Multithreaded search in c - c

Question

I'm supposed to have two threads that search for the minimum element in an array: the first one searches the first half, and the second thread searches the other half. However, when I run my code, it seems that it chooses a thread randomly. I'm not sure what I'm doing wrong, but it probably has to do with the "mid" part. I tried dividing an array into two, finding the midpoint and then writing the conditions from there, but I probably went wrong somewhere. I also tried putting array[i] in the conditions, but in that case only thread2 executes.
EDIT: I'm really trying my best here, but I'm not getting anywhere. I edited the code in a way that made sense to me, and I probably typecasted "min" wrong but now it doesn't even execute it just gives me an error, even though it compiles just fine. I'm just a beginner, and while I do understand everything you guys are talking about, I have a hard time implementing the ideas, so really, any help with fixing this is appreciated!
EDIT2: Okay so the previous code made no sense at all, I do apologize but I was exhausted while writing it. Anyway, I came up with something else that works partially! I split the array into two halves, however only the first element is accessible when using the pointer. But would it work if the whole array was being accessed and if so how can I fix that then?
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <string.h>
#define size 20
void *smallest(void *arg);
pthread_t th, th2;
int array[size], i, min;
int main(int argc, char *argv[]) {
srand ( time(NULL) );
for(i = 0; i < size; i++)
{
array[i] = (rand() % 100)+1;
printf("%d ", array[i]);
}
int *array1 = malloc(10 * sizeof(int));
int *array2 = malloc(10 * sizeof(int));
memcpy(array1, array, 10 * sizeof(int));
memcpy(array2, array + 10, 10 * sizeof(int));
printf("\nFirst half gives %d \n", *array1);
printf("Second half gives %d \n", *array2);
pthread_create(&th, NULL, smallest, (void*) array1);
pthread_create(&th2, NULL, smallest, (void*) array2);
pthread_join(th, NULL);
pthread_join(th2, NULL);
//printf("\nFirst half gives %d\n", array1);
//printf("Second half gives %d\n", array2);
if (*array1 < *array2) {
printf("\nThread1 finds the number\n");
printf("The smallest element is %i\n", *array1);
}
else {
printf("\nThread2 finds the number\n");
printf("The smallest element is %i\n", *array2);
}
return 0;
}
void *smallest(void* arg){
int *array = (int*)arg;
min = array[0];
for (i = 0; i < size; i++) {
if (array[i] < min) {
min = array[i];
}
}
pthread_exit(NULL);
}

Answer 1

The code you've set up never runs more than one thread. Notice that if you run the first branch of the if statement, you fire off one thread to search half the array, wait for it to finish, then continue onward, and if the else branch executes, the same thing happens in the second half of the array. Fundamentally, you probably want to rethink your strategy here by having the code always launch two threads and join each of them only after both threads have started running.
The condition within your if statement also seems like it's mistaken. You're asking whether the middle element of the array is greater than its index. I assume this isn't what you're trying to do.
Finally, the code you have in each thread always looks at the entire array, not just a half of it. I would recommend rewriting the thread routine so that its argument represents the start and end indices of the range to take the minimum of. You would then update the code in main so that when you fire off the thread, you specify which range to search.
I would structure things like this:
Fire off a thread to find the minimum of the first half of the array.
Fire off a thread to find the minimum of the second half of the array.
Join both threads.
Use the results from each thread to find the minimum.
As one final note, since you'll have two different threads each running at the same time, you'll need to watch for data races as both threads try to read or write the minimum value. Consider having each thread use its exit code to signal where the minimum is and then resolving the true minimum back in main. This eliminates the race condition. Alternatively, have one global minimum value, but guard it with a mutex.

Answer 2

1) You´re redeclaring the global variables in the main function, so there´s actually no point in declaring i, low, high, min:
int array[size], i, low, high, min;
The problem you´re having is with the scope of the variables when you redeclare the variables in the main function, the global ones with the same name become "invisible"
int *low = array;
int *high = array + (size/2);
int mid = (*low + *high) / 2;
So when you run the threads all the values of your variables(low, high, min;
) are 0, this is because they are never actually modified by the main and because they start in 0 default(startup code,etc).
Anyways I wouldn´t really recommend(it´s really frowned upon) using global variables unless it´s a really small proyect for personal use.
2) Another crucial problem is that you´re ignorning the main idea behind threads which is running both simultaneously
if (array[mid] > mid) {
pthread_create(&th, NULL, &smallest, NULL);
pthread_join(th, NULL);
printf("\nThread1 finds the number\n");
}
else if (array[mid] < mid) {
pthread_create(&th2, NULL, &smallest, NULL);
pthread_join(th2, NULL);
printf("\nThread2 finds the number\n");
}
You´re actually only running one thread when executing.
Try something like this:
pthread_create(&th, NULL, &smallest, NULL);
pthread_create(&th2, NULL, &smallest, NULL);
pthread_join(th2, NULL);
pthread_join(th, NULL);
3) You are trying to have two threads access the same variable this can result in undefined behaviour, you MUST use a muthex to avoid a number from not actually being stored.
This guide is pretty complete regarding mutexes but if you need anyhelp please let me know.

Answer 3

This is a single threaded version of what you are asking.
#include <stdio.h>
#include <stdlib.h>
/*
I can not run pthread on my system.
So this is some code that should kind of work the same way
*/
typedef int pthread_t;
typedef int pthread_attr_t;
typedef void*(*threadfunc)(void*);
int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine)(void*), void *arg)
{
start_routine(arg);
return 0;
}
int pthread_join(pthread_t thread, void **value_ptr)
{
return 0;
}
struct context
{
int* begin;
int* end;
int* result;
};
//the function has to be castable to the threadfunction type
//that way you do not have to worry about casting the argument.
//be careful though - if something does not match these errors may be hard to track
void * smallest(context * c) //signature needet for start routine
{
c->result = c->begin;
for (int* current = c->begin; current < c->end; ++current)
{
if (*current < *c->result)
{
c->result = current;
}
}
return 0; // not needet with the way the argument is set up.
}
int main(int argc, char *argv[])
{
pthread_t t1, t2;
#define size 20
int array[size];
srand(0);
for (int i = 0; i < size; ++i)
{
array[i] = (rand() % 100) + 1;
printf("%d ", array[i]);
}
//prepare data
//one surefire way of messing up in multithreading is sharing data between threads.
//even a simple approach like storing in a variable who is accessing will not solve the issues
//to properly lock data you would have to dive into the memory model.
//either lock with mutexes or memory barriers or just don' t share data between threads.
context c1;
context c2;
c1.begin = array;
c1.end = array + (size / 2);
c2.begin = c1.end + 1;
c2.end = array + size;
//start threads - here your threads would go
//note the casting - you may wnt to wrap this in its own function
//there is error potential here, especially due to maintainance etc...
pthread_create(&t1, 0, (void*(*)(void*))smallest, &c1); //without typedef
pthread_create(&t2, 0, (threadfunc)smallest, &c2); //without typedef
pthread_join(t1, 0);//instead of zero you could have a return value here
pthread_join(t1, 0);//as far as i read 0 throws the return value away
//return value could be useful for error handling
//evaluate
if (*c1.result < *c2.result)
{
printf("\nThread1 finds the number\n");
printf("The smallest element is %i\n", *c1.result);
}
else
{
printf("\nThread2 finds the number\n");
printf("The smallest element is %i\n", *c2.result);
}
return 0;
}
Edit:
I edited some stubs in to give you an idea of how to use multithreading.
I never used pthread but this should likely work.
I used this source for prototype information.