File Implementation in the Linux Disk space are divided into a small size of space called disk block. A block size varies in different system. Numbers of blocks are used for saving a file. To manage a file system, information on which blocks are used for a file must be saved for file management.
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File Implementation in the Linux Linux system use i-Node file implementation method. Each file is associated with an i-node. Information for a file is saved in its inode: such as attributes, block numbers used by the file
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COSC350 System Software, Fall 2015
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File Implementation in the Linux
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Directory Linux use Hierarchical directory System A Linux directory is a special file that acts as a container for other files and even other directories. The directory entry provide the information needed to find the file disk blocks or subdirectory
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Linux Hierarchical Directory Root directory of File System
/
bin
dev
tom
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etc
home
mary
scot
sbin
mike
COSC350 System Software, Fall 2015
root
usr
Directory Below root
User home directory
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File System partition
partition
i-node list
partition
Directory and data blocks
Boot block Super block i-node
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i-node
…
i-node
COSC350 System Software, Fall 2015
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File System Data and Directory Block
i-node list
Data block
Direct block
Data block
Direct Block
i-node number
i-node
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i-node
…
Data block
File name
i-node
COSC350 System Software, Fall 2015
i-node number
File name
7
File Implementation in the Linux
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File System
Every i-node has link count that contains the number of directory entries that point to the inode. A file be deleted when the link count is 0. Unlinking means delete a file entry from a directory. If link count is > 1, unlinking file does not delete the file. The link count is contained in the st_nlink in the stat data structure
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COSC350 System Software, Fall 2015
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File System struct stat { mode_t st_mode; /*file type & mode (permissions) */ ino_t st_ino; /* i-node number */ dev_t st_dev; /* device number (file system) */ dev_t st_rdev; /* device number for special files */ nlink_t st_nlink; /* number of links */ uid_t st_uid; /* user ID of owner */ gid_t st_gid; /* group ID of owner */ off_t st_size; /* size in bytes, for regular files */ time_t st_atime; /* time of last access */ time_t st_mtime; /* time of last modification */ time_t st_ctime; /* time of last file status change */ blksize_t st_blksize; /* best I/O block size */ blkcnt_t st_blocks; /*number of 512 byte blocks allocated */ mode_t st_attr; /* The DOS-style attributes for this file */ }; 10/12/2015
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File System The i-node contains most of information (attributes) about the file: the file type, the file’s access permission bit,…and so on. But only two information are stored in the directory entry: the filename and the inode number. When move a file from one directory to another directory need only change directory entry point.
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COSC350 System Software, Fall 2015
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File System Data and Directory Block
i-node list
Data block
Direct block
Data block
Direct block
sxwang
home
/ i-node number
Direct Block
home
i-node number
sxwang i-node number
i-node
i-node
…
i-node
foo
i-node
Move /home/sxwang/foo to /home/foo 10/12/2015
COSC350 System Software, Fall 2015
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File System Data and Directory Block
i-node list
Data block
Direct block
Data block
Direct block
i-node
i-node
…
i-node
sxwang
home
/ i-node number
Direct Block
home
i-node number
sxwang
i-node number
foo
i-node
Move /home/sxwang/foo to /home/foo 10/12/2015
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The link() System Call Any file can have multiple directory entries pointing to its i-node. The way we can create a link to an existing file is with the link system call. Prototype:
#include int link (const char *existingpath, const char *newpath) Returns: 0 if OK,
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COSC350 System Software, Fall 2015
return -1 on error
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The link() System Call #include int link (const char *existingpath, const char *newpath) Returns: 0 if OK, return -1 on error
The link system call create a new directory entry newpath that references the existing file existingpath. The creation and increment of the link count be done automatically by kernel. Only a super-user process can create a new link that points to a directory. Because link system calls to a directory can cause loops in the file system.
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The link() System Call /home/sxwang
Lecture
cosc 220
cosc350
lecture
chapter2
lec11
link.c
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The link() System Call /home/sxwang
Lecture
cosc 220
cosc350
lecture
chapter2
lec11
link.c
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/* link.c */ #include #include #include #include #include void err_sys(char *str) { printf ("%s\n",str); exit (1); } int main () { struct stat buff; /* get attributes for the file link.c*/ if (stat ("link.c", &buff) < 0) err_sys("stat error for foo"); printf ("link count for a file link.c was %d \n", buff.st_nlink); /* increase the number of link by one */ if (link ("/home/sxwang/Lecture/cosc350/lecture/lec11/link.c", "/home/sxwang/Lecture/cosc350/lecture/link.c")
