System Calls and Libraries

• The book defines a system call as a controlled entry point into the kernel.
  • This allows the process to request the kernel perform an action
  • This is the kernel Application Programming Interface (API)
• This includes
  • Creating a new process
  • I/O
  • Interprocess communications.
• He suggests man 2 syscalls
• System Calls
  • Change the processor state from user mode to kernel mode
  • There are a fixed set, the implementation is actually by number
• When a system call is made
  • A wrapper function is employed to make the system call just like a normal procedure call
  • The arguments are taken from the stack and placed in registers by the wrapper function.
  • The system call number is written to a register.
  • On an X86 the trap instruction is called.
    • This is the actual switch to kernel space
  • The kernel then
    • Saves the registers on the kernel stack
    • Calls the appropriate service routine based on the system call number.
    • restores the registers
    • Returns control to the wrapper function in user mode.
  • If there is an error, the global variable errno is set.
  • An appropriate return value is set.
• Library functions
  • For convenience, there are a number of library functions provided to access the kernel
  • These generally do some setup then perform a system call
  • For example, brk is the way to manipulate the heap
  • But malloc and free and new and delete are library functions which simplify this task.