CS 105 Tour of the Black Holes of Computing Processes Topics cs105 Process context switches Creating and destroying processes Processes Def A process is 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 address space Each program seems to have exclusive use of main memory How are these Illusions maintained 2 Process executions interleaved multitasking Address spaces managed by virtual memory system CS 105 Logical Control Flows Each process has its own logical control flow Process A Process B Process C Time 3 CS 105 Concurrent Processes Two processes run concurrently are concurrent if their flows overlap in time Otherwise they are sequential Examples Concurrent A B A C Sequential B C Process A Process B Process C Time 4 CS 105 User View Concurrent Processes Control flows for concurrent processes are physically disjoint in time Except on multi CPU machines However we can think of concurrent processes as running in parallel with each other Process A Process B Process C Time 5 CS 105 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 Time kernel code context switch user code kernel code context switch user code 6 CS 105 Private Address Spaces Each process has its own private address space 0xffffffff 0xc0000000 0x40000000 kernel virtual memory code data heap stack user stack created at runtime read write segment data bss 7 0 esp stack pointer memory mapped region for shared libraries run time heap managed by malloc 0x08048000 memory invisible to user code read only segment init text rodata unused brk loaded from the executable file CS 105 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 Huh Run that by me again 8 CS 105 Fork Example 1 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 input and output file descriptors Relative ordering of their print statements undefined void fork1 int x 1 Parent and child will get private copies 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 9 CS 105 Fork Example 2 Key Points Both parent and child can continue forking void fork2 printf L0 n fork printf L1 n fork printf Bye n 10 L0 L1 Bye Bye L1 Bye Bye CS 105 Fork Example 3 Key Points 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 L0 11 L1 L2 Bye Bye L2 Bye Bye L2 Bye Bye L2 Bye Bye CS 105 Fork Example 4 Key Points 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 12 Bye Bye L0 L1 L2 Bye Bye CS 105 Fork Example 5 Key Points 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 13 Bye L2 L1 L0 Bye Bye Bye CS 105 exit Destroying Process void exit int status Exits a process Normally return with status 0 success atexit registers functions to be executed upon exit void cleanup void printf cleaning up n void fork6 atexit cleanup fork exit 0 14 CS 105 Zombies Idea When process terminates still consumes system resources Various tables maintained by OS to store exit status 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 15 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 CS 105 Zombie Example 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 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 defunct 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 16 ps shows child process as defunct Killing parent allows child to be reaped CS 105 Nonterminating Child Example 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 linux forks 8 exit 0 Terminating Parent PID 6675 Running Child PID 6676 linux ps PID TTY TIME 6585 ttyp9 00 00 00 6676 ttyp9 00 00 06 6677 ttyp9 00 00 00 linux kill 6676 linux ps PID TTY TIME 6585 ttyp9 00 00 00 6678 ttyp9 00 00 00 17 CMD tcsh forks ps CMD tcsh ps Child process still active even though parent has terminated Must kill explicitly or else will keep running indefinitely CS 105 wait Synchronizing with children int wait int child status 18 Suspends current process until one of its children terminates Return value is pid of child process that terminated If child status NULL then integer it points to will be set to indicate why child terminated CS 105 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 0 19 HC Bye HP CT Bye CS 105 Wait Example If multiple children completed will take in arbitrary order Can use macros WIFEXITED and WEXITSTATUS to get information about exit 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 terminated abnormally n wpid 20 CS 105 Waitpid waitpid pid status options …
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