head->id = 10. However, after using printf function, the output is something like "id value: 85472". Why doesn't it print 10 directly? Where did I do it wrong?

Thanks in advice,

The source is as below.

Code:

#include <stdlib.h>
#include <stdio.h>
struct c_list{
        struct c_list *next;
        int id;
};
void
init(struct c_list *head)
{
        head = malloc(sizeof(struct c_list));
        //head->next = head;
        head->id = 10;
}

int main(int argc, char **argv)
{
        struct c_list *head;
        init(head);
        printf("id value: %d\n", head->id );
        return 0;
}

Mara · 07-30-2007, 02:18 PM

You can see what the pointer (head) is in the main function and in the init. You'll see that you do not have the allocated structure in main, but just the default value (it's better to set head to NULL at the beginning, that will show you it is not what you want). To get the allocated structure to main you can either use struct c_list **head for init or return struct c_list *.

Corrected code:

Code:

struct c_list *
init(void)
{
        struct c_list *head = malloc(sizeof(struct c_list));
        /* you should check for NULL here */
        //head->next = head;
        head->id = 10;
        return head;
}

int main(int argc, char **argv)
{
        struct c_list *head = NULL;
        head = init();
        if (head)
                printf("id value: %d\n", head->id );
        return 0;
}