File Access cont'

1
File Access (cont.)

• define OPEN_MAX 20 / max files open at once
  /
• extern FILE _iobOPEN_MAX
• define stdin (_iob0)
• define stdout (_iob1)
• define stderr (_iob2)
• enum _flags
• _READ 01, / file open for reading /
• _WRITE 02, / file open for writing /
• _UNBUF 04, / file is unbuffered /
• _EOF 010, / EOF has occurred on this
  file /
• _ERR 020 / error occurred on this file
  /

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Case study

• List directory list file size of files or
  directory
• for directory list file size of all files under
  the directory recursively
• Storage System
• Malloc vs Free implementation using system call

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Unix file system structure

• Directory
• A file that contains a list of filenames and
  inode numbers which are index into the inode
  list.
• Inode
• All information about a file except its name is
  kept.

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Unix file system structure
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File system call

• ltsys/type.hgt ltsys/stat.hgt
• int stat(char , struct stat )
• Take a filename and returns all of the
  information in the inode for that file, -1 for
  error.
• int fstat(int fd, struct stat )
• Take a file descriptor and returns all of the
  information in the inode for that file, -1 for
  error.

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Our data structures for directory
One for directory, one for directory entry
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System Level data structure for directory entry
includeltsys/dir.hgt
8
Function relationship
Opendir
Closedir
Readdir
dirwalk
fsize
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fsize list the size and name of files or
directories
10
Type of file
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Dirwalk recursively list name and size for files
inside directories
12
Opendir open a directory
13
Closedir close a directory
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Readdir read directory
15
Storage system

• Free storage is kept as a list of free blocks
• Each block contains a header (size, a pointer to
  the next block) and the space itself.
• Blocks are kept in order of increasing storage
  address and the last block points to the first

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How to find free blocks

• Malloc
• Looks for a block which satisfies the request and
  the block will be deleted from the free list and
  returned to the user.
• Free
• Find a proper place to insert the block being
  freed.

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Block format

• Block head
• A pointer to the next block
• A record of block size
• Alignment
• All blocks are multiple of header size

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A block returned by malloc
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malloc
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malloc(2)
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morecore
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sbrk(n)

• Unix system call sbrk(n) returns a pointer to n
  more bytes of storage. Return -1 for no space.
• Used in Morecore

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free
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Pointer functions

• Syntax
• type ( name) (argument list)
• Compare function prototypes
• type name (argument list)

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Pointer functions Usage
int find_average(int a) int i, sum0
for ( i0 ilt20 i) sum ai return
sum/20
int find_max(int a) int i int maxa0
for ( i0 ilt20 i) max max gt ai
? max ai return max
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Pointer functions Usage (cont.)

• int main()
• int job, result, score20
• score20.
• int (func)(int )
• scanf(d, job)
• func job 1 ? find_average find_max
• result func(score)
• return 0