User Level Software

• library calls
  • users generally make library cals that then make the system calls
  • example:
    • int count=write(fd,buffer,n);
    • write function is run at the user level
    • simply takes parameters and makes a system call
  • another example:

    • printf("My age: %d \n",age);
      

    • takes a string, reformats it, and then calls the write system call
• spooling
  • user program places data in a special directory
  • a daemon (background program) takes data from directoryy and outputs it to a device
    • the user doesn't have permission to directly access the device
    • daemon runs as a privileged user
  • prevents users from tying up resources for extented periods of time; printer example
  • OS never has to get involved in working with the I/O device