Signals

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CS 241 Section Week 8(10/29/09)
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Outline

• MP5 Overview
• Files I/O
• UNIX File Systems
• inodes
• Directories
• Links

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MP5 Overview
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MP5 Overview

• You are to create a deadlock resilient
  semaphore library. You should implement six
  functions.
• Since we only allow one instance of each
  resource, you do not need to implement the
  Bankers algorithm for deadlock prevention. You
  may use a resource allocation graph instead.
• In deadlock detection mode, once a deadlock is
  detected, you need only send a SIGINT signal. The
  library does NOT need to worry about how SIGINT
  is handled.
• The given test cases are far from complete. You
  should derive your own test cases.

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Files and I/O
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Unix File Structure
/ /bin/ /home/ /home/someuser/
/home/someuser/somefile.txt /usr/
/usr/bin/ /usr/lib/
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Internal File Structure

• A file is just a series of bytes

T
h
i
s
w
n
i
x
f
i
l
e
.
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Internal File Structure

• A file is just a series of bytes

T
h
i
s
w
n
i
x
f
i
l
e
.
Start of File
End of File
Current Offset
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I/O Libraries in C
stdio fopen, fread, fwrite, fclose,
... (buffered I/O)
User Process
open, read, write, close, select, poll,
... (direct to kernel I/O)
kernel system call handler
Kernel
file I/O
terminal I/O
pipe I/O
network I/O
audio I/O
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I/O Libraries in C
stdio fopen, fread, fwrite, fclose,
... (buffered I/O)
User Process
open, read, write, close, select, poll,
... (direct to kernel I/O)
kernel system call handler
Kernel
file I/O
terminal I/O
pipe I/O
network I/O
audio I/O
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Buffered I/O Advantages

• Weve previously used
• printf()
• fprintf()
• Why use buffers?
• I/O operations are SLOW!
• Every time you write just one byte, you dont
  want to have to access your hard drive.

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File Descriptors

• The UNIX operating system uses a file descriptor
  table to store information about open files

0
stdin
(sof, eof, offset values, etc)
1
stdout

2
stderr

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/usr/home/myfile.txt

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open()

• int open(const char pathname, int
  flags)
• returns an int, which is the file descriptor
• takes in either a relative or full path name
• various flag options allow a file to only be
  appended to (O_APPEND), opened as write only
  (O_WRONLY), and more.

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open()

• To open a file for reading
• int ifd open(./input.txt, O_RDONLY)
• To open OR create a file for writing, with given
  permissions
• int ofd open(output.txt,
  O_WRONLY O_CREAT, S_IRUSR
  S_IWUSR)

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fopen()

• FILE fopen( const char filename,
  const char mode)
• Rather than an int (file descriptor), fopen
  returns a FILE stream.

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File Permissions

• In UNIX, the file permissions system is
  relatively basic.
• Each file has a single owner and a single group
  associated with it.
• Each file also has permissions associated with
  itself for the owner, members of the group the
  file is in, and for everyone else.

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File Permissions

• These permissions are stored as a
  three-octal-digit number (000 to 777).

7
5
5
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File Permissions

• The most-significant number is the owners
  permission.

Owner
7
5
5
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File Permissions

• The middle number is the groups permission.

Group
7
5
5
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File Permissions

• The least-significant number is everyone elses
  permission.

Other
7
5
5
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File Permissions

• Each octal number is simply three bits a read
  bit, a write bit, and an execute bit.

7
5
5
Read
1
1
1
Write
1
0
0
Execute
1
1
1
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File Permissions

• Thus
• 755 means everyone can read and execute by file,
  but only the owner can write to (edit) my file
• 644 means everyone can read my file, only the
  owner can write to my file, and no one can
  execute it
• 660 means only members of the files group and
  the files owner may read or edit the file
  others cannot even read it

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Other C file commands!

• close(int fd)
• Close the file associated with the given file
  descriptor number.
• Can you close stdout? Try it.
• fclose(FILE stream)
• fclose can close stdout.

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Other C file commands!

• ssize_t read(int fd, void buf,
  size_t count)
• Read up to count bytes from a file descriptor
  into the buffer buf.
• ssize_t write(int fd, void buf,
  size_t count)
• Write count bytes to a file descriptor from the
  buffer buf.

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Buffered I/O versions

• size_t fread(void ptr, size_t size,
  size_t count, FILE
  stream)
• Read up to countsize bytes from a file
  descriptor into the buffer ptr.
• size_t fwrite(void ptr, size_t size,
  size_t count, FILE
  stream)
• Write countsize bytes to a file descriptor from
  the buffer ptr.

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Other C file commands!

• off_t lseek(int fd, off_t offset,
  int whence)
• Seek to a different point in the file.
• lseek(fd, 4, SEEK_SET)
• Seek four bytes after the beginning of the file.
• lseek(fd, -4, SEEK_END)
• Seek four bytes before the end of the file.
• lseek(fd, 16, SEEK_CUR)
• Seek sixteen bytes ahead of the current position.

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Other C file commands!

• int fseek(FILE stream, long int
  offset, int origin)
  
• fseek(stream, 4, SEEK_SET)
• Seek four bytes after the beginning of the file.
• fseek(stream, -4, SEEK_END)
• Seek four bytes before the end of the file.
• fseek(stream, 16, SEEK_CUR)
• Seek sixteen bytes ahead of the current position.

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UNIX File Systems
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UNIX File Systems
inode per-file data structure Advantage Efficient
for small files Flexible if the size
changes Disadvantage File must fit in a single
disk partition
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UNIX File Systems
inode (continued) Storing Large Files
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Directories are files too!

• Directories, like files, have inodes with
  attributes and pointers to disk blocks

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Directories are files too!

• Directories, like files, have inodes with
  attributes and pointers to disk blocks
• Each directory contains the name and i-node for
  each file in the directory.

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Directories are files too!

• Directories, like files, have inodes with
  attributes and pointers to disk blocks
• Each directory contains the name and i-node for
  each file in the directory.

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Directory functions
include ltunistd.hgt Change the directory int
chdir(const char path) Get the current working
directory char getcwd(char buf, size_t size)
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Directory reading functions
include ltdirent.hgt Open the directory DIR
opendir(const char dirname) Close the
directory int closedir(DIR dirp) Read the
directory struct dirent readdir(DIR dirp)
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Whats in a directory entry?
struct dirent Member Fields char
d_name256 Null-terminated file name ino_t
d_ino inode number unsigned char
d_reclen Length of this record unsigned char
d_type Type of file (DT_REG, DT_DIR,
DT_FIFO, DT_SOCK, DT_CHR,
DT_BLK, DT_UNKNOWN)
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Example 1

• Use opendir and readdir to print all the
  filenames in the current directory
• include ltdirent.hgt
• DIR dir
• struct dirent entry
• dir opendir(.)
• while(entry readdir(dir))
• printf(s\n,entry-gtd_name)
• closedir(dir)
• Remember to include error checking!!

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Example 2

• Modify Example 1 to use the member fields of
  struct dirent to display the inode for each file,
  as well as whether the file is a directory or a
  regular file.
• include ltdirent.hgt
• DIR dir
• struct dirent entry
• dir opendir(.)
• while(entry readdir(dir))
• printf(s lu\n,entry-gtd_name, entry-gtd_ino)
• if(entry-gtd_type DT_DIR)
• printf(Directory )
• else if(entry-gtd_type DT_REG)
• printf(File )
• closedir(dir)
• Remember to include error checking!!

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More Directory Functions
include ltdirent.hgt Set the position of next
readdir void seekdir(DIR dir, off_t
offset) Set the position back to the start of
the directory void rewinddir(DIR dirp) Get the
current location of directory stream off_t
telldir (DIR dir)
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Warning! Warning!
opendir and readdir are NOT thread-safe. DO
NOT open two directories at the same time!
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How to recursively traverse a directory tree

• Open the directory (opendir)
• Read each entry (readdir)
• If the file is a directory (d_type DT_DIR),
  store it (e.g. in an array of strings).
• Close the directory (closedir)
• Traverse each saved subdirectory, EXCEPT '.' and
  '..'

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File information stat
Use the stat functions to view the files inodes
attributes. include ltsys/stat.hgt include
ltsys/types.hgt include ltunistd.hgt For a
file int stat(const char restrict path, struct
stat restrict buf) For a link int lstat(const
char restrict path, struct stat restrict
buf) For a file descriptor int fstat(int
fildes, struct stat buf)
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Useful fields and macros in struct stat

• stat.st_size
• File size, in bytes
• stat.st_mode
• File type
• S_ISDIR(stat.st_mode)
• Is this a directory?
• User permissions
• Etc.
• stat.st_mtime
• Time of last modification

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Example 3

• Modify Example 2 to also give file information
  about each file.
• How large is each file?
• Which files are world-readable?
• Which files have been modified in the last 24
  hours?
• Hint man 2 stat

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include ltstdio.hgt include ltdirent.hgt include
lttime.hgt include ltsys/types.hgt include
ltsys/stat.hgt include ltunistd.hgt include
lterrno.hgt int main(int argc, char argv)
DIR dir struct dirent entry time_t now
time(NULL) if((dir opendir(".")) NULL)
perror("Can't open directory")
exit(-1) while((entry readdir(dir)) !
NULL) struct stat status
if(entry-gtd_type DT_DIR)
printf("Directory ") else printf("File
") printf("s is at inode d\n",
entry-gtd_name, entry-gtd_ino)
if(stat(entry-gtd_name, status))
perror("\tCan't get stat info") continue
printf("\t d bytes, ",
status.st_size) if(status.st_mode
S_IROTH) printf("World-readable, ")
if(difftime(now,status.st_mtime)lt86400)
printf("Recently updated, ")
closedir(dir)
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Links
Hard Link Directory Entry e.g. all regular
files Symbolic Link Also called a Soft Link A
special file that serves as a reference to
another file
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Link Functions
include ltunistd.hgt To create a new link int
link(const char oldpath, const char
newpath) Same as ln To remove an entry from
the directory int unlink(const char
path) Same as rm Returns 0 if successful, -1
with errno set if unsuccessful
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Hard Link Example
Command Line ln /dirA/name1 /dirB/name2 C Code
Segments if (link("/dirA/name1", "/dirB/name2")
-1) perror("Failed to make a new link in
/dirB")
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Hard Link Example (contd)
Q What happens if /dirA/name1 is deleted and
recreated?
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Hard Link Example (contd)
A /dirA/name1 and /dirB/name2 are now two
distinct files.
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Symbolic Link Function
include ltunistd.hgt To create a symbolic
link int symlink(const char oldpath, const
char newpath) Same function as command ln
s Returns 0 if successful, -1 with errno set if
unsuccessful
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Soft Link Example
Command Line ln s /dirA/name1 /dirB/name2 C
Code Segments if (symlink("/dirA/name1",
"/dirB/name2") -1) perror("Failed to create
a symbolic link in /dirB")
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Soft Link Example (contd)
Q What happens if /dirA/name1 to is deleted and
recreated?
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Soft Link Example (contd)
A /dirA/name1 has a different inode, but
/dir/name2 still links to it.
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Link number

• The link number (the st_nlink field in stat)
  tells how many directory entries link to this
  inode. The link number is
• Set to 1 when a file is created
• Incremented when link is called
• Decremented when unlink is called
• The link number appears in the second column of
  the output of ls l. Try it!
• The link number only counts hard links, not soft
  links.