fcntl 的学习过程

  
    

   
     #include 
   
     <
   
     sys
   
     /
   
     types.h
   
     >
   
     
#include 
   
     <
   
     sys
   
     /
   
     stat.h
   
     >
   
     
#include 
   
     <
   
     fcntl.h
   
     >
   
     
#include 
   
     "
   
     ourhdr.h
   
     "
   
     
#include 
   
     "
   
     error.c
   
     "
   
     
#include 
   
     "
   
     f10.17.c
   
     "
   
     

   
     static
   
      
   
     void
   
      lockabyte(
   
     const
   
      
   
     char
   
      
   
     *
   
     , 
   
     int
   
     , off_t );


   
     int
   
      lock_reg( 
   
     int
   
      fd, 
   
     int
   
      cmd, 
   
     int
   
      type, off_t offset, 
   
     int
   
      whence, off_t len )
{
 
   
     struct
   
      flock 
   
     lock
   
     ;

 
   
     lock
   
     .l_type 
   
     =
   
      type; 
   
     /*
   
      F_RDLCK, F_WRLCK, F_UNLCK 
   
     */
   
     
 
   
     lock
   
     .l_start 
   
     =
   
      offset; 
   
     /*
   
      byte offset, relative to l_whence 
   
     */
   
     
 
   
     lock
   
     .l_whence 
   
     =
   
      whence; 
   
     /*
   
      SEEK_SET, SEEK_CUR, SEEK_END 
   
     */
   
     
 
   
     lock
   
     .l_len 
   
     =
   
      len; 
   
     /*
   
      bytes ( 0 means to EOF )
   
     */
   
     

 
   
     return
   
      ( fcntl( fd, cmd, 
   
     &
   
     lock
   
     ) );
}


   
     int
   
      main( 
   
     void
   
      )
{
 
   
     int
   
      fd;
 pid_t pid;

 
   
     /*
   
      create a file and write two bytes to it 
   
     */
   
     
 
   
     if
   
      ( ( fd 
   
     =
   
      creat( 
   
     "
   
     templock
   
     "
   
     , FILE_MODE ) ) 
   
     <
   
      
   
     0
   
      )
 err_sys( 
   
     "
   
     create error
   
     "
   
      );

 
   
     if
   
      ( write( fd, 
   
     "
   
     ab
   
     "
   
     , 
   
     2
   
      ) 
   
     !=
   
      
   
     2
   
      )
 err_sys( 
   
     "
   
     write error
   
     "
   
      );

 TELL_WAIT();
 
   
     if
   
      ( ( pid 
   
     =
   
      fork() ) 
   
     <
   
      
   
     0
   
      )
 err_sys( 
   
     "
   
     fork error
   
     "
   
      );
 
   
     else
   
      
   
     if
   
      ( pid 
   
     ==
   
      
   
     0
   
      ) { 
   
     /*
   
      child 
   
     */
   
     
 lockabyte( 
   
     "
   
     child
   
     "
   
     , fd, 
   
     0
   
      );
 TELL_PARENT( getppid() );
 WAIT_PARENT();
 lockabyte( 
   
     "
   
     child
   
     "
   
     , fd, 
   
     1
   
      );
 } 
   
     else
   
      {
 lockabyte( 
   
     "
   
     parent
   
     "
   
     , fd, 
   
     1
   
      );
 TELL_CHILD( pid );
 WAIT_CHILD();
 lockabyte( 
   
     "
   
     parent
   
     "
   
     , fd, 
   
     0
   
      );
 }
 exit( 
   
     0
   
      );
}


   
     static
   
      
   
     void
   
      lockabyte( 
   
     const
   
      
   
     char
   
      
   
     *
   
     name, 
   
     int
   
      fd, off_t offset )
{
 
   
     if
   
      ( writew_lock( fd, offset, SEEK_SET, 
   
     1
   
      ) 
   
     <
   
      
   
     0
   
      )
 err_sys( 
   
     "
   
     %s: writew_lock error
   
     "
   
     , name );

 printf( 
   
     "
   
     %s: got the lock, byte %d\n
   
     "
   
     , name, offset );
}