Chapter#21 IO : File and Directory

Chapter#21 IO : File and Directory

- Files and Directories

- Unix file IO

- Links

- Mount

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Files and Directories

Storage Virtualization

File : read/write가능한 linear array of bytes

- content : creator에 의해 만들어짐 e.g. text/binary

- inode number로 identified됨 (pid와 유사)

Directory : file과 directories를 묶기 위한 special file

- content : list of (user-readable name, inode #) pairs를 가짐

- hierarchical oraganization ( tree, acyclic-graph, graph )

- inode number로 indentififed됨

 

트리구조이므로 최상위 directory는 root ( '/'로 나타냄 )

 

Pathname :

- absolute pathname : /home/~~/~~~

- relative pathname : ~~/~~~

Working Directory : 

- 모든 process는 working directory가 있다 ( pwd로 확인 )

Home Directory : 

- log in된 origin directory

 

Unix File : normal file만 아니라 peripheral device와 inter-process communication channels도 file로 간주

- /dev/stdin, /dev/stdout, /dev/rmt0 (tape device), /dev/sda2 (disk partition), /dev/tty2 (terminal), /proc (kernel data structure) 등등

- FIFO, PIPE, SOCKET

- 모두 고유 inode number 가짐

Unix Directory : 

- (user-readable name, inode #) pairs를 가짐

- directory의 각 entry는 file or directories로 간주됨

- 마찬가지로 고유 inode number 가짐

 

Ownership

- 각 file은 particular user의 소유이다

- Owner은 file의 permission을 설정할 수 있다

Permissions

- 누가 어떻게 file에 access할 수 있는지 설정

Superuser(or administrator) : rwx에 상관없이 모든 파일에 access 가능

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Unix file IO

Creaing Files

int fd; // file descriptor
if ((fd = open("foo", O_CREAT | O_WRONLY | O_TRUNC)) < 0){
    perror("open");
    exit(1);
}

O_CREAT : creat file

O_WRONLY : only write

O_TRUNC : make the file size zero ( = remove any existing content )

open() returns file descripter : an integer, used to access files

- return -1 : perror("open");

 

Closing Files

int fd; // file descriptor
int retval; // return value
if((retval = close(fd)) < 0){
    perror("close");
    exit(1);
}

 

Reading and Writing Files

1. open() call

2. 반복적으로 read(), write() call

3. close() call

 

strace 명령어를 통해 어떤 call들이 이뤄졌는지 확인 가능

 

open(file descriptor, flags) : return file descriptor. 0/1/2 = stdin/stdout/stderr

read(file descriptor, buffer pointer, size of buffer) : return 읽은 byte 수

write(file descriptor, buffer pointer, size of buffer) : return 쓴 byte 수

 

current offset : read/write가 현재 어디서 시작할지 나타냄

- Implicitly : read/write N bytes를 하면 current offset += N

- Explitly : lseek()

 

off_t lseek(int fileds, off_t offset, int whence);

fildes : file descriptor

offset : position the file offset to particular location

whence : determine how seek is performed

- SEEK_SET : 0 + OFFSET

- SEEK_CUR : CUROFFSET+OFFSET

- SEEK_END : ENDPOINT + OFFSET

 

Renaming Files

rename(char* old, char* new)

ex) > mv foo bar

ex) rename("foo.txt.tmp", "foo.txt");

 

Getting Information About Files

stat(), fstat() : show file metadata

struct stat {
    dev_t st_dev; /* ID of device containing file */
    ino_t st_ino; /* inode number */
    mode_t st_mode; /* protection */
    nlink_t st_nlink; /* number of hard links */
    uid_t st_uid; /* user ID of owner */
    gid_t st_gid; /* group ID of owner */
    dev_t st_rdev; /* device ID (if special file) */
    off_t st_size; /* total size, in bytes */
    blksize_t st_blksize; /* blocksize for filesystem I/O */
    blkcnt_t st_blocks; /* number of blocks allocated */
    time_t st_atime; /* time of last access */
    time_t st_mtime; /* time of last modification */
    time_t st_ctime; /* time of last status change */
};

ex) > stat file

indoe structure : disk에 저장된 file system에 의해 관리된 type of information

 

Removing Files

ex) > rm file

ex) unlink("file")

 

Making Directoreis

ex) > mkdir dir

ex) mkdir("dir", 0777)

처음에 ls하면 . (itself)와 .. (parent)를 가짐

 

Reading Directories

// sample code of ls
int main(int argc, char *argv[]){
    DIR *dp = opendir(".");			// open current directory
    assert(dp!=NULL);
    struct dirent *d;
    while((d = readdir(dp)) != NULL){		// read one directory entry
        print("%d %s\n", (int) d->d_ino, d->d_name);
    }
    closedir(dp);				// close current directory
    return 0;
}

struct dirent {
    char 		d_name[256];	/* filename */
    ino_t 		d_ino;		/* inode number */
    off_t 		d_off;		/* offset to the next direct */
    unsigned short 	d_reclen;	/* length of this record */
    unsigned char 	d_type;		/* type of file */
}

 

Deleting Directories

rmdir() : empty directory인 경우만 가능. 아니면 fail

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Links

Hard Links

link(old pathname, new one)

- same file을 refer함

- ex) > ln file file2

작동 : 

- create another name in directory

- refer same inode number of origin

제거(remove file) :

- 해당 file inode를 refer하는 파일 개수를 count

- unlink() call되면 count 1 감소

- count가 0이 되면, 실질적으로 inode와 data block 제거

여러 file system을 사용하는 경우 문제 발생할 수 있음

 

Symbolic Links(Soft Link)

hard link는 다른 partition에 파일 생성 불가

ex) > ln -s file file2

새 inode 값을 가지며 content는 원본 file pathname ( = 4 bytes, longer pathname -> larger bits )

-> Dangling reference : 원본이 지워지면 link가 남을 수 있음 (trash)

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Mount

mkfs tool : make a file system

- 빈 파일 시스템 씀. root에서 시작

- input : device (e.g. /dev/sda1), file system(e.g. ext3)

 

mount()

- 기존 directory를 mount point를 가리키게 함

ex) > mount -t ext3 /dev/sda1 /home/users

= /home/users/ 는 /dev/sda1의 root를 가리킴

ex)

/dev/sda1 on / type ext3 (rw)
proc on /proc type proc (rw)
sysfs on /sys type sysfs (rw)
/dev/sda5 on tmp type ext3 (rw)
/dev/sda7 on var /vice/cache type ext3 (rw)
tmpfs on / dev shm type tmpfs (rw)
AFS on /afs type afs (rw)

ext3 : standard disk-based file system

proc : file system for accessing information about current process

tmpfs : file system for temporary files

AFS : distributed file system

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학교 강의를 토대로 개인 공부 목적으로 작성되어 오타가 및 오류가 있을 수 있으며, 문제가 될 시 삭제될 수 있습니다.