Question

Explain the errors in the C code that cause the output NOT to be -10. (Hint: There are 2 errors):

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
  
#define SIZE 5
void *threadFunc(void *arg);
int *changingVal;
int a[SIZE]; // Assume a[] = { 13444, 3320, 31020, 3302, 31313 };

int main() {
int i;
int min;
int * changeVal = malloc (sizeof(int*));
*changeVal = -10;
// Thread creation
pthread_t thread1;
pthread_attr_t attr;
pthread_attr_init(&attr);
  
for (i = 0; i < SIZE; i++) {
a[i] = rand();
printf("%d\n", a[i]);
}
min = a[0];
  
pthread_create(&thread1,&attr,threadFunc, &changeVal);

#pragma omp parallel for num_threads(1)
for (i = 1; i < SIZE; i++) {
if (a[i] < min) {
pthread_join(thread1, NULL);
#pragma omp critical

{
// compare a[i] and min again because min
// could have been changed by another thread after   
// the comparison outside the critical section
if (a[i] < min)
min = a[i];
}
}
}
printf("Min number found:\t%d\n", min);
}

void * threadFunc(void *arg) {
changingVal = (int*) arg;
a[3] = *changingVal;
pthread_exit(0);
}

Answer 1

Updated Code is in BOLD!!

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
  
#define SIZE 5
void *threadFunc(void *arg);
int *changingVal;
int a[SIZE]; // Assume a[] = { 13444, 3320, 31020, 3302, 31313 };

int main() {
int i;
int min;
int * changeVal = malloc (sizeof(int*));
*changeVal = -10;
// Thread creation
pthread_t thread1;
pthread_attr_t attr;
pthread_attr_init(&attr);
  
for (i = 0; i < SIZE; i++) {
a[i] = rand();

//printf("%d\n", a[i]); We need not to print here, instead print after updation
}
min = a[0];
//pthread_create(&thread1,&attr,threadFunc, &changeVal); , This was error
pthread_create(&thread1,&attr,threadFunc, changeVal);

for (i = 0; i < SIZE; i++) {
printf("%d\n",a[i]);
}

#pragma omp parallel for num_threads(1)
for (i = 1; i < SIZE; i++) {
if (a[i] < min) {
pthread_join(thread1, NULL);
#pragma omp critical

{
// compare a[i] and min again because min
// could have been changed by another thread after   
// the comparison outside the critical section
if (a[i] < min)
min = a[i];
}
}
}
printf("Min number found:\t%d\n", min);
}

void * threadFunc(void *arg) {
changingVal = (int*) arg;
a[3] = *changingVal;
pthread_exit(0);
}

The first error was that we are passing the reference of the changeVal -

pthread_create(&thread1,&attr,threadFunc, &changeVal);  

Instead, we need to pass the value of changeVal ,i.e, -10

pthread_create(&thread1,&attr,threadFunc, changeVal);  

The second error was we are printing the value of "a" before the updation

for (i = 0; i < SIZE; i++) {
a[i] = rand();
//printf("%d\n", a[i]); Comment this to avoid this line to run!!
}

pthread_create(&thread1,&attr,threadFunc, changeVal);

Instead we will print after updation

pthread_create(&thread1,&attr,threadFunc, changeVal);

for (i = 0; i < SIZE; i++) {
printf("%d\n",a[i]);
}