system call for file system

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Chapter 5System Calls for File

System


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System calls that return file descriptors for use inother system call;

System calls that use the namei algorithm toparse a path name;

System calls that assign and free inodes , using

algorithm ialloc and ifree; System calls that set or change the attribute of a

file;

System calls that do I/O to and from a process ,using algorithms alloc ,free, and the bufferallocation algorithms;

System calls that change the structure of the filesystem;

S stem calls that allow a rocess to chan e its


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The kernel searches the file system for the file

name parameters using namei algorithm; It checks permissions for opening the file after it

finds the in-core inode and allocates an entry inthe file table for open file;

The file table entry contains a pointer to the inodeof the open file and a field that indicates the byteoffset in the file where the kernel expects the nextread or write begin;

The kernel initializes the offset 0 during the opensystem call;

The kernel allocate an entry in a private table inthe process u area called the user file descriptor

table;


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Algorithm open

Inputs : file name

type of open

file permissions (for creation type ofopen)

Output : file descriptor

1) convert file name to inode(algorithm namei);

2) if file does not exist or not permitted access

return error;

allocate the table entry for inode , initialize count,offset;

allocate user file descriptor entry , set pointer to

file table entry;


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Example:

fd1=open(/etc/passwd, O_RDONLY);

fd2=open(local, O_RDWR);

fd3=open(/etc/passwd, O_WRONLY);


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01

2

3

4

5

6

7

User filedescriptor table

::

count1read

::

count

1 rd-wr

::

count1

write

Filetable

Count : 2/etc/passwd

Count:1Local

Inodetable


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Example

process 1

fd1=open(/etc/passwd,O_RDONLY);

fd2=open(local, O_RDWR); fd3=open(/etc/passwd,

O_WRONLY);

Process 2 fd1=open(/etc/passwd,

O_RDONLY);

fd2=open(private, O_RDONLY);


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0

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2

3

4

5

:

:

User file descriptortable

::

count:1

read

::

count:1

rd-write

::

count:1

read

count:1write

Filetable


Count:1Local

Count:1private

Inodetable

0

1

2

3

4

:

:

:


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Read Syntax :

Number=read( fd , buffer, count)

fd : the file descriptor return by open

Buffer : address of a data structure in the userprocess that will contain the read data onsuccessful completion of the call

Count: number of bytes the user wants to read

Number : number of bytes actually read


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The kernel gets the file table entry that corresponds to the userfile descriptor

It now sets the I/O parameters in the u area.

Mode indicates read or writeCount count of bytes to read or writeOffset byte offset in file

Address target address to copy data, inuser or kernel memoryFlag indicates if address is in user or

kernel memory

figure 5.6 I/O parameters saved in U area


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After the kernel sets the I/O parameters in the uarea;

It follows the pointer from the file table entry tothe inode ,looking the inode before it reads the file;

the algorithm now goes into loop until the read issatisfied;

The kernel converts the file byte offset into a blocknumber, using algorithm bmap

And it notes the byte offset in the block where theI/O should begin and how many bytes in the block

it should read.


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Algorithm read

Input : user file descriptor

address of buffer in user processnumber of bytes to read

Output : count of bytes into user space

1) get file table entry from user file descriptor; 2) check file accessibility;

3) set parameters in u area for user address ,bytecount , I/O to user;

4) get inode from file table;

5) lock inode;

6) set byte offset in u area from file table offset;


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7) while could not satisfied

convert file offset to disk block (algorithmbmap);

calculate offset into block , number of bytes toread;

8) if number bytes to read is 0

/*trying to read end of file */

break;

9) read block (algorithm breada if with read

ahead , algorithm bread otherwise); 10) copy data from system buffer to user address;

11) update u area fields for file offset ,read count ,address to write into user space;

Release buffer;


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Unlock inode

Update file table offset for next read;

return (total number of bytes read);


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Write Syntax :

Number=write( fd , buffer , count);

fd : the file descriptor return by open

Buffer : address of a data structure in the userprocess that will contain the read data on

successful completion of the call Count: number of bytes the user wants to read

Number : number of bytes actually read


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The algorithm for writing a regular file is similar tothat for reading a regular file.

However , if the file dose not contain a block thatcorresponds to the byte offset to be written, thekernel allocates a new block using algorithm alloc

And assigns the block number to the correctposition in the inode s table of contents.

if the byte offset is that of an indirect block , thekernel may have to allocate several blocks for use

as indirect blocks and data blocks. The inode is locked for the duration of the write;

Because the kernel may change the inode whenallocating new blocks;


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Allowing other processes to access to the file could

corrupt the inode ,if several processes allocateblocks simultaneously for the same byte offsets;

When the write is complete, the kernel updatesthe file size entry in the inode if the file has grownlarger;


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Process 1

fd=open(/home/stu/p.c , O_WRONLY);

Write(fd,hello,5);

Process 2

fd=open(/home/stu/p.c, O_RDONLY);

read(fd,buf,5);


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Adjusting the position of file I/O LSEEK

The ordinary use of read and write system callsprovide sequential access to a file ,but processes

can use the lseek system call to position the I/Oand allow random access to a file .

Syntax :

position = lseek (fd , offset,reference);

fd : file descriptor identifying the file

offset : a byte offset reference : indicate whether offset should be

considered from the file , from the currentposition of the read/write offset , or

from end of file.


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The lseek system call has nothing to do with theseek operation that positions a disk arm over aparticular disk sector;

To implement lseek , the kernel simply adjusts theoffset value in the file table;

Subsequent read or write system calls use the filetable as their starting byte offset;


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Close A process closes an open file when it no longer

wants to access it; syntax:

Close(fd);

fd : file descriptor for open file

The kernel does the close operation bymanipulating the file descriptor and thecorresponding file table and inode table entries.


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If the reference count of the file table entry isgreater than 1 because of dup or fork systemcalls , then other user file descriptors reference the

file table entries; The kernel decrement count and close completes;

User file descriptor File Inode


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0

1

2

3

4

5

:

:

User file descriptortable

::

count:1

read

::

count:1

rd-write

::

count:0

count:1write

Filetable


Count:1Local

Count:0private

Inodetable

0

1

2

NULL

NULL

:

:

:


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FILE CREATION The open system call gives a process access to an

existing file, but the creat system call creates anew file in the system.

Syntax :

fd=creat(pathname , modes);

Pathname: file name modes: file permission

File descriptor: open system call returns an integercalled user file descriptor


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If no such file previously existed , the kernelcreates a new file with the specified name andpermission mode;

If the file already existed , the kernel truncates thefile (releases all existing data blocks and sets thefile size to 0);


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Algorithm Creat

Input : file name

permission settings

Output : file descriptor

1) get inode for file name ( algorithm namei);

2) if file already exists

if not permitted access

release inode(algorithm iput);

return(error);

3) else

assign free inode from file system(algorithm

ialloc)


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4) allocate file table entry for inode , initializedcount;

5) if file did exist at time of create

free all file blocks (algorithm free)

unlock(inode);

return(user file descriptor);


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CREATION OF SPECIAL FILES

The system call mknod creates special file in thesystem , including :

named pipes;

Device files;

Directories;

It similar to creat in that the kernel allocates an

inode for the file.


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Algorithm makenew node

Inputs : node(filename)

file type

permissions


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Assign free inode from file system for new node( ialloc );

Create new directory entry in parent directory;include new inode name and newly assignedinode number;

Release parent directory inode number(iput);

3)If (new node is block or char special file)

write major , minor numbers into inodestructure;

release new node inode(iput);


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system call for file system

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