CODEWITHSUNDEEP
ckernel
do_os
do_os

Reputation: 127

invalid type argument of '->'

#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/module.h>

int start(void){

    printk("starting module\n");
    printk("pid\ttask_name\tstate\n");

    struct task_struct *task;
    struct list_head *list;

    list_for_each(list,&init_task->children){
        task=list_entry(list,struct task_struct,sibling);
        printk("%5d\t%s\t%li\n",task->pid,task->comm,task->state);
    }
    return 0;
}


void ending(void){
    printk("closing module\n");
}

module_init(start);
module_exit(ending);

When I compile, an error this below shows up

error: invalid type argument of ‘->’ (have ‘struct task_struct’)
  list_for_each(list,&init_task->children){
                               ^
include/linux/list.h:408:14: note: in definition of macro ‘list_for_each’
  for (pos = (head)->next; pos != (head); pos = pos->next)

I think the error msg is trying to say that "&init_task" should have "struct task_struct" but doesn't this suffice? As far as I know, variable 'init_task' is externally declared in 'linux/sched.h' which means that the error shouldn't be caused by not being able to locate or find 'init_task'.

Since a pointer should come in front of '->', using '&init_task' seems right.

Could someone point out what I'm missing here?

---update: solved ---

reconsidering operator precedences of '->' and '&', the proper use is either '(&init_task)->children' or 'init_task.children'. However, even with this changed, an error came up:

include/linux/list.h:408:31: error: invalid operands to binary != (have ‘struct list_head *’ and ‘struct list_head’)
  for (pos = (head)->next; pos != (head); pos = pos->next)

This error msg pointed out that I should be providing the address of 'children' in 'struct task_struct'. Thus, I changed the problematic line as:

list_for_each(list,&(init_task.children)){

which solved the problem and the compilation went smoothly.

Upvotes: 2

Views: 1935

Answers (1)

Eugene Sh.
Eugene Sh.

Reputation: 18361

init_task is a structure, not pointer. So you should either convert it to the pointer prior dereferencing (&init_task)->children, or access children using the period notation init_task.children. The &init_task->children is denoting the address of the children field of the structure pointed by init_task, if it were pointer.

Upvotes: 5

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