1. Task Control: Signals and Alarms Chapter 7 and 8
B. Ramamurthy
Amrita-UB-MSES-2013
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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
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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
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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
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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.
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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.
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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.
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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.
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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
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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
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11. Example Code int retVal; 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);
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12. 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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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
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14. Signal Value Action Comment
SIGHUP Term Hangup detected on controlling terminal
or death of controlling process
SIGINT Term Interrupt from keyboard
SIGQUI Core Quit from keyboard
SIGILL Core Illegal Instruction
SIGABR Core Abort signal from abort(3)
SIGFP Core Floating point exception
SIGKILL Term Kill signal
SIGSEG Core Invalid memory reference
SIGPIPE Term Broken pipe: write to pipe with no readers
SIGALRM Term Timer signal from alarm(2)
SIGTERM 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.
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15. Realtime signals
Linux supports real-time signals as originally defined in the POSIX.1b real-time extensions (and now included in POSIX ). 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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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.
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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
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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
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19. Sample programs Starting new tasks in linux: page 165
Programs in pages: on signals and alarms
See demos directory for first
See page 175 for the second program
See page 178 … for the third program
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20. Pingpong
Parent
PSIG 43
Child
CSIG 42
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21. Observe in pingpong.c pause(): indefinite
sleep(): sleep is random/finite time
While loop
Signal handlers
Re-arming of the signals
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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
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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
5/18/2013