System Interface

1
System Interface

• Interface that provides services from the OS
• More efficient
• Access facility which is not in the library
• Unix System Call
• File System
• Storage System

2
File System

• A popular but also complex subsystem
• File descriptors (pseudo files, like stdin and
  stdout)
• Low level IO
• File Management
• Examples

file pointer
EOF
3
A little about File Attributes

• Unix file system ,each file is associated with 9
  bits permission information.
• -rwxr-xr-x 1 lina cisgrad 120320 Sep 27
  1438 lec01.ppt
• -rw-r--r-- 1 lina cisgrad 133632 Oct 9
  2334 lec02.ppt
• read, write and execute
• an octal digit, describes the permission to a set
  of users
• Three sets of different users, owner, group and
  others
• Some UNIX commands chmod, etc to change the
  attributes
• chmod 755

4
File Descriptor

• File pointer FILE fp
• point to a structure that contains information
  about the file
• Used in library functions
• File descriptor int fd
• non negative integer
• used to identify a file
• used in low level I/O (system call)

5
File Descriptor

• File pointer vs File descriptor
• standard input stdin 0
• standard output stdout 1
• standard error stderr 2
• All three are automatically opened when running a
  program
• Low level I/O can also use redirection and pipe
  to change the default setting of the three files.

6
Low level I/O read and write

• Read and Write
• getc() and putc() getchar() and putchar()
• int read(int fd, char buf, int n)
• Return -1 for error return 0 for EOF.
• int write(int fd, char buf, int n)
• Error return value does not equal to n

include stdio.h int main () char
buf1024 int n while ((n read(0,
buf, 1024)) gt 0) write ( 1, buf, n)
return 0
include stdio.h int getchar () char c
return (read(0, c, 1) 1 ) ? c EOF
7
Open

• includeltfcntl.hgt
• int fd
• fdopen (name, flags, perms)
• flags O_RDONLY O_WRONLY O_RDWR
• perms always 0 for uses of open
• Flags are defined in ltfcntl.hgt
• can not open non-existing files, -1 for errors

8
Creat

• create new files or rewrite old ones
• int fd
• fdcreat (name, perms)
• If file exists, creat will discard previous
  contents
• -1 for error
• use 9 bit file attributes as perms when creating
  files.
• This function also opens the new file

9
Close , Unlink

• int close(int fd)
• free file descriptor for reuse.
• correspond to fclose in standard library.
• int unlink(char path)
• remove a file form the file system.

10
Examples
include stdio.h / A copy program/ int main
() char f1 file1, f2file2
char buf1024 int fd1, fd2, n if
((fd1 open(f1, O_RDONLY, 0)) -1)
usage(unable to opening source file s\n, f1)
if ((fd2 creat(f2, 0666)) -1)
usage(unable to creating new file s \n, f2)
while ((n read(fd1, buf, 1024) )gt 0)
write ( fd2, buf, n) return 0
include ltstdio.hgt includeltstdarg.hgt void
usage(char fmt, ) va_list args
va_start(args, fmt) fprintf(stderr, error
) vfprintf(stderr, fmt, args)
va_end(args) fprint(usage srcfile
destfile\n)
11
Random Access-Lseek

• lseek
• long lseek (int fd, long offset, int origin)
• Origin 0, 1, 2, determines where to start
  offset.
• 0 beginning 1 current location 2 the end.
• compare with
• int fseek(FILE fp, long offset, int origin)

12
Example
include stdio.h / append file1 to file2/ int
main () char f1 file1, f2file2
int fd1, fd2, n char buf1024 if ((fd1
open(f1, O_RDONLY, 0)) -1) usage(
unable to opening source file s\n, f1) if
((fd2 creat(f2, 0666) ) -1)
usage(unable to creating new file s \n, f2)
else lseek(fd2, 0L, 2) while ((n
read(fd1, buf, 1024) ) gt 0) write ( fd2,
buf, n) return 0
13
Lab4 Assignment

• From lina_at_cse.ohio-state.edu Saturday Nov 18
  183355 2005
• Return-Path li.655_at_osu.edu
• How are you? ltNote email bodygt
• From lina_at_cse.ohio-state.edu Sunday Nov 19
  183255 2005
• Return-Path ltli.655_at_osu.edugtI am fine. ltNote
  email bodygt

14
Lab4 Assignment

• /home/2/lina lab4 mailbox.lina
• 1From lina_at_cse.ohio-state.edu Saturday Nov 18
  183355 2005
• 2 From lina_at_cse.ohio-state.edu Sunday Nov 19
  183255 2005
• Please choose which email you want to read
• 1
• How are you?
• Please choose which email you want to read
• 2
• I am fine.
• Please choose which email you want to read
• 0
• Thanks for using my simple mail reader. Bye.

15
Lab4 Assignment

• When started, read from a mailbox and print out
  the ordered list of 'From ' lines from all the
  emails.(30 points)
• Ask the user about which email he or she wants to
  read.
• When having received a choice, print out the BODY
  
• of that email.(35 points)
• Repeat a) -- c) unless the user says 0, at which
  point
• your utility command exits.(40 points)

16
Lab4 Assignment

• Read mailbox (a file)
• FILE fopen (char name, char mode)
• long ftell(FILE stream)
• int fseek(FILE fp, long offset, int origin)
• char fgets(char line, int maxline, FILE fp)
• int fclose(FILE fp)