Exceptional Control Flow: Exceptions and Processes

Control flow

  • branch and jumps => program state
  • can’t react to changes in system state
    • event outside the system: data from disk, keyboard input, network adapter
    • instruction divides by zero
    • timer expires
    • …

Exceptional Control Flow

  • ubiquitous at all level of computer system

Asynchronous Exceptions (Interrupts)

  • processor’s interrupt pin
  • example:
    • Timer interrupt used by kernel
    • I/O interrupt

Synchronous Exception

  • Traps

    • Intentional
    • Example: system call, looks like a call, but in fact transferring control to the kernel

  • Faults

    • page faults, recoverable
    • protection faults, unrecoverable

  • Aborts

    • unintentional and unrecoverable
    • Example: illegal instruction, parity error

Processes

provides two abstractions:

  • Logical control flow
    • seems to have exclusive use of the cpu
    • by context switch
  • Private address space
    • seems to have exclusive use of the memory
    • by virtual memory

fork example

fork runs in the kernel, it copy every things in the process, including code and data, opened file descriptors, and start a new process.

fork call once and return twice, one in the parent process, one in the child process.

pid = Fork()
if (pid == 0) { // child code
    // ...
}

// parent code
// ...

/* execution flow:

(uer space) parent call fork 
    -> (kernel space) do stuff, copy, scheduling 
        -> (user space) parent process (next instruction, returned) -> ...
        -> (user space) child process (next instruction, returned) -> ...
*/

Zombie

child process exited but not reaped by the parent, becomes zombie (i.e., defunct)

  • wait: Synchronizing with Children
  • waitpid: ..

execve: run a different program

int execve(char* filename, char* argv[], char* envp[])

  • Load and runs in the current process
  • Overwrites code, data and stack
    • retain PID, open files and signal context
  • called once and never returns except error

todo: change font