Exercises for TDDD36,
Processes I
September 22, 2011
1. Forking processes (based on [1] Excercise 3.4)
Using the program shown below, what will be the output at Line A?
#include
#include
#include
#include
<sys/types.h>
<stdio.h>
<unistd.h>
<stdlib.h>
int value = 5;
int main() {
pid_t pid;
}
pid = fork();
if (pid == 0) { /* child */
value += 15;
} else if (pid > 0) { /* parent */
wait(NULL);
printf("PARENT: value %d \n", value); /* Line A */
exit(0);
}
2. Which techniques could be used in the program of Exercise 1 in order to
get value 20 in the printed output?
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3. Följande pseudokod använder sig semaforoperationerna wait och signal
med målet att åstadkomma ömsesidig uteslutning av en gemensam resurs.
Ger implementeringen önskad resultat?
Isåfall, förklara varför.
I annat
fall, presentera ett spår från programmets körning där oönskat beteende
uppstår.
Process P1
signal(mutex)
Critical_section_1
signal(mutex)
Non_critical_section_1
end
Process P2
wait(mutex)
Critical_section_2
signal(mutex)
Non_critical_section_2
end
4. Mutual Exlusion (based on [1] Excercise 6.1)
The rst known correct software solution to the critical-section problem
for two processes was developed by Dekker. The two processes
P0
and
P1 ,
share the following variables:
boolean flag[2]; /* initially false */
int turn;
/* initially 0 or 1 */
The structure of process
Pi (i = 0, 1) is shown below.
It uses
j 6= i ∈ {0, 1}
to refer to the other process' variables. Prove that the algorithm satises
the mutual exclusion requirement and that it is free from starvation.
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do {
flag[i] = TRUE;
while (flag[j]) {
if (turn == j) {
flag[i] = FALSE;
while (turn == j)
; //do nothing
}
}
flag[i] = TRUE;
//critical section
turn = j;
flag[i] = FALSE;
//remainder section
} while (TRUE);
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5. Deadlock avoidance (Banker's algorithm). Given the following snapshot
of a system:
P0
P1
P2
P3
P4
Allocation
---------A B C
0 1 2
1 0 3
2 1 0
4 1 1
1 0 0
Max
--A B C
1 2 3
4 1 5
3 3 4
7 2 5
3 1 1
Available
--------A B C
2 3 2
a) Is the system in a safe state?
b) If process
P2
requests the resources (1, 2, 0), can the request be imme-
diately granted?
c) If, instead, process
P3
requests the resources (1, 1, 1), can the request
be immediately granted?
References
[1] Abraham Silberschatz, Greg Gagne, and Peter Baer Galvin.
System Concepts.
Operating
John Wiley & Sons, seventh edition, 2005.
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