copyright ©: nahrstedt, angrave, abdelzaher1 processes and threads

Copyright ©: Nahrstedt, Angrave, Abdelzaher 1

Processes and Threads

Copyright ©: Nahrstedt, Angrave, Abdelzaher

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The Multi-Process Astronomy Server

Consider the following problem: A server is made up of one “input process” and multiple “worker

processes”. The input process performs a loop that reads multiple file names

from disk. Each named file contains a very large scientific data set (with observation data on all stars in some galaxy).

The process passes each file to a different newly forked child that opens the named file and performs some math on the data (to compute the trajectories of the observed stars). The results are written by the child to a results file for that galaxy. The child then exists with a code that indicates success or failure.

The parent (input process) waits for the children. Each time a child exists, the parent checks the exit code and restarts the child if the code indicates that an error has occurred.

What do you need to know to write this up?


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Waiting for Children to Return The wait() Function

wait function allows parent process to wait (block) until child finishes then returns the id and status of that child

wait function causes the caller to suspend execution until child’s status is available

waitpid function allows a parent to wait for a particular child


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Waiting for Children

#include <errno.h>#include <sys/wait.h>

pid_t childpid;

…

childpid = wait(NULL);if (childpid != -1) printf(“waited for child with pid %ld\n”, childpid);


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Dead Children may become Zombies

When a process terminates but its parent does not wait for it?

Zombie (pid entry remains in system process table)

Zombies unlike orphans do not consume many resources.


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Zombie Removal

Professional code installs signal handler for signal SIGCHLD … which issues a wait() call


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Orphans

When the parent process terminates first? “Child is orphaned” Child is “re-parented” to init process Child continues to consume resources init process (id=1) waits for children


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Status Values for wait(int *stat_loc)

The following macros are available for checking the return status of a child:

#include <sys/wait.h>

WIFEXITED(int stat_val) WEXITSTATUS(int stat_val) WIFSIGNALED(int stat_val) WTERMSIG(int stat_val) WIFSTOPPED(int stat_val) WSTOPSIG(int stat_val)

They are used in pairs.If WIFEXITED returns true, the child executed normally and the return status (at most 8 bits) can be gotten with WEXITSTATUS.


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Simple Example

int status; childpid = wait(&status); if (childpid == -1) perror("Failed to wait for child");

else if (WIFEXITED(status) && 0==WEXITSTATUS(status)) printf(“Child terminated normally”);


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Typical Child Creation Code#include <stdio.h>#include <unistd.h>#include <sys/types.h>#include <sys/wait.h>

int main (void) { pid_t childpid; /* set up signal handlers here ... */ childpid = fork(); if (childpid == -1) { perror("Failed to fork");return 1;}

if (childpid == 0) fprintf(stderr, "I am child %ld\n", (long)getpid()); else if (wait(NULL) != childpid) fprintf(stderr, "A signal must have interrupted the wait!\n");

else fprintf(stderr, "I am parent %ld with child %ld\n", (long)getpid(), (long)childpid);

return 0;}


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exec() Function

Exec function allows child process to execute code that is different from that of parent

Exec family of functions provides a facility for overlaying the process image of the calling process with a new image.

Exec functions return -1 and sets errno if unsuccessful


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Exec Example

#include <stdio.h>#include <unistd.h>#include <sys/types.h>#include <sys/wait.h>int main (void) { pid_t childpid; childpid = fork(); if (childpid == -1) { perror("Failed to fork"); return 1; } if (childpid == 0) { /*child code*/ execl(“/bin/ls”, “ls”, “-l”, NULL); perror(“Child failed to exec ls”); return 1; } if (wait(NULL) != childpid) { /* parent code */ perror(“Parent failed to wait due to signal or error”); return 1; }return 0;}


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Background Processes and Daemons

Shell – command interpreter creates background processes if line ends with & When shell creates a background process, it

does not wait for the process to complete before issuing a prompt and accepting another command

Ctrl-C does not terminate background process Daemon is a background process that

normally runs indefinitely


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Processes and Threads


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PropertyProcesses created with fork

Threads of a process Ordinary function calls

variablesget copies of all variables

share global variables share global variables

IDs get new process IDsshare the same process ID but have unique thread ID

share the same process ID (and thread ID)

Communication

Must explicitly communicate, e.g.pipesor use small integer return value

May communicate with return valueor shared variablesif done carefully

May communicate with return valueor shared variables(don't have to be careful)

Parallelism (one CPU)

Concurrent Concurrent Sequential

Parallelism (multiple CPUs)

May be executed simultaneously

Kernel threads may be executed simultaneously

Sequential

Threads versus Processes


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Thread Components

address space: code and global variables open files semaphores signals timers process ID

A thread has its own program counter and stack, but shares a number of resources with its process and other threads of the process:


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Thread Components


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Creating a Thread

When a new thread is created it runs concurrently with the creating process.

When creating a thread you indicate which function the thread should execute.


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Normal function call


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Threaded function call


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Pthread OperationsPOSIX function description

pthread_create

create a thread

pthread_detach

set thread to release resources

pthread_equal

test two thread IDs for equality

pthread_exit exit a thread without exiting process

pthread_kill send a signal to a thread

pthread_join wait for a thread

pthread_self find out own thread ID

copyright ©: nahrstedt, angrave, abdelzaher1 processes and threads

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