UNIX directories

1
Interprocess communication
2
1. Pipes

• A form of interprocess communication Between
  processes that have a common ancestor
• Typical use
• Pipe created by a process
• Process calls fork()
• Pipe used between parent and child

3
Differences between versions

• All systems support half-duplex
• Data flows in only one direction
• Many newer systems support full duplex
• Data flows in two directions
• For portability, assume only half-duplex

4
Creating a pipe

• include ltunistd.hgt
• int pipe(int filedes2)
• Returns 0 if ok, -1 on error
• Returns two file descriptors
• filedes0 is open for reading
• filedes1 is open for writing
• Output of filedes1 is input to filedes0

5
After the pipe() call
0 stdin
1 stdout
2 stderr
3
4
5
Filedes 0 1
3 4
6
The process then calls fork()
0 stdin
1 stdout
2 stderr
3
4
5
0 stdin
1 stdout
2 stderr
3
4
5
Child
Parent
7
And then .

• We close the read end in one process
• And close the write end in the other process
• To get .

8
Parent writing to child
0 stdin
1 stdout
2 stderr
3
4 X
5
0 stdin
1 stdout
2 stderr
3 X
4
5
Child
Parent
9
Child writing to parent
0 stdin
1 stdout
2 stderr
3 X
4
5
0 stdin
1 stdout
2 stderr
3
4 X
5
Child
Parent
10
After one end of the pipe is closed

• Reading from a empty pipe whose write end has
  been closed returns 0 (indicating EOF)
• Writing to a pipe whose read end has been closed
  generates a SIGPIPE signal
• If we ignore the signal or catch and return,
  handler returns -1, and errno set to EPIPE

11
Example

• include ltunistd.hgt
• include ltstdio.hgt
• int main(void)
• int n // to keep track of num
  bytes read
• int fd2 // to hold fds of both
  ends of pipe
• pid_t pid // pid of child process
• char line80 // buffer to hold text
  read/written

12
Continued

• if (pipe(fd) lt 0) // create
  the pipe
• perror("pipe error")
• if ((pid fork()) lt 0) // fork
  off a child
• perror("fork error")
• else if (pid gt 0) //
  parent process
• close(fd0) // close
  read end
• write(fd1, "hello world\n", 12) //
  write to it

13
continued

• else // child
  process
• close(fd1) // close
  write end
• n read(fd0, line, 80) // read
  from pipe
• write(1, line, n) // echo
  to screen
• exit(0)

14
dup() and dup2

• include ltunistd.hgt
• int dup(int filedes)
• int dup2(int filedes, int filedes2)
• Both will duplicate an existing file descriptor
• dup() returns lowest available file descriptor,
  now referring to whatever filedes refers to
• dup2() - filedes2 (if open) will be closed and
  then set to refer to whatever filedes refers to

15
DUP

• Duplicate a file descriptor (system call)
• int dup( int fd )
• duplicates fd as the lowest unallocated
  descriptor
• Commonly used to redirect stdin/stdout
• Example redirect stdin to foo
• int fd
• fd open(foo, O_RDONLY, 0)
• close(0)
• dup(fd)
• close(fd)

16
DUP2

• For convenience
• dup2( int fd1, int fd2 )
• use fd2(new) to duplicate fd1 (old)
• closes fd2 if it was in use
• Example redirect stdin to foo
• fd open(foo, O_RDONLY, 0)
• dup2(fd,0)
• close(fd)

17
Pipes and Standard I/O

• int pid, p2
• if (pipe(p) -1)
• exit(1)
• pid fork()
• if (pid 0)
• close(p1)
• dup2(p0,0)
• close(p0)
• ... read from stdin ...
• else
• close(p0)
• dup2(p1,1)
• close(p1)
• ... write to stdout ...
• wait(status)

18
Pipes and Exec()

• int pid, p2
• if (pipe(p) -1)
• exit(1)
• pid fork()
• if (pid 0)
• close(p1)
• dup2(p0,0)
• close(p0)
• execl(...)
• else
• close(p0)
• dup2(p1,1)
• close(p1)
• ... write to stdout ...
• wait(status)

19
ls more example

• When command shells interprets ls more, it
• 1. Invokes the pipe( ) system call let us assume
  that pipe( ) returns the file descriptors 3 (the
  pipe's read channel ) and 4 (the write channel ).
• 2. Invokes the fork( ) system call twice.
• 3. Invokes the close( ) system call twice to
  release file descriptors 3 and 4.

20
ls more example

• The first child process, which must execute
  the ls program, performs the following
  operations
• 1. Invokes dup2(4,1) to copy file descriptor 4 to
  file descriptor 1. From now on, file descriptor 1
  refers to the pipe's write channel.
• 2. Invokes the close( ) system call twice to
  release file descriptors 3 and 4.
• 3. Invokes the execve( ) system call to execute
  the /bin/ls program. By default, such a program
  writes its output to the file having file
  descriptor 1 (the standard output), that is, it
  writes into the pipe.

21
ls more example

• The second child process must execute the more
  program therefore, it performs the following
  operations
• 1. Invokes dup2(3,0) to copy file descriptor 3 to
  file descriptor 0. From now on, file descriptor 0
  refers to the pipe's read channel.
• 2. Invokes the close( ) system call twice to
  release file descriptors 3 and 4.
• 3. Invokes the execve( ) system call to execute
  /bin/more. By default, that program reads its
  input from the file having file descriptor (the
  standard input) that is, it reads from the pipe.

22
popen and pclose

• include ltstdio.hgt
• FILE popen(const char cmdstring, const char
  type)
• The popen( ) function receives two parameters
• the cmdstring pathname of an executable file
• a type string specifying the direction of the
  data transfer (r or w).
• It returns the pointer to a FILE data structure
  (fp).
• popen runs cmdstring with output or input
  directed to fp based on value of type parameter.
• Handle the dirty work of creating pipe, forking
  child, closing unused ends, executing shell to
  run program, waiting for command to terminate
• int pclose(FILE fp)
• The pclose( ) function, which receives the file
  pointer returned by popen( ) as its parameter,
  simply invokes the wait4( ) system call and waits
  for the termination of the process created by
  popen( ).

23
Other techniques for IPC

• Socket programming
• We have seen this before in internet engineering.
• It is mainly used for IPC between two network
  processes.
• Semaphores
• Seen before.
• Shared memory
• shmget() for Create/Access Shared Memory
• shmat() for Accessing Shared Memory
• shmctl() for controlling shared memory
• shmdt() for deleting shared memory
• Messages
• msgget( KEY, IPC_CREATIPC_EXCL) for
  Create/access
• msgctl( id, IPC_RMID ) for Control
• Send/receive
• msgsnd( id, buf, text_size, flags )
• msgrcv( id, buf, max_size, flags )
• FIFO
• mkfifo( ) specifically to create a FIFO.
• Once created, a FIFO can be accessed through the
  usual open( ), read( ), write( ), and close( )
  system calls.

24
Directory structure
25
Directory Structure

• A directory file is a sequence of lines each
  line holds an i-node number and a file name.
• The data is stored as binary, so we cannot simply
  use cat to view it

26

• I-node
• The administrative information about a file is
  kept in a structure known as an inode.
• Inodes in a file system, in general, are
  structured as an array known as an inode table.
• An inode number, which is an index to the inode
  table, uniquely identifies a file in a file
  system.

27
i-node and Data Blocks
28
2. Links

• 2.1 What is a Link?
• 2.2 Creating a Link
• 2.3 Seeing Links
• 2.4 Removing a Link
• 2.5 Symbolic Links
• 2.6 Implementation

29
2.1. What is a Link?

• A link is a pointer to a file.
• Useful for sharing files
• a file can be shared by giving each person their
  own link (pointer) to it.

30
2.2. Creating a Link

• ln existing-file new-pointer
• Jenny types
• ln draft /home/bob/letter

/
/home/bob/draftand /home/jenny/letter
home
bob
jenny
memo
planning
31

• Changes to a file affects every link cat
  file_a This is file A. ln file_a file_b
  cat file_b This is file A.
• vi file_b
• cat file_bThis is file B after the change.
  cat file_aThis is file B after the change.

32
2.3. Seeing Links

• Compare status information
• ls -l file_a file_b file_c file_d-rw-r--r--
  2 dkl 33 May 24 1052 file_a-rw-r--r-- 2 dkl 33
  May 24 1052 file_b-rw-r--r-- 1 dkl 16 May 24
  1055 file_c-rw-r--r-- 1 dkl 33 May 24 1057
  file_d
• Look at inode number
• ls -i file_a file_b file_c file_d3534 file_a
  3534 file_b 5800 file_c 7328 file_d

33
2.4. Removing a Link

• Deleting a link does not remove the file.
• Only when the file and every link is gone will
  the file be removed.

34
2.5. Symbolic Links

• The links described so far are often called hard
  links
• a hard link is a pointer to a file which must be
  on the same file system
• A symbolic link is an indirect pointer to a file
• it stores the pathname of the pointed-to file
• it can link across file systems

35

• Jenny types
• ln -s shared /home/dkl/project

/
/home/jenny/sharedand /home/dkl/project
home
dkl
jenny
separate file system
memo
planning
36

• Symbolic links are listed differently
• ln -s pics /home/mh/img
• ls -lF pics /home/mh/img
• drw-r--r-- 1 dkl staff 981 May 24 1055 pics
• lrwxrwxrxw 1 dkl staff 4 May 24
  1057/home/mh/img --gt pics

37
2.6 Link Creation, Update Removal
cp bob new
ln bob new
ln -s bob new
bob
abc
abc
abc
update new
delete new
?
new
bob
new
bob
bob
new
abc
XY
abc
XY
abc
abc
XY
continued
38
2.7 link() and unlink()

• include ltunistd.hgt
• int link( const char oldpath, const char
  newpath )
• Meaning of link( abc, xyz )

fred.html
120
abc
207
135
bookmark.c
xyz
207

continued
39

• unlink() clears the directory record
• usually means that the i-node number is set to 0
• The i-node is only deleted when the last link to
  it is removed the data block for the file is
  also deleted (reclaimed) no process have the
  file opened

40
Example unlink

• include ltstdio.hgt
• include ltsys/stat.hgt
• include ltsys/types.hgt
• include ltfcntl.hgt
• int main(void)
• if( open( "tempfile", O_RDWR ) lt 0 )
• perror( "open error )
• exit( 1 )
• if( unlink( "tempfile ) lt 0 )
• perror( "unlink error )
• exit( 1 )
• printf( "file unlinked\n )
• exit(0)

41
symlink()

• include ltunistd.hgt
• int symlink(const char oldpath, const char
  newpath)
• Creates a symbolic link named newpath which
  contains the string oldpath.
• Symbolic links are interpreted at run-time.
• Dangling link may point to an non-existing
  file.
• If newpath exists it will not be overwritten.

42
readlink()

• include ltunistd.hgt
• int readlink( const char path, char buf,
• size_t bufsiz )
• Read value of a symbolic link (does not follow
  the link).
• Places the contents of the symbolic link path in
  the buffer buf, which has size bufsiz.
• Does not append a NULL character to buf.
• Return value
• The count of characters placed in the buffer if
  it succeeds.
• -1 if an error occurs.

43
3. Subdirectory Creation

• mkdir uga causes
• the creation of a uga directory file and an
  i-node for it
• an i-node number and name are added to the parent
  directory file

120
fred.html
207
abc
135
bookmark.c
201
uga

44
4. . and ..

• . and .. are stored as ordinary file names
  with i-node numbers pointing to the correct
  directory files.
• Example

dkl
book
memos
continued
45
In more detail
Directory ben
123
.
247
..
260
book
401
memos
Directory book
Directory memos
260
.
401
.
123
..
123
..
566
chap1
800
kh
567
chap2
810077
kd
chap3
590
mw
590
46
5. mkdir()

• include ltsys/types.hgtinclude
  ltfcntl.hgtinclude ltunistd.hgtint mkdir(char
  pathname, mode_t mode)
• Creates a new directory with the specified mode
  return 0 if ok, -1 on error

continued
47

• . and .. entries are added automatically
• mode must include execute permissions so the
  user(s) can use cd.
• e.g. 0755

48
6. rmdir()

• include ltunistd.hgtint rmdir(char pathname)
• Delete an empty directoryreturn 0 if ok, -1 on
  error.
• Will delay until other processes have stopped
  using the directory.

49
7. Reading Directories

• include ltsys/types.hgtinclude ltdirent.hgtDIR
  opendir(char pathname)struct dirent
  readdir(DIR dp)int closedir(DIR dp)

returns apointer if ok, NULL on error
returns apointer if ok, NULL at end or on error
50
dirent and DIR

• struct dirent
• long d_ino / i-node number /char
  d_nameNAME_MAX1 / fname /off_t d_off
  / offset to next rec /unsigned short
  d_reclen / record length /
• DIR is a directory stream (similar to FILE)
• when a directory is first opened, the stream
  points to the first entry in the directory

51
Example listdir.c
List the contents of the current directory.

• include ltstdio.hgtinclude ltdirent.hgtint
  main()
• DIR dp struct dirent dir if( (dp
  opendir(.)) NULL )
• fprintf( stderr, Cannot open dir\n
  ) exit(1)

continued
52

• / read entries / while( (dir
  readdir(dp)) ! NULL )
• / ignore empty records / if(
  dir-gtd_ino ! 0 ) printf( s\n,
  dir-gtd_name )
• closedir( dp ) return 0 / end main
  /

53
8. chdir()

• include ltunistd.hgt
• int chdir( char pathname )
• int fchdir( int fd )
• Change the current working directory (cwd) of the
  calling process return 0 if ok, -1 on error.

54
Example cd to /tmp

• Part of to_tmp.c
• if( chdir(/tmp ) lt 0 printf( chdir
  error\n) else printf( In /tmp\n )

55
Directory Change is Local

• The directory change is limited to within the
  program.
• e.g.
• pwd/usr/lib to_tmp / from last slide
  /In /tmp pwd/usr/lib

56
9. getcwd()

• include ltunistd.hgtchar getcwd(char buf, int
  size)
• Store the cwd of the calling process in
  bufreturn buf if ok, NULL on error.
• buf must be big enough for the pathname string
  (size specifies the length of buf).

57
Example

• include ltstdio.hgtinclude ltunistd.hgtinclude
  ltdirent.hgt / for NAME_MAX /int main()
• char nameNAME_MAX1 if( getcwd( name,
  NAME_MAX1 ) NULL ) printf( getcwd
  error\n ) else printf( cwd s\n,
  name )

58
10. Walking over Directories

• 'Visit' every file in a specified directory and
  all of its subdirectories
• visit means get the name of the file
• Apply a user-defined function to every visited
  file.

59
Function Prototypes

• include ltftw.hgt/ ftw means file tree walk,
  starting at directory /int ftw( char
  directory, MyFunc fp, int depth )/
  apply MyFunc() to each visited file /typedef
  int MyFunc( const char file, struct stat
  sbuf, int flag )

continued
60

• depth is the maximum number of directories that
  can be open at once. Safest value is 1, although
  it slows down ftw().
• Result of ftw() 0 for a successful visit of
  every file, -1 on error.

61
MyFunc Details

• The file argument is the pathname relative to the
  start directory
• it will be passed to MyFunc() automatically by
  ftw() as it visits each file
• sbuf argument is a pointer to the stat
  information for the file being examined.

continued
62

• The flag argument will be set to one of the
  following for the item being examined
• FTW_F Item is a regular file.
• FTW_D Item is a directory.
• FTW_NS Could not get stat info for item.
• FTW_DNR Directory cannot be read.
• If the MyFunc function returns a non-zero value
  then the ftw() walk will terminate.

63
Example shower.c
Print the names of all the files found below the
current directory.

• include ltstdio.hgtinclude ltsys/types.hgtinclud
  e ltsys/stat.hgtinclude ltftw.hgtint shower(const
  char file, const struct stat sbuf, int
  flag)void main() ftw(., shower, 1)

continued
64

• int shower(const char file, const struct
  stat sbuf, int flag) if (flag FTW_F)
  / is a file / printf("Found s\n",
  file) return 0