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Exceptional Control Flow
Part I
CSCE 230J
Computer Organization
Dr. Steve Goddard
http://cse.unl.edu/~goddard/Courses/CSCE230J
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Giving credit where credit is due
Most of slides for this lecture are based on
slides created by Drs. Bryant and
O’Hallaron, Carnegie Mellon University.
I have modified them and added new
slides.
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Topics
Exceptions
Process context switches
Creating and destroying processes
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Control Flow
inst 1 inst 2 inst 3 … instn
Computers do Only One Thing
From startup to shutdown, a CPU simply reads and executes
(interprets) a sequence of instructions, one at a time.
This sequence is the system’s physical control flow (or flow
of control ).
Physical control flow
Time
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Altering the Control Flow
Up to Now: two mechanisms for changing control flow:
Jumps and branches
Call and return using the stack discipline.
Both react to changes in program state.
Insufficient for a useful system
Difficult for the CPU to react to changes in system state.
data arrives from a disk or a network adapter. Instruction divides by zero User hits ctl-c at the keyboard System timer expires
System needs mechanisms for “exceptional control
flow”
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Exceptional Control Flow
Mechanisms for exceptional control flow exists at all levels of a computer system.
Low level Mechanism
exceptions change in control flow in response to a system event (i.e., change in system state) Combination of hardware and OS software
Higher Level Mechanisms
Process context switch Signals Nonlocal jumps (setjmp/longjmp) Implemented by either: OS software (context switch and signals). C language runtime library: nonlocal jumps.
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System context for exceptions
Local/IO Bus
Memory
Network adapter
IDE disk controller
Video adapter
Display Network
Processor Interrupt controller
SCSI
controller
SCSI bus
Serial port controller
Parallel port controller
Keyboard controller
Keyboard Mouse Modem Printer
disk disk CDROM
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Exceptions
An exception is a transfer of control to the OS in response to some event (i.e., change in processor state)
User Process OS
exception exception processing by exception handler
exception return (optional)
event current next
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Interrupt Vectors
Each type of event has a unique exception number k Index into jump table (a.k.a., interrupt vector) Jump table entry k points to a function (exception handler). Handler k is called each time exception k occurs.
interrupt vector 0 1 2 ...
n-
code for exception handler 0
code for exception handler 1
code for exception handler 2
code for exception handler n-
...
Exception numbers
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Asynchronous Exceptions (Interrupts)
Caused by events external to the processor
Indicated by setting the processor’s interrupt pin
handler returns to “next” instruction.
Examples:
I/O interrupts
hitting ctl-c at the keyboard arrival of a packet from a network arrival of a data sector from a disk
Hard reset interrupt
hitting the reset button
Soft reset interrupt
hitting ctl-alt-delete on a PC
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Synchronous Exceptions
Caused by events that occur as a result of executing an instruction:
Traps
Intentional Examples: system calls, breakpoint traps, special instructions Returns control to “next” instruction
Faults
Unintentional but possibly recoverable Examples: page faults (recoverable), protection faults (unrecoverable). Either re-executes faulting (“current”) instruction or aborts.
Aborts
unintentional and unrecoverable Examples: parity error, machine check. Aborts current program
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Trap Example
User Process OS
exception Open file
return
int pop
Opening a File
User calls open(filename, options)
Function open executes system call instruction int
OS must find or create file, get it ready for reading or writing
Returns integer file descriptor
0804d070 <__libc_open>:
... 804d082: cd 80 int $0x 804d084: 5b pop %ebx ...
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Context Switching
Processes are managed by a shared chunk of OS code
called the kernel
Important: the kernel is not a separate process, but rather
runs as part of some user process
Control flow passes from one process to another via a
context switch.
Process A code
Process B code user code
kernel code user code
kernel code
user code
Time
context switch
context switch
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Private Address Spaces
Each process has its own private address space.
kernel virtual memory (code, data, heap, stack)
memory mapped region for shared libraries
run-time heap (managed by malloc)
user stack (created at runtime)
0 unused
%esp (stack pointer)
memory invisible to user code
brk
0xc
0x
0x
read/write segment (.data, .bss) read-only segment (.init, .text, .rodata)
loaded from the executable file
0xffffffff
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fork : Creating new processes
int fork(void)
creates a new process (child process) that is identical to the
calling process (parent process)
returns 0 to the child process
returns child’s pid to the parent process
if (fork() == 0) { printf("hello from child\n"); } else { printf("hello from parent\n"); }
Fork is interesting (and often confusing) because it is called once but returns twice
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Fork Example #
void fork1() { int x = 1; pid_t pid = fork(); if (pid == 0) { printf("Child has x = %d\n", ++x); } else { printf("Parent has x = %d\n", --x); } printf("Bye from process %d with x = %d\n", getpid(), x); }
Key Points
Parent and child both run same code
Distinguish parent from child by return value from fork
Start with same state, but each has private copy
Including shared output file descriptor Relative ordering of their print statements undefined
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Fork Example #
void fork2() { printf("L0\n"); fork(); printf("L1\n"); fork(); printf("Bye\n"); }
Key Points
Both parent and child can continue forking
L0 L
L
Bye
Bye
Bye
Bye
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Fork Example #
void fork3() { printf("L0\n"); fork(); printf("L1\n"); fork(); printf("L2\n"); fork(); printf("Bye\n"); }
Key Points
Both parent and child can continue forking
L1 L
L
Bye
Bye
Bye
Bye
L1 L
L
Bye
Bye
Bye
Bye
L
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Fork Example #
void fork4() { printf("L0\n"); if (fork() != 0) { printf("L1\n"); if (fork() != 0) { printf("L2\n"); fork(); } } printf("Bye\n"); }
Key Points
Both parent and child can continue forking
L0 L
Bye
L
Bye
Bye
Bye
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Fork Example #
void fork5() { printf("L0\n"); if (fork() == 0) { printf("L1\n"); if (fork() == 0) { printf("L2\n"); fork(); } } printf("Bye\n"); }
Key Points
Both parent and child can continue forking
L0 Bye
L
Bye
Bye
Bye
L
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exit : Destroying Process
void exit(int status)
exits a process
Normally return with status 0
atexit() registers functions to be executed upon exit
void cleanup(void) { printf("cleaning up\n"); }
void fork6() { atexit(cleanup); fork(); exit(0); }
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Zombies
Idea
When process terminates, still consumes system resources
Various tables maintained by OS
Called a “zombie”
Living corpse, half alive and half dead
Reaping
Performed by parent on terminated child
Parent is given exit status information
Kernel discards process
What if Parent Doesn’t Reap?
If any parent terminates without reaping a child, then child
will be reaped by init process
Only need explicit reaping for long-running processes
E.g., shells and servers
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linux> ./forks 7 & [1] 6639 Running Parent, PID = 6639 Terminating Child, PID = 6640 linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6639 ttyp9 00:00:03 forks 6640 ttyp9 00:00:00 forks 6641 ttyp9 00:00:00 ps linux> kill 6639 [1] Terminated linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6642 ttyp9 00:00:00 ps
Zombie
Example
ps shows child
process as “defunct”
Killing parent allows
child to be reaped
void fork7() { if (fork() == 0) { / Child / printf("Terminating Child, PID = %d\n", getpid()); exit(0); } else { printf("Running Parent, PID = %d\n", getpid()); while (1) ; / Infinite loop / } }
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linux> ./forks 8 Terminating Parent, PID = 6675 Running Child, PID = 6676 linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6676 ttyp9 00:00:06 forks 6677 ttyp9 00:00:00 ps linux> kill 6676 linux> ps PID TTY TIME CMD 6585 ttyp9 00:00:00 tcsh 6678 ttyp9 00:00:00 ps
Nonterminating
Child
Example
Child process still active
even though parent has
terminated
Must kill explicitly, or else
will keep running
indefinitely
void fork8() { if (fork() == 0) { / Child / printf("Running Child, PID = %d\n", getpid()); while (1) ; / Infinite loop / } else { printf("Terminating Parent, PID = %d\n", getpid()); exit(0); } }
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Summarizing
Exceptions
Events that require nonstandard control flow
Generated externally (interrupts) or internally (traps and faults)
Processes
At any given time, system has multiple active processes
Only one can execute at a time, though
Each process appears to have total control of processor + private
memory space
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Summarizing (cont.)
Spawning Processes
Call to fork
One call, two returns
Terminating Processes
Call exit
One call, no return
Reaping Processes
Call wait or waitpid
Replacing Program Executed by Process
Call execl (or variant)
One call, (normally) no return
