b. ramamurthy pag e 1 task control: signals and alarms chapter 7 and 8 7/2/2015
TRANSCRIPT
B. RAMAMURTHY
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Task Control:Signals and Alarms
Chapter 7 and 8
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Multi-taskingPage 2
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 messagesHow to (software) interrupt a process? signals
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ExamplesPage 3
Consider g++ myProg.c You want to kill this process after you started the
compilation..hit cntrl-CConsider execution of a program called “badprog”
>badprogIt 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
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Linux ProcessesPage 4
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-taskingChapter 8: Task communication and
synchronization
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Process creationPage 5
• 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.
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Termination of a processPage 6
• 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.
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Process controlPage 7
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.
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Process control (contd.)Page 8
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.
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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
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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
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Example Code
1. int retVal;
2. printf(" Just one process so far\n");3. printf(" Invoking/Calling fork() system call\n");
4. retVal = fork(); /* create new process*/
5. if (retVal == 0)6. printf(" I am the child %d \n",getpid());
7. else if (retVal > 0)8. printf(" I am the parent, child has pid %d \n", retVal);
9. else10. printf(" Fork returned an error %d \n", retVal);
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Input/output Resources
What is standard IO?These are resources allocated to the
process at the time of creation:From Wikipedia/Standard_streams
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SignalsPage 13
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 othersSIGHUP, SIGINT, SIGILL, SIGTRAP, SIGFPE,
SIGKILLSIGALRM (sent by kernel to a process after an
alarm timer has expired)SIGTERM signal (signal id, function) simply arms the signal
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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.
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.
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Intercept SignalsPage 16
Task1
Task2
Two essential parameters are destination process identifierand the signal code number: kill (pid, signal)Signals are a useful way of handling intermittent data arrivals or rare errorconditions.
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Handling SignalsPage 17
Look at the examples:Catching SIGALRM Ignoring SIGALRMsigtest.csigHandler.cpingpong.cSee /usr/include/sys/iso/signal_iso.h for
signal numbers
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Signals and AlarmsPage 18
#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 tctime(&t); will convert time to ascii formatAlarm 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
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Sample programsPage 19
Starting new tasks in linux: page 165Programs in pages: 174-180 on signals and
alarmsSee demos directory for the codeSee page 175 for the second programSee page 178 … for the third program
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PingpongPage 20
Parent
Child
PSIG 43
CSIG 42
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Observe in pingpong.c
pause(): indefinitesleep(): sleep is random/finite timeWhile loopSignal handlersRe-arming of the signals
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Volatile Page 22
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
Registers in devices are abstracted for programmatic access as “volatile” type
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Summary
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We studied signals and alarms and their specification and example programs