Multithreading in C - Final created thread overtakes output of other threads? - c

Question

I'm a beginner at C, and am trying out multithreading. I wrote a program to try and calculate values of Euler's phi function through the use of multiple threads. I'm using brute force, checking each individual number less than the given input for common factors.
#include <stdio.h>
#include <pthread.h>
#include <math.h>
#include <stdlib.h>
#include <unistd.h>
int has_common_factor(int number1, int number2);
void *phi_function(void *);
pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;
int out = 0;
int NTHREADS,inp;
int main(int argc, char *argv[])
{
int c = 0;
while(1){
c = getopt(argc, argv, "p:n:");
if (c == -1){
break;
}
switch(c){
case 'p': inp = atoi(optarg);
break;
case 'n': NTHREADS = atoi(optarg);
break;
default: printf("Invalid Option");
return 1;
}
}
//pthread_t thread_id[NTHREADS];
pthread_t *thread_id;
thread_id = (pthread_t *)malloc(sizeof(pthread_t) * NTHREADS);
int i, j, arg;
for (i = 0; i < NTHREADS; i++)
{
arg = i * inp / NTHREADS + 1;
pthread_create(&thread_id[i], NULL, phi_function, (void *)&arg);
}
for (j = 0; j < NTHREADS; j++)
{
pthread_join(thread_id[j], NULL);
}
printf("Final value: %d\n", out);
}
void *phi_function(void *ptr)
{
printf("threadid: %ld\n",pthread_self());
int i;
int *min;
min = (int *)ptr;
FILE *fptr;
sleep(*min);
char filename[100];
sprintf(filename,"output%d", *min);
fptr = fopen(filename,"w");
for (i = *min; i < *min + inp / NTHREADS; i++)
{
int j;
if (has_common_factor(i, inp) == 0) {
pthread_mutex_lock(&mutex1);
out++;
printf("i = %d\n",i);
pthread_mutex_unlock(&mutex1);
fprintf(fptr,"i = %d\n",i);
}
}
fclose(fptr);
}
int has_common_factor(int number1, int number2) {
int j;
for (j=2; j<=number1; j++) {
if (number1 %j == 0 && number2 %j ==0) {
return 1;
}
}
return 0;
}
I used threading by splitting the input into N different equal sized ranges, and making each of N threads check each range for relatively prime integers. I also printed out all of the relatively prime integers detected to check for any errors. The program works fine for 1 integer, giving the correct outputs for all the numbers I checked, but something interesting happens when using more than 1 thread.
After compiling and running with two threads, using 2 threads and an input of 20, like this:
./thread -p 20 -n 2
This is the output:
threadid: 140235244418816
threadid: 140235236026112
i = 11
i = 13
i = 17
i = 19
i = 11
i = 13
i = 17
i = 19
Final value: 8
As you can see, it gets the correct count for the number of relatively prime integers, but only outputs the integers detected in the range of the second thread, seemingly overriding that of the first. Both threads must have been run since there are two unique threadids. Using 1 thread results in the following output:
threadid: 140235236026112
i = 1
i = 3
i = 7
i = 9
i = 11
i = 13
i = 17
i = 19
Final value: 8
What is happening/what can I do to fix this?

Answer 1

for (i = 0; i < NTHREADS; i++)
{
arg = i * inp / NTHREADS + 1;
pthread_create(&thread_id[i], NULL, phi_function, (void *)&arg);
}
You pass the same parameter, &arg, to every thread. You need to pass different parameters to each thread.
A good pattern to use is this:
Call malloc to allocate an object to hold the parameters for a thread.
Fill in the structure.
Call pthread_create, passing it the value you got from malloc.
In the thread, extract the values from the structure and free it when you're done with it.
This ensures you pass a different value to each thread.