Thread synchronisation using semaphore for c - c

Question

I am writing a c program to create three threads(1,2,3) such that at any given point of time only one thread must execute and print the output on console in the order 123123123123.........
I am making use of semaphore for synchronization.
I am having an issue with the code i have written. the code doesn't print it in the order 123123... the order is random and stops after a while.
#include<stdio.h>
#include<semaphore.h>
#include<pthread.h>
#include<unistd.h>
#include<assert.h>
#include<stdlib.h>
sem_t sem_1;
void *func1(void *arg){
int err1=0;
while(1){
err1=sem_wait(&sem_1);
assert(err1==0);
printf("thread 1\n");
}
//return NULL;
}
void *func2(void *arg){
int err2=0;
while(1){
err2=sem_wait(&sem_1);
assert(err2==0);
printf("thread 2\n");
}
// return NULL;
}
void *func3(void *arg){
int err3=0;
while(1){
err3=sem_wait(&sem_1);
assert(err3==0);
printf("thread 3\n");
}
// return NULL;
}
int main(){
pthread_t *t1,*t2,*t3;
int i=0,rc=0,c1=0,c2=0,c3=0;
t1=(pthread_t *)malloc(sizeof(*t1));
t2=(pthread_t *)malloc(sizeof(*t2));
t3=(pthread_t *)malloc(sizeof(*t3));
i=sem_init(&sem_1,0,1);
assert(i==0);
c1=pthread_create(t1,NULL,func1,NULL);
assert(c1==0);
c2=pthread_create(t2,NULL,func2,NULL);
assert(c2==0);
c3=pthread_create(t3,NULL,func3,NULL);
assert(c3==0);
while(1){
rc=sem_post(&sem_1);
assert(rc==0);
sleep(1);
}
return 0;
}

Answer 1

Why would you even expect them in in an order?
Your threads do things inbetween the different waits, and according to the system scheduler, these can take different amount of time.
printf is a buffered operation that gives you exclusive access to a shared resource. So in addition to your semaphore there is a hidden lock somewhere, that also regulates the progress of your threads. Don't expect the prints to appear in order, even if the calls themselves are.
Then, for the end, using assert here is really a very bad idea. All sem_t operations can (and will) fail spuriously, so it is really bad to abort, just because such a failure.
In fact, sem_t is really a low level tool, and you should never use it without checking return values and without having a fall back strategy if such a call fails.

Answer 2

Of course. I can't see any order related synchronization mechanism.
One of the options to achieve the strict order is to have one semaphore per thread:
sem_t sem[3];
// ...
while(1) {
for(n = 0; n < 3; n++) {
rc=sem_post(&sem[n]);
assert(rc==0);
}
sleep(1);
}

Answer 3

Below Code will help to solve the problem, one semaphore for each thread used to achieve synchronization. The initial value of the semaphore does the job. used usleep to launch thread in sequence manner for needed output. The same can be done by using Mutex lock and cond variables
//declaring semaphore
sem_t sema1;
sem_t sema2;
sem_t sema3;
void* t1(void *arg)
{
for(int j=0;j<10;j++)
{
sem_wait(&sema1);
printf("Thread 1 \n");
sem_post(&sema2);
}
printf("T1 return\n");
return NULL;
}
void* t2(void *arg)
{
for(int j=0;j<10;j++)
{
sem_wait(&sema2);
printf("Thread 2 \n");
sem_post(&sema3);
}
printf("T2 return\n");
return NULL;
}
void* t3(void *arg)
{
for(int j=0;j<10;j++)
{
sem_wait(&sema3);
printf("Thread 3 \n");
sem_post(&sema1);
}
printf("T3 return \n");
return NULL;
}
int main()
{
pthread_t tid1, tid2,tid3;
sem_init(&sema1,0,1);
sem_init(&sema2,0,0);
sem_init(&sema3,0,0);
pthread_create(&tid1, NULL, t1, NULL);
usleep(100);
pthread_create(&tid2, NULL, t2, NULL);
usleep(100);
pthread_create(&tid3, NULL, t3, NULL);
pthread_join(tid3, NULL);
pthread_join(tid2, NULL);
pthread_join(tid1, NULL);
sem_destroy(&sema1);
sem_destroy(&sema2);
sem_destroy(&sema3);
return 0;
}