OS PROGRAM -2 WITH THEORY

THEORY:

     System calls                      Function

      open              open an existing file or create a new file

      read              Read data from a file

      write             Write data to a file

      lseek             Move the read/write pointer to the

 specified location

      close             Close an open file

Open() system call:

used to open an existing file or to create a new file if it does not   

 exist already.

 Syntax:

int open(const char *pathname, int flags);

int open(const char *pathname, int flags, mode_t mode);

Read()  System call: 

read from a file descriptor

Syntax:

ssize_t read(int fd, void *buf, size_t nbytes);

Write()  System call:

 write to a file descriptor

syntax:

ssize_t write(int fd, const void *buf, size_t nbytes);

Close() System call: 

 closes a file

Syntax:

int close(int fd);

lseek() System call: 

allows random access to a file by reposition the offset for next read or write

Syntax:

off_t lseek(int fd, off_t offset, int reference);

PRE_WORK:

CREATE A FILE(eg:file)

(USING cat COMMAND)

PROGRAM:

*/

#include<stdio.h>

#include<fcntl.h>

#include<string.h>

#include<stdlib.h>

main()

{

      int fd,wcount,i;

      char buf[]="linux lab";

      fd=open("file",O_RDWR|O_CREAT,0755);

     if(fd==-1)

      {

            printf("cannot open file");

            exit(1);

      }    

      wcount=write(fd,buf,strlen(buf));

      if(wcount!=strlen(buf))

      {

            printf("write failed not enough space");

            exit(2);

      }

            lseek(fd,0,SEEK_SET);

            read(fd,0,wcount);

            printf("file contents %s\n",buf);

            close(fd);

     

}