What is the "current" in Linux kernel source?

Question

I'm studying about Linux kernel and I have a problem.

I see many Linux kernel source files have current->files. So what is the current?

struct file *fget(unsigned int fd)
{
     struct file *file;
     struct files_struct *files = current->files;

     rcu_read_lock();
     file = fcheck_files(files, fd);
     if (file) {
             /* File object ref couldn't be taken */
             if (file->f_mode & FMODE_PATH ||
                 !atomic_long_inc_not_zero(&file->f_count))
                     file = NULL;
     }
     rcu_read_unlock();

     return file;
 }

Answer 1

It's a pointer to the current process (i.e. the process that issued the system call).

On x86, it's defined in arch/x86/include/asm/current.h (similar files for other archs).

#ifndef _ASM_X86_CURRENT_H #define _ASM_X86_CURRENT_H  #include <linux/compiler.h> #include <asm/percpu.h>  #ifndef __ASSEMBLY__ struct task_struct;  DECLARE_PER_CPU(struct task_struct *, current_task);  static __always_inline struct task_struct *get_current(void) {     return percpu_read_stable(current_task); }  #define current get_current()  #endif /* __ASSEMBLY__ */  #endif /* _ASM_X86_CURRENT_H */ 

More information in Linux Device Drivers chapter 2:

The current pointer refers to the user process currently executing. During the execution of a system call, such as open or read, the current process is the one that invoked the call. Kernel code can use process-specific information by using current, if it needs to do so. [...]

Answer 2

Current is a global variable of type struct task_struct. You can find it's definition at [1].

Files is a struct files_struct and it contains information of the files used by the current process.

[1] http://students.mimuw.edu.pl/SO/LabLinux/PROCESY/ZRODLA/sched.h.html