struct task_struct {
        /* ... */
        struct files_struct *files;
        /* ... */
};

struct files_struct {
	struct fdtable *fdt;
};

struct fdtable {
	unsigned int max_fds;
	struct file **fd;
};

struct file {
	struct path	  f_path;
	struct inode	  *f_inode;
	atomic_long_t	  f_count;
	unsigned int 	  f_flags;
	fmode_t		  f_mode;
	loff_t		  f_pos;
	const struct cred *f_cred;
};

stdout
fd1

fd1
stdout

pos   = 0
count = 1

pos   = 0
count = 1

pos   = 0
count = 1

pos   = 0
count = 1

MOUNT(8)                  BSD System Manager's Manual                 MOUNT(8)

NAME
     mount -- mount file systems

SYNOPSIS
     mount [-adfruvw] [-t lfs | external_type]
     mount [-dfruvw] special | mount_point
     mount [-dfruvw] [-o options] [-t lfs | external_type] special mount_point

NAME
       SSHFS - filesystem client based on ssh

SYNOPSIS
   mounting
       sshfs [user@]host:[dir] mountpoint [options]

   unmounting
       umount mountpoint

$> sudo mkdir /mnt/remote_dir
$> sudo sshfs username@xxx.xxx.xxx.xxx:/some/path/on/remote/serv \
              /mnt/remote_dir
$> cd /mnt/remote_dir
$> # you are on remote server

$> cat /proc/filesystems
sysfs
rootfs
ramfs
proc
tmpfs
securityfs
pipefs
ext3
ext2
ext4
vfat
fuse
...

struct fs_super_block {
        int32_t block_count;
        int32_t free_block_count;
        int32_t block_size;
        int32_t flags;
        int32_t mount_time;
        /* ... */
        int16_t magic;
        /* ... */
};

#define RAMFS_MAGIC		0x858458f6
#define TMPFS_MAGIC		0x01021994
#define EXT2_SUPER_MAGIC	0xEF53
#define EXT3_SUPER_MAGIC	0xEF53
#define EXT4_SUPER_MAGIC	0xEF53
#define MINIX_SUPER_MAGIC	0x137F
#define MSDOS_SUPER_MAGIC	0x4d44
#define NFS_SUPER_MAGIC		0x6969

/**
 * Linux kernel,
 * fs/fat/fat.h
 * 30.09.2018.
 */
struct fat_entry {
	int entry;
	union {
		u8 *ent12_p[2];
		__le16 *ent16_p;
		__le32 *ent32_p;
	} u;
	int nr_bhs;
	struct buffer_head *bhs[2];
	struct inode *fat_inode;
};

/** uapi/linux/msdos_fs.h */
struct msdos_dir_entry {
	__u8	name[MSDOS_NAME];/* name and extension */
	__u8	attr;		/* attribute bits */
	__u8    lcase;		/* Case for base and extension */
	__u8	ctime_cs;	/* Creation time, centiseconds (0-199) */
	__le16	ctime;		/* Creation time */
	__le16	cdate;		/* Creation date */
	__le16	adate;		/* Last access date */
	__le16	starthi;	/* High 16 bits of cluster in FAT32 */
	__le16	time,date,start;/* time, date and first cluster */
	__le32	size;		/* file size (in bytes) */
};

/**
 * Linux kernel,
 * fs/ext2/ext2.h
 * 30.09.2018
 * 53 lines.
 */
struct ext2_inode {
	__le16	i_mode;
	__le16	i_uid;
	__le32	i_size;
	__le32	i_atime;
	__le16	i_links_count;
	__le32	i_blocks;
	__le32	i_flags;
	__le32	i_block[15];
        /* ... */
};

struct ext2_group_desc
{
	__le32	bg_block_bitmap;
	__le32	bg_inode_bitmap;
	__le32	bg_inode_table;
	__le16	bg_free_blocks_count;
	__le16	bg_free_inodes_count;
	__le16	bg_used_dirs_count;
	__le16	bg_pad;
	__le32	bg_reserved[3];
};

#define EXT2_MIN_BLOCK_SIZE 1024
#define	EXT2_MAX_BLOCK_SIZE 4096
#define	EXT2_NDIR_BLOCKS    12
#define	EXT2_IND_BLOCK	    EXT2_NDIR_BLOCKS
#define	EXT2_DIND_BLOCK	    (EXT2_IND_BLOCK + 1)
#define	EXT2_TIND_BLOCK	    (EXT2_DIND_BLOCK + 1)
#define	EXT2_N_BLOCKS	    (EXT2_TIND_BLOCK + 1)

/**
 * Linux kernel.
 * include/linux/fs.h
 * 30.09.2018.
 * 33 lines.
 */
struct super_operations {
   	struct inode *(*alloc_inode)(struct super_block *sb);
	void (*destroy_inode)(struct inode *);

   	void (*dirty_inode) (struct inode *, int flags);
	int (*write_inode) (struct inode *, struct writeback_control *wbc);
	int (*drop_inode) (struct inode *);
	void (*evict_inode) (struct inode *);
	void (*put_super) (struct super_block *);
	int (*sync_fs)(struct super_block *sb, int wait);
	int (*freeze_super) (struct super_block *);
	int (*freeze_fs) (struct super_block *);
	int (*thaw_super) (struct super_block *);
	int (*unfreeze_fs) (struct super_block *);
	int (*statfs) (struct dentry *, struct kstatfs *);
	int (*remount_fs) (struct super_block *, int *, char *);
	void (*umount_begin) (struct super_block *);

	int (*show_options)(struct seq_file *, struct dentry *);
	int (*show_devname)(struct seq_file *, struct dentry *);
	int (*show_path)(struct seq_file *, struct dentry *);
	int (*show_stats)(struct seq_file *, struct dentry *);
	long (*free_cached_objects)(struct super_block *,
				    struct shrink_control *);
};

write(fd, buf, size);

file = find_file(fd);
file->write(buf, size);

#include <stdio.h>
#include <dirent.h>

int main()
{
	DIR *dir = opendir(".");
	struct dirent *dirent = readdir(dir);
	while (dirent != NULL) {
		printf("name = %s, inode number = "\
                       "%d, type = %d\n",
                       dirent->d_name,
                       (int) dirent->d_ino,
		       (int) dirent->d_type);
		dirent = readdir(dir);
	}
	closedir(dir);
	return 0;
}

#include <stdio.h>
#include <sys/stat.h>
#include <unistd.h>

int main(int argc, char **argv) {
	struct stat st;
	stat(argv[1], &st);
	printf("inode = %d, protection = %d, links = "\
		"%d, uid = %u, size = %d, blocks = "\
		"%d\n", (int)st.st_ino, (int)st.st_mode,
		(int)st.st_nlink, (unsigned)st.st_uid,
		(int)st.st_size, (int)st.st_blocks);
	if ((st.st_mode & S_IFDIR) == S_IFDIR)
		printf("the file is directory\n");
	if ((st.st_mode & S_IFREG) == S_IFREG)
		printf("the file is regular\n");
	if ((st.st_mode & S_IFLNK) == S_IFLNK)
		printf("the file is symbolic link\n");

	if ((st.st_mode & S_IRUSR) == S_IRUSR)
		printf("can read it\n");
	if ((st.st_mode & S_IWUSR) == S_IWUSR)
		printf("can write it\n");
	if ((st.st_mode & S_IXUSR) == S_IXUSR)
		printf("can execute it\n");

	printf("my uid: %d\n", (int)getuid());
	return 0;
}

/* ... */
while (read(fd, buf, size) != 0) {
    /* do something ... */
}
/* ... */

$> # Template:
$> # cat /sys/block/{device_name}/queue/scheduler
$>
$> cat /sys/block/sda/queue/scheduler 
noop deadline [cfq]

read(fd, buf, size);

int
printf(const char * restrict format, ...);

int
fprintf(FILE * restrict stream, const char * restrict format, ...);

int
fputs(const char *restrict s, FILE *restrict stream);

int
fflush(FILE *stream);