Implementing Processes and Threads © 2004, D. J. Foreman

Required Software for Threads UNIX (Linux, OpenBSD, FreeBSD, Solaris) Exported POSIX API or use "Pthreads" API gcc or g++ with -lpthread -lposix4 -lthread Windows (98/ME/NT/XP/Vista/7…) WIN32 API – not POSIX compliant Pthreads.DLL – freeware sources.redhat.com/pthreads-win32 Copy pthread.dll to C:\windows Keep .h files wherever you want them © 2004, D. J. Foreman

Exploring the Abstraction Loc 0 Loc n Loc 0 Loc n Loc 0 Loc n User i Processes & RAM User j Processes & RAM User k Processes & RAM CPU i CPU j CPU k Abstract Actual RAM User i Processes User j Processes User k Processes Loc 0 Loc n CPU Page space © 2004, D. J. Foreman

Process Manager Responsibilities Define & implement the essential characteristics of a process and thread Algorithms for behavior Process state data Define the address space (and thus available content) Manage the resources (I/O, RAM, CPU) Tools to manipulate processes & threads Tools for scheduling the CPU Tools for inter-thread synchronization Handling deadlock Handling protection © 2004, D. J. Foreman

Resources What is a "resource" R={Rj|0<=j<=m} Requestable blocking object or service Reusable – CPU, RAM, disk space, etc Non-reusable (consumable) Data within a reusable resource R={Rj|0<=j<=m} Rj is one type of resource, such as RAM C={cj>=0| Rj R(0<=j<m)} cj is the # of available units of Rj © 2004, D. J. Foreman

Resource Mgmt Model  {Mgr} : Rj (Mgr(Rj) gives ki<=ci units of Rj to Pn) Pn may only request i units of Rr Pn may only request limited units of Rn Why do we need the set notation? Formalized descriptions can lead to deadlock detection and prevention algorithms © 2004, D. J. Foreman

Win NT/2K/XP,7,8 Process Mgmt Split into 2 facilities: NT Kernel Object mgmt Interrupt handling Thread scheduling NT Executive All other Process aspects FYI: See "Inside Windows 2000", 3e, Solomon & Russinovich, Ch. 6, MS Press, 2000 © 2004, D. J. Foreman

Fork } Parent creates a child: Main() { PID=fork(…); // spawn a child If (PID==0) child_code(); Else if (PID==NULL) error_code(); Else parent_code; waitpid(PID); // waits for only this child } void child_code() { } void error code() { } void parent_code() { } © 2013, D. J. Foreman

The Address Space Boundaries of memory access H/W can help (DAT) (more later) Multiprogramming possible without H/W!!!! Self-relocation Pre-load relocation Both use true addresses FIXED at load time NO paging, but MAY have swapping Windows 3.1 IBM OS/VS1 © 2004, D. J. Foreman

Address Binding Given: Where are X, y, z and q?? int function X(y,z) {Int q; return ff(y,z)} Void function M {X(3,4);} Where are X, y, z and q?? How does X get control from M? What happens if there is an interrupt BETWEEN M's call to X and X starting? © 2004, D. J. Foreman

Address Binding-2-fixed Gather all files of the program Arrange them in RAM in linear fashion Determine runloc for the executable Find all address constants (functions and External data) Find all references to those constants Modify the references in RAM Store as an executable file Run at the pre-determined location in RAM © 2004, D. J. Foreman

Address Binding-3-dynamic Perform "fixed binding", but in step 3, use a value of "zero" In step 6, mark as "relocatable" For step 8, before actually transferring control, REPEAT 4-6 using the actual runloc determined by the loader Same for DLL members © 2004, D. J. Foreman

Address Binding-4 DLL's How does a program find a DLL it didn't create? Each DLL member has a specific name System has list of DLL member names When DLL is requested, system fetches module and dynamically binds it to memory, but NOT to the caller! System transfers control to DLL member © 2004, D. J. Foreman

Address Binding - 5 How does the system make it look as if each abstract machine starts at 0? How does the system keep user spaces apart How does the system protect address spaces © 2004, D. J. Foreman

Context Switching Power on, ROM reads bootstrap program from head 0 of device ROM transfers control to the program Bootstrap program reads the loader Loader reads the kernel Kernel gets control and initializes itself Kernel loads User Interface Kernel waits for an interrupt Kernel starts a process, then waits again © 2004, D. J. Foreman

Context Switching - 2 Device requests interrupt ROM inspects system for ability to accept If interrupts are masked off, exit Future interrupts may be queued by hardware Or devices may be informed to re-try If interrupts are allowed: set status (in RAM, control store, etc) Atomically: load new IC, privileged mode, interrupts masked off, set storage protection off Kernel processes the interrupt © 2004, D. J. Foreman

Context Switching - 3 The actual Context Switch: Save all user state info: Registers, IC, stack pointer, security codes, etc Load kernel registers Access to control data structures Locate the interrupt handler for this device Transfer control to handler, then: Restore user state values Atomically: set IC to user location in user mode, interrupts allowed again © 2004, D. J. Foreman

Questions to ponder Why must certain operations be done atomically? What restrictions are there during context switching? What happens if the interrupt handler runs too long? Why must interrupts be masked off during interrupt handling? © 2004, D. J. Foreman

What is a "Handle"? Application requests an object A window, a chunk of RAM, a file, etc. Must give application a way to access it Done via a "handle" A counter (file handles) An address in user RAM (structures) Always a "typed" variable Helps insure correct usage (except "C" doesn't enforce typed usage) © 2004, D. J. Foreman