Strange error(dereferencing pointer to incomplete type)

Question

void get_cwd(char* buf)
{
    char *result;

  current->fs->pwd;
    result = get_dentry_path(current->fs->pwd);

    memcpy(buf, result, strlen(result)+1);

    kfree(result);
}

error: dereferencing pointer to incomplete type

The error points to current->fs->pwd;

includes:

#include <asm/stat.h>
#include <linux/fs.h>
#include <linux/file.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/string.h>
#include <linux/dirent.h>
#include <linux/sched.h>
#include <asm/uaccess.h>
#include <asm/current.h>
#include <linux/path.h>

If I type current->fs; on 5th line gcc don't give error on this line. The problem is with pwd field.

Answer 1

http://lxr.linux.no/#linux+v2.6.33/include/linux/fs_struct.h#L11 -- this should work; current should be a pointer to struct task struct, which should contain a pointer to struct fs_struct fs, which should contain struct path pwd. Perhaps you need to include fs_struct.h so you can see the contents of struct fs_struct.

Answer 2

error: dereferencing pointer to incomplete type means that you're attempting to access data within an opaque data structure. An opaque data structure is usually just a typedef in a header file (.h*), with the real definition in the implementation file (.c*) and only accessible to the implementation. This is used to hide the implementation details and only provide access to the elements via the interface API provided by the header.

http://en.wikipedia.org/wiki/Opaque_pointer

Answer 3

Well, the error message and your experiments obviously mean that current->fs is a pointer to an incomplete type. That's all there is to it. Why do you consider it "strange"?