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Task Control: Signals and Alarms Chapter 7 and 8

Learn how to create and control multiple tasks, synchronize processes, and communicate among them using XINU and Linux. Understand process creation, termination, and control mechanisms with examples and code snippets in a Unix environment.

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Task Control: Signals and Alarms Chapter 7 and 8

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  1. Task Control:Signals and AlarmsChapter 7 and 8 B. Ramamurthy Amrita-UB-MSES-2013-12

  2. Multi-tasking • How to create multiple tasks? Ex: Xinu create() • How to control them? • ready() • resched() • How to synchronize them? How to communicate among them? • XINU: semaphores, send and receive messages • How to (software) interrupt a process? signals Amrita-UB-MSES-2013-12

  3. Examples • Consider g++ myProg.c • You want to kill this process after you started the compilation..hit cntrl-C • Consider execution of a program called “badprog” >badprog It core dumps .. What happened? The error in the program results in a signal to kernel to stop and dump the offending code • Consider “kill –p <pid>” • Kill issues a termination signal to the process identified by the pid Amrita-UB-MSES-2013-12

  4. Linux Processes • Similar to XINU Procs. • Lets understand how to create a linux process and control it. • Chapter 7 and 8 of text book. • Chapter 7 : multi-tasking • Chapter 8: Task communication and synchronization Amrita-UB-MSES-2013-12

  5. Process creation • Four common events that lead to a process creation are: 1) When a new batch-job is presented for execution. 2) When an interactive user logs in / system initialization. 3) When OS needs to perform an operation (usually IO) on behalf of a user process, concurrently with that process. 4) To exploit parallelism an user process can spawn a number of processes. Amrita-UB-MSES-2013-12

  6. Termination of a process • Normal completion, time limit exceeded, memory unavailable • Bounds violation, protection error, arithmetic error, invalid instruction • IO failure, Operator intervention, parent termination, parent request, killed by another process • A number of other conditions are possible. • Segmentation fault : usually happens when you try write/read into/from a non-existent array/structure/object component. Or access a pointer to a dynamic data before creating it. (new etc.) • Bus error: Related to function call and return. You have messed up the stack where the return address or parameters are stored. Amrita-UB-MSES-2013-12

  7. Process control • Process creation in unix is by means of the system call fork(). • OS in response to a fork() call: • Allocate slot in the process table for new process. • Assigns unique pid to the new process.. • Makes a copy of the process image, except for the shared memory. • both child and parent are executing the same code following fork() • Move child process to Ready queue. • it returns pid of the child to the parent, and a zero value to the child. Amrita-UB-MSES-2013-12

  8. Process control (contd.) • All the above are done in the kernel mode in the process context. When the kernel completes these it does one of the following as a part of the dispatcher: • Stay in the parent process. Control returns to the user mode at the point of the fork call of the parent. • Transfer control to the child process. The child process begins executing at the same point in the code as the parent, at the return from the fork call. • Transfer control another process leaving both parent and child in the Ready state. Amrita-UB-MSES-2013-12

  9. Process Creation (contd.) Parent process create children processes, which, in turn create other processes, forming a tree of processes Generally, process identified and managed via a process identifier (pid) Resource sharing Parent and children share all resources Children share subset of parent’s resources Parent and child share no resources Execution Parent and children execute concurrently Parent waits until children terminate Amrita-UB-MSES-2013-12

  10. Process Termination Process executes last statement and asks the operating system to delete it (exit) Output data from child to parent (via wait) Process’ resources are deallocated by operating system Parent may terminate execution of children processes (abort) Child has exceeded allocated resources Task assigned to child is no longer required If parent is exiting Some operating system do not allow child to continue if its parent terminates All children terminated - cascading termination Amrita-UB-MSES-2013-12

  11. Example Code • intretVal; • printf(" Just one process so far\n"); • printf(" Invoking/Calling fork() system call\n"); • retVal = fork(); /* create new process*/ • if (retVal == 0) • printf(" I am the child %d \n",getpid()); • else if (retVal > 0) • printf(" I am the parent, child has pid %d \n", retVal); • else • printf(" Fork returned an error %d \n", retVal); Amrita-UB-MSES-2013-12

  12. Input/output Resources • What is standard IO? • These are resources allocated to the process at the time of creation: • From Wikipedia/Standard_streams Amrita-UB-MSES-2013-12

  13. Signals • Signals provide a simple method for transmitting software interrupts to UNIX process • Signals cannot carry information directly, which limits their usefulness as an general inter-process communication mechanism • However each type of signal is given a mnemonic name; Ex: SIGINT • See signal.h for others • SIGHUP, SIGINT, SIGILL, SIGTRAP, SIGFPE, SIGKILL • SIGALRM (sent by kernel to a process after an alarm timer has expired) • SIGTERM • signal (signal id, function) simply arms the signal Amrita-UB-MSES-2013-12

  14. Signal Value Action Comment ------------------------------------------------------------------------- SIGHUP 1 Term Hangup detected on controlling terminal or death of controlling process SIGINT 2 Term Interrupt from keyboard SIGQUI 3 Core Quit from keyboard SIGILL 4 Core Illegal Instruction SIGABR 6 Core Abort signal from abort(3) SIGFP 8 Core Floating point exception SIGKILL 9 Term Kill signal SIGSEG 11 Core Invalid memory reference SIGPIPE 13 Term Broken pipe: write to pipe with no readers SIGALRM 14 Term Timer signal from alarm(2) SIGTERM 15 Term Termination signal SIGUSR1 30,10,16 Term User-defined signal 1 SIGUSR2 31,12,17 Term User-defined signal 2 SIGCHLD 20,17,18 Ign Child stopped or terminated SIGCONT 19,18,25 Cont Continue if stopped SIGSTOP 17,19,23 Stop Stop process SIGTSTP 18,20,24 Stop Stop typed at tty SIGTTIN 21,21,26 Stop tty input for background process SIGTTOU 22,22,27 Stop tty output for background process The signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored. Amrita-UB-MSES-2013-12

  15. Realtime signals • Linux supports real-time signals as originally defined in the POSIX.1b real-time extensions (and now included in POSIX.1-2001). Linux supports 32 real-time signals, numbered from 32 (SIGRTMIN) to 63 (SIGRT- MAX) • Main difference is that these are queued and not lost. • Realtime signals are delivered in guaranteed order. Amrita-UB-MSES-2013-12

  16. Intercept Signals Task1 Task2 Two essential parameters are destination process identifier and the signal code number: kill (pid, signal) Signals are a useful way of handling intermittent data arrivals or rare error conditions. Amrita-UB-MSES-2013-12

  17. Handling Signals • Look at the examples: • Catching SIGALRM • Ignoring SIGALRM • sigtest.c • sigHandler.c • pingpong.c • See /usr/include/sys/iso/signal_iso.h for signal numbers Amrita-UB-MSES-2013-12

  18. Signals and Alarms #include <signal.h> unsigned int alarm( unsigned int seconds ); alarm(a); will start a timer for a secsonds and will interrupt the calling process after a secs. time(&t); will get you current time in the variable t declared as time_t t ctime(&t); will convert time to ascii format Alarm has a sigaction function that is set for configuring the alarm handler etc. sigaction(SIGALRM, &act, &oldact) ; the third paramter is for old action configuration Amrita-UB-MSES-2013-12

  19. Sample programs • Starting new tasks in linux: page 165 • Programs in pages: 174-180 on signals and alarms • See demos directory for first • See page 175 for the second program • See page 178 … for the third program Amrita-UB-MSES-2013-12

  20. Pingpong Parent PSIG 43 Child CSIG 42 Amrita-UB-MSES-2013-12

  21. Observe in pingpong.c • pause(): indefinite • sleep(): sleep is random/finite time • While loop • Signal handlers • Re-arming of the signals Amrita-UB-MSES-2013-12

  22. Volatile • A variable should be declared volatile whenever its value could change unexpectedly. In practice, only three types of variables could change: • Memory-mapped peripheral registers • Global variables modified by an interrupt service routine • Global variables within a multi-threaded application Amrita-UB-MSES-2013-12

  23. Summary • We studied signals and alarms • Their specification and example programs • Signals in pthread is different. We will discuss this next class. Amrita-UB-MSES-2013-12

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