Accessing OS Services

• The mechanism used to provide access to OS services (i.e., enter the operating system and perform a ``privileged operation'') is commonly known as a system call. The (only) difference between a ``procedure call'' and a ``system call'' is that a system call changes the execution mode of the CPU (to supervisor mode) whereas a procedure call does not.
• System call interface: A set of functions that are called by (user) programs to perform specific tasks. System call groups:
  • Process control, fork(), exec(), wait(), abort()
  • File manipulation, chmod(), link(), stat(), creat()
  • Device manipulation, open(), close(), ioctl(), select()
  • Information maintenance, time(), acct(), gettimeofday()
  • Communications, socket(), accept(), send(), recv()

Figure 4: System Call