sched.h, offsetof( ) function

[Andu]

I need to find an offset of a pointer within a structure.

I'm using Linux, and I need to find the offset of a pointer member withing task_struct which is defined in sched.h as follows

struct task_struct {
    volatile long state;    /* -1 unrunnable, 0 runnable, >0 stopped */
    struct thread_info *thread_info;
    atomic_t usage;
    unsigned long flags;    /* per process flags, defined below */
    unsigned long ptrace;
 
    int lock_depth;        /* BKL lock depth */
 
#if defined(CONFIG_SMP) && defined(__ARCH_WANT_UNLOCKED_CTXSW)
    int oncpu;
#endif
    int prio, static_prio;
    struct list_head run_list;
    prio_array_t *array;
 
.... /*ect*/
 
...
 
#define next_task(p)    list_entry((p)->tasks.next, struct task_struct, tasks)
#define prev_task(p)    list_entry((p)->tasks.prev , struct task_struct, tasks)
 
#define for_each_process(p) \
    for (p = &init_task ; (p = next_task(p)) != &init_task ; )
 

The code snippet I am using is the following. I am using the 'offsetof( ) function to obtain the offset of the given pointer declared in the said structure.

#include <stddef.h>
#include </usr/include/linux/sched.h>
 
int
main
()
{
 
    int A;
    A = offsetof(struct task_struct, next_task);
    printf ("%d \n", A);
}
 

I have tried this piece of code by declaring a small simple struct within the program, and it has worked.
However, ever since I've included the sched.h headerfile, I have been getting a neverending series of compliation errors from all the other h files included in sched.h
Am i making incorrect use of the headerfiles?

I am using gcc with a Bash shell.

[JosAH]

Am i making incorrect use of the headerfiles?

Yes you are: you shouldn't include that file all by itself, i.e. it needs other
files that will include that file for you and define other necessary hulla-baloo
needed by the files included by that sched.h file. I'm not at my manuals now
so I can't help you any further with that now; one more thing: offsetof() is
innocent in this all and your use of it is correct ;-)

kind regards,

Jos

[Andu]

Yes, i was afraid of that....

I am at a loss at how to solve that, because i did try copying and pasting all the includes that sched.h has onto my program and i even went as far as including headerfiles that were included in the includes, and still i got a neverending list of errors.

Header files have always eluded me.....

But glad to know that the offsetof( ) is the innocent party :)

[JosAH]

Yes, i was afraid of that....

I am at a loss at how to solve that, because i did try copying and pasting all the includes that sched.h has onto my program and i even went as far as including headerfiles that were included in the includes, and still i got a neverending list of errors.

Header files have always eluded me.....

But glad to know that the offsetof( ) is the innocent party :)

I'm still not at my manuals, but here's a little trick/hack I use sometimes:
find some kernel sources (.c files) and grep for "task_struct" in them.
Those .c files certainly compile so they have included the correct .h files.

Just do as the Romans do and copy those #include directives to your own
.c file(s). See if they compile. If they do leave out an #include directive and
try again. You can figure out what #include directives to use in your own
sources then. I know it's a dirty hack/trick; but without manuals everything
is allowed ;-)

Basically those #include files are simple: they are simply included in your
own compilation unit. If you draw a graph off all that include stuff, it's just
a DAG (Directed Acyclic Graph) and the #ifndef FOO #define Foo #endif
thingies prevent cyclic include directives.

kind regards,

Jos

[Andu]

Although that sounded like a brilliant plan, I've tried it. It didn't work.. and I can't understand why that didn't work to be honest :S

I did try specifically as you said, I looked up quite a number of kernel source codes that referenced the task_struct, i copied their includes, and it still didn't work.. I was getting the exact same list of errors as before.

The funny thing is that when i cross referenced the different source code files that referenced task_struct, the only include that they had in common was the sched.h file...

This problem really seems to have it's heart set on tormenting me!

Do you know if there's somewhere I can look up exactly how to do this?

thanks a lot for your help :)