Revision as of 16:35, 17 December 2022

Using pthreads

Single thread example

The example below shows the syntax of pthreads. The thread doesn't do anything useful.

compile with: gcc pt1.c -o pt -pthread

 1 #include <stdio.h>
 2 #include <pthread.h>
 3 
 4 long counter;
 5 void * adder(void * data) {
 6         for (int i=0; i < 1000000; i++)
 7                 counter++;
 8 }
 9 
10 int main(void) {
11         int i;
12         pthread_t pt;
13 
14         counter=0;
15         pthread_create(&pt, NULL, adder, NULL);
16         pthread_join(pt, NULL);
17         printf("Value of counter is %ld\n", counter);
18         return(0);
19 }

Using multiple threads

Using multiple threads simultaneously can give race conditions as the code below shows when running.

 1 #include <stdio.h>
 2 #include <pthread.h>
 3 
 4 #define THREADS 2
 5 
 6 long counter;
 7 void * adder(void * data) {
 8         for (int i=0; i < 1000000; i++)
 9                 counter++;
10 }
11 
12 int main(void) {
13         int i;
14         pthread_t pt[THREADS];
15 
16         counter=0;
17 
18         for (i=0; i < THREADS; i++) {
19                 pthread_create(&pt[i], NULL, adder, NULL);
20                 printf("Created thread %d\n",i+1);
21         }
22 
23         for (i=0; i < THREADS; i++) {
24                 pthread_join(pt[i], NULL);
25                 printf("Joined thread %d\n",i+1);
26         }
27 
28         printf("Value of counter is %ld\n", counter);
29         return(0);
30 }

Running the code two times gave this output as example: Note: The two wrong counter values.

heth@emb3:~/bin$ pt2
Created thread 1
Created thread 2
Joined thread 1
Joined thread 2
Value of counter is 1032119
heth@emb3:~/bin$ pt2
Created thread 1
Created thread 2
Joined thread 1
Joined thread 2
Value of counter is 1081391

Releasing resources used by a pthread

To release the resources - memory for stack and householding - it is necessary to call either pthread_detach or pthread_join. The resources are not released when the pthread exits without one of these calls.

pthread_join is called by the process or another thread.
pthread_detach is typically called by the thread itself to release the resources when the thread terminates

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