Virtlab:Linuxové okénko

Z VirtlabWiki

seznam dobrych rad, osvedcenych postupu, a pod.

Zaznam cinnosti na konzoli

pokud delam praci pres konzoli(ssh) a chci to mit jako reprodukovatelny postup a zaroven jsem lenivy si vsechno psat co jsem udelal, tak existuje programek script ktery je schopen nahravat konzoli a k tomu je scriptreplay ktery umi nahravku prehrat

programy v linuxu:

script - nahrava konzoli (ulozi do souboru nahravku)

scriptreplay - prehrava (cte ze souboru nahravku)

Psaní robustních shell skriptů

Realizace kritické sekce v Bash

LOCKFILE=$SPOJ_DIR/spojovac.lock

# Implementace kriticke sekce s pouzitim LOCKFILE
# noclobber mode: redirection fails if (a regular) file to be written to exists
if ( set -o noclobber; echo "$$" > "$LOCKFILE") 2> /dev/null;
then
  trap 'rm -f "$LOCKFILE"; exit $?' INT TERM EXIT
  <CRITICAL SECTION>
  rm -f "$LOCKFILE"
  trap - INT TERM EXIT 
else
  echo "Failed to acquire LOCKFILE: $LOCKFILE."
  echo "Held by $(cat $LOCKFILE)"
fi
# Opsano z http://www.davidpashley.com/articles/writing-robust-shell-scripts.html

MUTEX promocí UNIXových semaforů

Vytvoření, použití a smazání semaforu (spustit jako první)

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <stdio.h>
#include <fcntl.h>

extern int errno;

int main(void) {

struct sembuf sb;
key_t key;
int semid;

if ((key=ftok ("/tmp/qqq", '1'))==-1) perror("FTOK:");
// zmenit soubor na neco rozumnejsiho z Virtlabu

// create one semaphore with no special flags
if ((semid = semget (key,1,IPC_CREAT| O_RDWR | 0777))==-1) perror("A:");

// set semaphore value to 1
sb.sem_num=0;
sb.sem_op=1;
sb.sem_flg=0;
if (semop (semid, &sb,1)==-1) perror("B:");

//  Wait until a mutex is released or proceed if it is free
//  Decrease value by 1 before proceeding

sb.sem_num=0;
sb.sem_op=-1;
sb.sem_flg=0;
printf("Waiting before semaphore\n");
if (semop (semid, &sb,1)==-1) perror("C:");
printf("Passed through semaphore. Waiting for input to release it.`\n");

getc(stdin);

// release mutex - increase value by 1
sb.sem_num=0;
sb.sem_op=1;
sb.sem_flg=0;
if (semop (semid, &sb,1)==-1) perror("D:");
printf("Semaphore released. Waiting for input to remove it\n");

getc(stdin);
getc(stdin); // odstrani CR

//remove the semaphore
if (semctl(semid,1,IPC_RMID)==-1) perror("E:");
printf("Semaphore removed\n");
return 0;
}

Synchronizace se semaforem (spustit poté)

1. include <sys/types.h>
2. include <sys/ipc.h>
3. include <sys/sem.h>
4. include <stdio.h>
5. include <fcntl.h>

extern int errno;

int main(void) {

struct sembuf sb; key_t key; int semid;

if ((key=ftok ("/tmp/qqq", '1'))==-1) perror("FTOK:"); // zmenit soubor na neco rozumnejsiho z Virtlabu

// create one semaphore with no special flags if ((semid = semget (key,1, 0777))==-1) perror("A:");

// Wait until a mutex is released or proceed if it is free // Decrease value by 1 before proceeding

sb.sem_num=0; sb.sem_op=-1; sb.sem_flg=0; printf("Waiting before semaphore\n"); if (semop (semid, &sb,1)==-1) perror("C:"); printf("Passed through semaphore. Waiting for input to release it.`\n");

getc(stdin);

// release mutex - increase value by 1 sb.sem_num=0; sb.sem_op=1; sb.sem_flg=0; if (semop (semid, &sb,1)==-1) perror("D:"); printf("Semaphore released\n"); return 0; }