読込みサンプル

[root@north dup]# cat ropen.c

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>

void    do_dup(void);
void    do_fopen(void);

char    buff[6];

void    main(int argc,char **argv)
{
    system("rm -f babakaka.txt");
    system("echo 0123456789 > babakaka.txt");

    if (!strcmp(argv[1], "dup")) {
        do_dup();
     }
    else {
        do_fopen();
     }
}

void    do_dup()
{

   	int fd1 =open("babakaka.txt", O_RDWR);
   	int fd2 = dup(fd1);

   	buff[5] = 0; read(fd1, buff, 5); printf("%s", buff);
   	buff[5] = 0; read(fd2, buff, 5); printf("%s\n", buff);
}

void    	do_fopen()
{
    int fd1 =open("babakaka.txt", O_RDWR);
    int fd2 =open("babakaka.txt", O_RDWR);

    buff[5] = 0; read(fd1, buff, 5); printf("%s", buff);
    buff[5] = 0; read(fd2, buff, 5); printf("%s\n", buff);
}

[root@north dup]# ./ropen.out fopen
0123401234

[root@north dup]# ./ropen.out dup
0123456789

書込みサンプル

[root@north dup]# cat wopen.c

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>

void    	do_dup(void);
void    	do_fopen(void);

void    main(int argc,char **argv)
{
   	system("rm -f babakaka.txt");
   	system("touch babakaka.txt");

   	if (!strcmp(argv[1], "dup")) {
   	    	do_dup();
   	}
   	else {
   	    	do_fopen();
   	}
}

void    	do_dup()
{
   	int fd1 =open("babakaka.txt", O_RDWR);
   	int fd2 = dup(fd1);

   	write(fd2, "01234", 5);
   	write(fd1, "56789\n", 6);
   	close(fd1);
   	close(fd2);

   	system("cat babakaka.txt");
}

void    	do_fopen()
{
    int fd1 =open("babakaka.txt", O_RDWR);
    int fd2 =open("babakaka.txt", O_RDWR);

    write(fd1, "01234", 5);
    write(fd2, "56789\n", 6);
    close(fd1);
    close(fd2);

    system("cat babakaka.txt");
}

[root@north dup]# ./wopen.out fopen
56789

[root@north dup]# ./wopen.out dup
0123456789

カーネル

file = current->files[引数ソースfid]
current->files[重複先fid] = file; 重複先fidが指定されてなければcurrent->files[]の空インデックス

struct file {
  :
       atomic_long_t           f_count;
       unsigned int            f_flags;
       fmode_t                 f_mode;
       loff_t                  f_pos;     <- 読み書き位置
       struct fown_struct      f_owner;
  :
};

SYSCALL_DEFINE1(dup, unsigned int, fildes)
{
       int ret = -EBADF;
       struct file *file = fget_raw(fildes);

       if (file) {
               ret = get_unused_fd();
               if (ret >= 0)
                       fd_install(ret, file);
               else
                       fput(file);
       }
       return ret;
}

struct file *fget_raw(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 (!atomic_long_inc_not_zero(&file->f_count))
                       file = NULL;
       }
       rcu_read_unlock();

       return file;
}

void fd_install(unsigned int fd, struct file *file)
{
       __fd_install(current->files, fd, file);
}

void __fd_install(struct files_struct *files, unsigned int fd, struct file *file)
{
       struct fdtable *fdt;
       spin_lock(&files->file_lock);
       fdt = files_fdtable(files);
       BUG_ON(fdt->fd[fd] != NULL);
       rcu_assign_pointer(fdt->fd[fd], file);
       spin_unlock(&files->file_lock);
}