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Transcript of CENG334-2013-W02a-ECF
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Carnegie Mellon
Exceptions and Processes
Slides adapted from:Gregory Kesden and Markus Pschel of Carnegie Mellon University
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Control Flow
inst1
inst2inst3
instn
Processors 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 CPUs control flow(orflow 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
Both react to changes inprogram state
Insufficient for a useful system:
Difficult to react to changes in system state
data arrives from a disk or a network adapter
instruction divides by zero
user hits Ctrl-C at the keyboard
System timer expires
System needs mechanisms for exceptional control flow
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Exceptional Control Flow
Exists at all levels of a computer system Low level mechanisms
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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Exceptions
An exceptionis a transfer of control to the OS in response tosome event (i.e., change in processor state)
Examples:
div by 0, arithmetic overflow, page fault, I/O request completes, Ctrl-C
User Process OS
exception
exception processing
by exception handler
return to I_current
return to I_next
abort
event I_currentI_next
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01
2...
n-1
Interrupt Vectors
Each type of event has a
unique exception number k
k = index into exception table
(a.k.a. interrupt vector)
Handler k is called each time
exception k occurs
ExceptionTable
code for
exception handler 0
code forexception handler 1
code for
exception handler 2
code for
exception handler n-1
...
Exception
numbers
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Asynchronous Exceptions (Interrupts)
Caused by events external to the processor Indicated by setting the processors interrupt pin
Handler returns to next instruction
Examples: I/O interrupts
hitting Ctrl-C at the keyboard
arrival of a packet from a network
arrival of data from a disk
Hard reset interrupt
hitting the reset button
Soft reset interrupt
hitting Ctrl-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), floating point exceptions 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: Opening File
User calls: open(filename, options)
Function openexecutes system call instruction int
OS must find or create file, get it ready for reading or writing
Returns integer file descriptor
0804d070 :. . .804d082: cd 80 int $0x80804d084: 5b pop %ebx
. . .
User Process OS
exception
open file
returns
intpop
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Fault Example: Page Fault
User writes to memory location That portion (page) of users memory
is currently on disk
Page handler must load page into physical memory
Returns to faulting instruction
Successful on second try
int a[1000];main ()
{a[500] = 13;
}
80483b7: c7 05 10 9d 04 08 0d movl $0xd,0x8049d10
User Process OS
exception: page fault
Create page and
load into memory
returns
movl
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Fault Example: Invalid Memory Reference
Page handler detects invalid address
Sends SIGSEGVsignal to user process
User process exits with segmentation fault
int a[1000];
main (){
a[5000] = 13;}
80483b7: c7 05 60 e3 04 08 0d movl $0xd,0x804e360
User Process OS
exception: page fault
detect invalid address
movl
signal process
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User mode vs. Kernel mode
Priviliged instructions
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Exception handlers
Return adress Depends on the class of exception
Current instruction (page fault)
Next instruction
Push some additional processor state Necessary to restart the interrupted program when the handler
returns
If the control is being transferred from user to kernel
All these items are pushed on the kernel stack
Exception handlers run in kernel mode
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Exception Table IA32 (Excerpt)
Exception Number Description Exception Class
0 Divide error Fault
13 General protection fault Fault
14 Page fault Fault
18 Machine check Abort
32-127 OS-defined Interrupt or trap
128 (0x80) System call Trap
129-255 OS-defined Interrupt or trap
Check pp. 183:
http://download.intel.com/design/processor/manuals/253665.pdf
Carnegie Mellon
http://download.intel.com/design/processor/manuals/253665.pdfhttp://download.intel.com/design/processor/manuals/253665.pdfhttp://download.intel.com/design/processor/manuals/253665.pdfhttp://download.intel.com/design/processor/manuals/253665.pdf -
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# write our string to stdout
movl $len,%edx # third argument: message lengthmovl $msg,%ecx # second argument: message to writemovl $1,%ebx # first argument: file handle (stdout)
movl $4,%eax # system call number (sys_write)int $0x80 # call kernel
# and exit
movl $0,%ebx # first argument: exit codemovl $1,%eax # system call number (sys_exit)int $0x80 # call kernel
.data # section declaration
msg:.ascii "Hello, world!\n" # our dear stringlen = . - msg # length of our dear string
int main(){
write(1, hello, world\n, 13);exit(0);
}
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Today
Exceptional Control Flow Processes
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Processes
Definition: Aprocessis an instance of a running program.
One of the most profound ideas in computer science
Not the same as program or processor
Process provides each program with two key abstractions:
Logical control flow
Each program seems to have exclusive use of the CPU
Private virtual address space
Each program seems to have exclusive use of main memory
How are these Illusions maintained?
Process executions interleaved (multitasking)
Address spaces managed by virtual memory system
well talk about this in a couple of weeks
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What is a process?
A process is the OS's abstraction for execution A process represents a single running application on the system
Process has three main components:
1) Address space
The memory that the process can access Consists of various pieces: the program code, static variables,
heap, stack, etc.
2) Processor state
The CPU registers associated with the running process
Includes general purpose registers, program counter, stack pointer,
etc.
3) OS resources
Various OS state associated with the process
Examples: open files, network sockets, etc.
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Process Address Space
The range ofvirtual memoryaddresses that theprocess can access
Includes the codeof the running
program The data of the
running program(static variablesand heap)
An execution stack
Local variablesand savedregisters foreachprocedure call
Stack
Heap
Initialized vars(data segment)
Code(text segment)
Address space
0x00000000
0xFFFFFFFF
Stack pointer
Program counte
Uninitialized vars(BSS segment)
(Reserved for OS)
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Process Address Space
Note!!! This is theprocess's own viewof the address space---
physicalmemory may not belaid out this way at
all. In fact, on systems
that supportmultiple runningprocesses, it's prettymuchguaranteed to lookdifferent.
The virtual memorysystem provides thisillusion to eachprocess.
Stack
Heap
Initialized vars(data segment)Code
(text segment)
Address space
0x00000000
0xFFFFFFFF
Stack pointer
Program counter
Uninitialized vars(BSS segment)
(Reserved for OS)
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Execution State of a Process
Each process has an execution state Indicates what the process is currently doing
Running:
Process is currently using the CPU
Ready:
Currently waiting to be assigned to a CPU That is, the process could be running, but another process is using the
CPU
Waiting (or sleeping):
Process is waiting for an event
Such as completion of an I/O, a timer to go off, etc. Why is this different than ready ?
As the process executes, it moves between these states
What state is the process in most of the time?
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Process State Transitions
What causes schedule and unschedule transitions?
New
Terminated
Ready
Running
Waiting
create
kill orexit I/O, page fault,
etc.
I/O done
scheduleunschedule
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g
Process Control Block
OS maintains a Process Control Block (PCB) for each process The PCB is a big data structure with many fields:
Process ID
User ID
Execution state
ready, running, or waiting Saved CPU state
CPU registers saved the last time the process was suspended.
OS resources
Open files, network sockets, etc.
Memory management info Scheduling priority
Give some processes higher priority than others
Accounting information
Total CPU time, memory usage, etc.
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g
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 Process B
user code
kernel code
user code
kernel code
user code
context switch
context switch
Time
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g
Context Switching The act of swapping a process state on or off the CPU is a context
switch
PC
Registers
PC
Registers
PID 1342State: Running
PC
Registers
PID 4277State: Ready
PC
Registers
PID 8109State: Ready
Save current CPU state
Currently running process
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g
Context Switching
The act of swapping a process state on or off the CPU is a contextswitch
PC
Registers
PC
Registers
PID 1342State: Ready
PC
Registers
PID 4277State: Ready
PC
Registers
PID 8109State: Ready
Suspend process
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Context Switching
The act of swapping a process state on or off the CPU is a contextswitch
PC
Registers
PC
Registers
PID 1342State: Ready
PC
Registers
PID 4277State: Running
PC
Registers
PID 8109State: Ready
Restore CPU state of new process
Pick next process
PC
Registers
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Context Switch Overhead
Context switches are not cheap Generally have a lot of CPU state to save and restore
Also must update various flags in the PCB
Picking the next process to runschedulingis also expensive
Context switch overhead in Linux 2.4.21 About 5.4 usec on a 2.4 GHz Pentium 4
This is equivalent to about 13,200 CPU cycles!
Not quite that many instructions since CPI > 1
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Context Switching in Linux
Process A
time
Process A is happ i ly ru nn ing a long. ..
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Context Switching in Linux
Process A
time
Timer interrupthandler
1) Timer interrupt f ires
2) PC saved on stac kUser
Kernel
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Context Switching in Linux
Process A
Timer interrupthandler
time
1) Timer interrupt f ires
2) PC saved on stack
Scheduler
4) Call sch edule() rou t ine
3) Rest o f CPU state
saved in PCB
User
Kernel
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Context Switching in Linux
Process A
Timer interrupthandler
time
1) Timer interrupt f ires
2) PC saved on stack
Scheduler5) Decide n extp rocess to run
4) Call sch edule() rou t ine
3) Rest o f CPU state
saved in PCBTimer interrupt
handler
6) Resume Proc ess B(suspended wi th in
t imer in terrupt hand ler !)
User
Kernel
Process B
7) Return from in terrupt
hand lerproc ess CPU
state restored
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State Queues
The OS maintains a set of state queues for each process state Separate queues for ready and waiting states
Generally separate queues for each kind of waiting process
e.g., One queue for processes waiting for disk I/O
Another queue for processes waiting for network I/O, etc.
PC
Registers
PID 4277State: Ready
PC
Registers
PID 4110State: Waiting
PC
Registers
PID 4002State: Waiting
PC
Registers
PID 4923State: Waiting
PC
Registers
PID 4391State: Ready
Ready queue
Disk I/O queue
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State Queue Transitions
PCBs move between these queues as their state changes When scheduling a process, pop the head off of the ready queue
When I/O has completed, move PCB from waiting queue to ready
queue
PC
Registers
PID 4277
State: Ready
PC
Registers
PID 4110
State: Waiting
PC
Registers
PID 4002
State: Waiting
PC
Registers
PID 4391
State: Ready
PC
Registers
PID 4923
State: Waiting
Ready queue
Disk I/O queue
PC
Registers
PID 4923
State: Ready
Disk I/O completes
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Process Creation
One process can create, or fork, another process The original process is the parent
The new process is the child
What creates the first process in the system, and when?
Parent process defines resources and access rights ofchildren
Just like real life ...
e.g., child process inherits parent's user ID
% pstree -p
init(1)-+-apmd(687)|-atd(847)|-crond(793)|-rxvt(2700)---bash(2702)---ooffice(2853)`-rxvt(2752)---bash(2754)
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UNIX fork mechanism
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PC
Registers
PID 4110
State: Ready
UNIX fork mechanism In UNIX, use the fork() system call to create a new process
This creates an exact duplicateof the parent process!!
Creates and initializes a new PCB
Creates a new address space Copies entire contents of parent's address space into the child
Initializes CPU and OS resources to a copy of the parent's
Places new PCB on ready queue
PC
Registers
PID 4109
State: Running
PC
Registers
PID 4277State: Ready
PC
Registers
PID 4391State: Ready
Ready queue
Process calls fork()PC
Registers
PID 4110
State: ReadyCopy state
PC
Registers
PID 4110
State: Ready
Add to end ofready queue
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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 childspidto the parent process
Fork is interesting (and often confusing) because
it is called oncebut returns twice
pid_t pid = fork();if (pid == 0) {printf("hello from child\n");
} else {printf("hello from parent\n");
}
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Understanding fork
pid_t pid = fork();if (pid == 0) {printf("hello from child\n");
} else {printf("hello from parent\n");
}
Process n
pid_t pid = fork();if (pid == 0) {printf("hello from child\n");
} else {printf("hello from parent\n");
}
Child Process m
pid_t pid = fork();if (pid == 0) {printf("hello from child\n");
} else {printf("hello from parent\n");
}
pid = m
pid_t pid = fork();if (pid == 0) {printf("hello from child\n");
} else {printf("hello from parent\n");
}
pid = 0
pid_t pid = fork();if (pid == 0) {printf("hello from child\n");
} else {printf("hello from parent\n");
}
pid_t pid = fork();if (pid == 0) {printf("hello from child\n");
} else {printf("hello from parent\n");
}
hello from parent hello from childWhich one is first?
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Fork Example #1
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);
}
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 #2
void fork2(){
printf("L0\n");fork();
printf("L1\n");fork();
printf("Bye\n");}
Both parent and child can continue forking
L0 L1
L1
Bye
Bye
Bye
Bye
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Fork Example #3
Both parent and child can continue forking
void fork3(){
printf("L0\n");fork();
printf("L1\n");fork();
printf("L2\n");fork();
printf("Bye\n");} L1 L2
L2
Bye
Bye
Bye
Bye
L1 L2
L2
Bye
Bye
Bye
Bye
L0
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Fork Example #4
Both parent and child can continue forking
void fork4(){
printf("L0\n");if (fork() != 0) {printf("L1\n");if (fork() != 0) {
printf("L2\n");fork();
}}
printf("Bye\n");}
L0 L1
Bye
L2
Bye
Bye
Bye
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Fork Example #4
Both parent and child can continue forking
void fork5(){
printf("L0\n");if (fork() == 0) {printf("L1\n");if (fork() == 0) {
printf("L2\n");fork();
}}
printf("Bye\n");}
L0 Bye
L1
Bye
Bye
Bye
L2
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Why have fork() at all?
Why make a copy of the parent process? Don't you usually want to start a new program instead?
Where might cloning the parent be useful?
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Why have fork() at all?
Why make a copy of the parent process? Don't you usually want to start a new program instead?
Where might cloning the parent be useful?
Web servermake a copy for each incoming connection
Parallel processingset up initial state, fork off multiple copies todo work
UNIX philosophy: System calls should be minimal.
Don't overload system calls with extra functionality if it is not
always needed.
Better to provide a flexible set of simple primitives and let
programmers
combine them in useful ways.
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Memory concerns
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Memory concerns So fork makes a copy of a process. What about memory usage?
Stack
Heap
Initialized vars(data segment)
Code(text segment)
Uninitialized vars(BSS segment)
(Reserved for OS)Parent
Stack
Heap
Initialized vars(data segment)
Code(text segment)
Uninitialized vars(BSS segment)
(Reserved for OS)Child #1
Stack
Heap
Initialized vars(data segment)
Code(text segment)
Uninitialized vars
(BSS segment)
(Reserved for OS)Child #2
Stack
Heap
Initialized vars(data segment)
Code(text segment)
Uninitialized vars(BSS segment)
(Reserved for OS)
Child #3
Stack
Heap
Initialized vars(data segment)
Code(text segment)
Uninitialized vars(BSS segment)
(Reserved for OS)Child #4
Stack
Heap
Initialized vars(data segment)
Code(text segment)
Uninitialized vars
(BSS segment)
(Reserved for OS)
Child #5
Stack
Heap
Initialized vars(data segment)
Code(text segment)
Uninitialized vars(BSS segment)
(Reserved for OS)Child #6
Stack
Heap
Initialized vars(data segment)
Code(text segment)
Uninitialized vars(BSS segment)
(Reserved for OS)
Child #7
Stack
Heap
Initialized vars(data segment)
Code(text segment)
Uninitialized vars(BSS segment)
(Reserved for OS)Child #8
Stack
Heap
Initialized vars(data segment)
Code(text segment)
Uninitialized vars
(BSS segment)
(Reserved for OS)Child #10
Stack
Heap
Initialized vars(data segment)
Code(text segment)
Uninitialized vars(BSS segment)
(Reserved for OS)Child #9
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Problematic?
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Problematic?
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Memory concerns
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Memory concerns OS aggressively tries to share memory between processes.
Especially processes that are fork()'d copies of each other
Copies of a parent process do not actually get a private copyof the address space...
... Though that is the illusion that each process gets.
Instead, they share the same physical memory, until one of them makesa change.
The virtual memory system is behind these shenanigans. We will discuss this in much detail later in the course
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exit: Ending a 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 doesnt reap? If any parent terminates without reaping a child, then child will be
reaped by initprocess
So, only need explicit reaping in long-running processes
e.g., shells and servers
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linux> ./forks 7 &[1] 6639Running Parent, PID = 6639
Terminating Child, PID = 6640linux>psPID TTY TIME CMD6585 ttyp9 00:00:00 tcsh6639 ttyp9 00:00:03 forks6640 ttyp9 00:00:00 forks
6641 ttyp9 00:00:00 pslinux>kill 6639[1] Terminatedlinux>psPID TTY TIME CMD6585 ttyp9 00:00:00 tcsh6642 ttyp9 00:00:00 ps
Zombie
Example
psshows child process asdefunct
Killing parent allows child to be
reaped by init
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 8Terminating Parent, PID = 6675Running Child, PID = 6676linux>ps
PID TTY TIME CMD6585 ttyp9 00:00:00 tcsh
6676 ttyp9 00:00:06 forks6677 ttyp9 00:00:00 pslinux>kill 6676linux>ps
PID TTY TIME CMD6585 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 keeprunning 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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wait: Synchronizing with Children
int wait(int *child_status)
suspends current process until one of its children terminates
return value is thepidof the child process that terminated
if child_status!= NULL, then the object it points to will be setto a status indicating why the child process terminated
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wait: Synchronizing with Children
void fork9() {int child_status;
if (fork() == 0) {printf("HC: hello from child\n");
}else {
printf("HP: hello from parent\n");wait(&child_status);printf("CT: child has terminated\n");
}printf("Bye\n");exit();
}
HP
HC Bye
CT Bye
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wait()Example If multiple children completed, will take in arbitrary order
Can use macros WIFEXITED and WEXITSTATUS to get information aboutexit status
void fork10(){
pid_t pid[N];int i;int child_status;for (i = 0; i < N; i++)
if ((pid[i] = fork()) == 0)exit(100+i); /* Child */
for (i = 0; i < N; i++) {pid_t wpid = wait(&child_status);
if (WIFEXITED(child_status))printf("Child %d terminated with exit status %d\n",
wpid, WEXITSTATUS(child_status));else
printf("Child %d terminate abnormally\n", wpid);}
}
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waitpid(): Waiting for a Specific Process
waitpid(pid, &status, options)
suspends current process until specific process terminates
various options (that we wont talk about)
void fork11(){
pid_t pid[N];
int i;int child_status;for (i = 0; i < N; i++)
if ((pid[i] = fork()) == 0)exit(100+i); /* Child */
for (i = 0; i < N; i++) {
pid_t wpid = waitpid(pid[i], &child_status, 0);if (WIFEXITED(child_status))printf("Child %d terminated with exit status %d\n",
wpid, WEXITSTATUS(child_status));else
printf("Child %d terminated abnormally\n", wpid);}
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fork() and execve()
How do we start a new program, instead of just a copy ofthe old program?
Use the UNIX execve() system call
int execve(const char *filename,
char *const argv [], char *const envp[]);
filename: name of executable file to run
argv: Command line arguments
envp: environment variable settings (e.g., $PATH, $HOME, etc.)
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fork() and execve()
execve() does not fork a new process! Rather, it replaces the address space and CPU state of the current
process
Loads the new address space from the executable file and starts it
from main()
So, to start a new program, use fork() followed by execve()
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execl andexecFamily
int execl(char *path, char *arg0, char *arg1, , 0)
Loads and runs executable atpathwith args arg0, arg1, pathis the complete path of an executable object file
By convention,arg0is the name of the executable object file Real arguments to the program start with arg1, etc.
List of args is terminated by a (char *)0argument
Environment taken from char **environ, which points to an arrayof name=value strings:
USER=ganger
LOGNAME=ganger
HOME=/afs/cs.cmu.edu/user/ganger
Returns -1if error, otherwise doesnt return!
Family of functions includesexecv, execve(basefunction),execvp, execl, execle, and execlp
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exec:Loading and Running Programs
main() {if (fork() == 0) {execl("/usr/bin/cp", "cp", "foo", "bar", 0);
}wait(NULL);printf("copy completed\n");exit();
}
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Summary
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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Summary (cont.)
Spawning processes
Call to fork
One call, two returns
Process completion
Call exit
One call, no return Reaping and waiting for Processes
Callwaitorwaitpid
Loading and running Programs
Call execl(or variant) One call, (normally) no return