I have an array boxIn[] of type char that contains R R G B G B O Y O O P R characters. boxIn[] is in shared memory. There is also a char* in shared memory, p, that has a value of one of the chars in the boxIn[] array.
There are two semaphores defined in the program as follows:
/* initialize semaphores */
if((sem_init(&sem, pshared, value)) == 1){ /* only 2(value) processes at the same time */
perror("Error initializing synch semaphore\n");
exit(1);
}
if((sem_init(&mutex, pshared, 1)) == 1){ /* only 1 processes at the same time */
perror("Error initializing synch semaphore\n");
exit(1);
}
I fork the children from a for loop:
/* fork child processes */
for(i=0; i<n ; i++){
pid = fork();
if(pid < 0){ /* check for error */
printf("Fork error.\n");
}
else if(pid == 0) break; /* child processes */
}
Then I let the children do some processing. At the moment I'm trying to get to my goal step by step. Therefore I change *p's value only once.
/******************************************************/
/****************** PARENT PROCESS ****************/
/******************************************************/
if(pid != 0){
while(wait()>0);
}
/******************************************************/
/****************** CHILD PROCESS *****************/
/******************************************************/
else{
while(*p != boxIn[i]); /* wait until it's my turn */
sem_wait(&sem);
printf("%c boxes are being painted.\n",boxIn[i]);
printf("Done painting.\n");
sleep(1);
sem_wait(&mutex);
if(*p=='R') *p='G';
sem_post(&mutex);
sem_post(&sem);
exit(1);
}
However I do get an unexpected output:
What I expect is:
R boxes are being painted.
R boxes are being painted.
Done.
Done.
R boxes are being painted.
Done.
G boxes are being painted.
G boxes are being painted.
Done.
Done.
And what I get is as follows:
R boxes are being painted.
Done painting.
R boxes are being painted.
Done painting.
R boxes are being painted.
Done painting.
G boxes are being painted.
Done painting.
G boxes are being painted.
Done painting.
varaquilex@computer ~/Dropbox/Courses/BLG312E - Computer Operating Systems/hw3 $ G boxes are being painted.
Done painting.
G boxes are being painted.
Done painting.
G boxes are being painted.
Done painting.
G boxes are being painted.
Done painting.
G boxes are being painted.
G boxes are being painted.
Done painting.
Done painting.
G boxes are being painted.
Done painting.
G boxes are being painted.
Done painting.
P boxes are being painted.
Done painting.
P boxes are being painted.
Done painting.
P boxes are being painted.
Done painting.
P boxes are being painted.
Done painting.
P boxes are being painted.
Done painting.
^C
Question is: why do I get more than 2 G boxes painted even though there are only 2 G boxes in the boxIn[] array, and more importantly how do I even get P boxes painted, not to mention there is only one P box in the array and I get many P boxes painted?
Note: I have been trying for a long time. Not to mention that I used shift+C to stop many programs before this one. I restarted terminal and got this output. More interesting point is, I tried to print the value of i along with the box being painted and i saw that the output was mixed up! I mean, there were some rows that show G boxes(3) are being painted and some others were G boxes are being painted even though the printf() was printf("%c boxes(%d) are being painted",boxIn[i],i);. This is awkward, I restarted the operating system and got a different output from the same program. Can someone explain me why this occurs and how can I prevent this from occurring once again?
