Question

I’m working on assignment for an operating systems class. We are given code to use to help us with our assignment, but I have little programming experience with C, and I can’t figure out how to use it. What I’m trying to do is print the information at the head of the list. The list is a list of structs defined as

typedef struct 
{
    char name[NAME_MAX];
    int  lifetime;
} pcb_t;

int 
List_head_info ( List_t *list, void **data )
{
     int all_ok = 0;

     *data = NULL;
     if ((list != NULL) && (list->head != NULL)) {
         *data = list->head->data;
         all_ok = 1;
     }

     return all_ok;
}

I tried to display them with:

printf("test: %s", List_head_info(&processes, (void *)pcb)->name);

but I am given the error invalid type argument a->a when compiling.

Answer 1

When you call List_head_info(), you will get back two things:

1. A pointer (void *) to the head data, or NULL.
2. A status indicating whether the pointer is non-NULL.

If it returns successfully, you can convert (coerce, or cast) the void * to a pcb_t * and then use that to print the data.

---

How would I do that specifically?

Probably something a bit like this:

List_t list;

...code to initialize and maybe add things to the list...

void *head_data = 0;
if (List_head_info(&list, &head_data))
{
    pcb_t *item = (pcb_t *)head_data;
    printf("Lifetime: %.2d; Name: %s\n", item->lifetime, item->name);
}

Strictly, the initialization of head_data is superfluous; the code in List_head_info() always sets the value at least once (to NULL or 0) and sometimes twice (the second time to the data component of the head item on the list).

This is ‘example code’ with enough information in it to compile. I’ve ‘reverse engineered’ the list structures enough to make sense; the actual implementation will be different, of course. This compiles cleanly under fairly stringent GCC warning levels, with GCC 4.1.2 and 4.7.0 on Mac OS X 10.7.4. AFAICS, it avoids some complex issues related to ‘strict aliasing’ which you really don’t want to have to worry about at this stage.

#include <stdio.h>

enum { NAME_MAX = 40 };

typedef struct Node Node;
struct Node
{
    void *data;
    Node *next;
};

typedef struct
{
    Node *head;
    Node *tail;
} List_t;

typedef struct 
{
    char name[NAME_MAX];
    int  lifetime;
} pcb_t;

extern int List_head_info(List_t *list, void **data);
extern void another_func(List_t processes);

void another_func(List_t list)
{
    void *head_data = 0;
    if (List_head_info(&list, &head_data))
    {
        pcb_t *item = (pcb_t *)head_data;
        printf("Lifetime: %.2d; Name: %s\n", item->lifetime, item->name);
    }
}

int 
List_head_info ( List_t *list, void **data )
{
    int all_ok = 0;

    *data = NULL;
    if ((list != NULL) && (list->head != NULL)) {
        *data = list->head->data;
        all_ok = 1;
    }

    return all_ok;
}