output value interleavely with two pthreads - c

Question

I want to create two thread which output interleave like below
Thread1:1=>Ping!
Thread2:2=>Pong!
Thread1:3=>Ping!
Thread1:4=>Ping!
Thread2:5=>Pong!
Thread2:6=>Pong!
Thread1:7=>Ping!
Thread2:8=>Pong!
Thread1:9=>Ping!
..........
until 50
and my code is below
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>
void* increment1(void* arg);
void* increment2(void* arg);
int count = 0;
sem_t sem;
int main() {
//variable initialize
pthread_t thread1, thread2;
int res1 = 0, res2 = 0;
int number = 0;
int i = 0;
//create semaphore
if (sem_init(&sem, 0, 1) == -1){
printf("Semaphore creation failed!!\n");
exit(EXIT_FAILURE);
}
for (i = 0; i < 25; ++i){
//create thread
res1 = pthread_create(&thread1, NULL, increment1, NULL);
if (res1 != 0) {
printf("Thread1 creation failed!!\n");
exit(EXIT_FAILURE);
}
//wait thread synchronization
pthread_join( thread1, NULL);
res2 = pthread_create(&thread2, NULL, increment2, NULL);
if (res2 != 0) {
printf("Thread2 creation failed!!\n");
exit(EXIT_FAILURE);
}
//wait thread synchronization
pthread_join( thread2, NULL);
}
exit(EXIT_SUCCESS);
}
void* increment1(void* arg) {
sem_wait(&sem);
count ++;
printf("Thread1:%d\nPing!\n",count);
fsync(fileno(stdout));
sem_post(&sem);
}
void* increment2(void* arg) {
sem_wait(&sem);
count ++;
printf("Thread2:%d\nPong!\n",count);
fsync(fileno(stdout));
sem_post(&sem);
}
But I think that what I do isn't using two thread in parallel and is wrong, what I use is sequential alternative executing two thread and it isn't not in parallel.(By using pthread_join, thread2 will execute after thread1 finish).
I try to using semaphore it seem that it cannot assure the thread execution order.
What I want to ask is
1.how to using semaphore to assure the two thread order?
2.how to pause the thread and resume it? I think that I do is create new two pthread in a loop cycle.
Thank in advance.

Answer 1

Add a second semaphore and initialize it to zero so that thread1 is forced to first. Then increment1 and increment2 keep signally that it is the other threads turn to go. You had some minor hangups with where joined things that you can figure out.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>
void* increment1(void* arg);
void* increment2(void* arg);
int count = 0;
sem_t sem1, sem2;
int main()
{
pthread_t thread[2];
int res = 0;
int number = 0;
int i = 0;
if (sem_init(&sem1, 0, 1) == -1)
{
printf("Semaphore creation failed!!\n");
exit(EXIT_FAILURE);
}
if (sem_init(&sem2, 0, 0) == -1)
{
printf("Semaphore creation failed!!\n");
exit(EXIT_FAILURE);
}
for (i = 0; i < 25; ++i)
{
res = pthread_create(&thread[0], NULL, increment1, NULL);
if (res != 0)
{
printf("Thread creation failed!!\n");
exit(EXIT_FAILURE);
}
res = pthread_create(&thread[1], NULL, increment2, NULL);
if (res != 0)
{
printf("Thread creation failed!!\n");
exit(EXIT_FAILURE);
}
for (int j = 0; j < 2; ++j)
{
pthread_join(thread[j], NULL);
}
}
exit(EXIT_SUCCESS);
}
void* increment1(void* arg)
{
sem_wait(&sem1);
count ++;
printf("Thread1:%d Ping!\n", count);
fsync(fileno(stdout));
sem_post(&sem2);
}
void* increment2(void* arg)
{
sem_wait(&sem2);
count ++;
printf("Thread2:%d Pong!\n", count);
fsync(fileno(stdout));
sem_post(&sem1);
}

Answer 2

Thank for you guys, I have modified it and it seems fine.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>
int count = 0;
void* increment_1(void* arg);
void* increment_2(void* arg);
sem_t sem_1, sem_2;
sem_t c_sem;
int main() {
//variable initialize
pthread_t thread1, thread2;
int res1 = 0, res2 = 0;
int number = 0;
int i = 0;
//create semaphore
if (sem_init(&c_sem, 0, 1) == -1){
printf("Semaphore creation failed!!\n");
exit(EXIT_FAILURE);
}
if (sem_init(&sem_1, 0, 1) == -1){
printf("Semaphore creation failed!!\n");
exit(EXIT_FAILURE);
}
if (sem_init(&sem_2, 0, 0) == -1){
printf("Semaphore creation failed!!\n");
exit(EXIT_FAILURE);
}
//create thread
res1 = pthread_create(&thread1, NULL, increment_1, NULL);
if (res1 != 0) {
printf("Thread1 creation failed!!\n");
exit(EXIT_FAILURE);
}
res2 = pthread_create(&thread2, NULL, increment_2, NULL);
if (res2 != 0) {
printf("Thread2 creation failed!!\n");
exit(EXIT_FAILURE);
}
//wait thread synchronization
pthread_join(thread1, NULL);
pthread_join(thread2, NULL);
exit(EXIT_SUCCESS);
}
void* increment_1(void* arg) {
while(1){
sem_wait(&sem_1);
sem_wait(&c_sem);
if(count == 50){
sem_post(&c_sem);
sem_post(&sem_2);
exit(EXIT_SUCCESS);
}
count ++;
printf("Thread1:%d->Ping!\n",count);
fsync(fileno(stdout));
sem_post(&c_sem);
sem_post(&sem_2);
}
}
void* increment_2(void* arg) {
while(1){
sem_wait(&sem_2);
sem_wait(&c_sem);
if(count == 50){
sem_post(&c_sem);
sem_post(&sem_1);
exit(EXIT_SUCCESS);
}
count ++;
printf("Thread2:%d->Pong!\n",count);
fsync(fileno(stdout));
sem_post(&c_sem);
sem_post(&sem_1);
}
}