Semaphores and threads

[komalwaseem]

I'm trying to run this code on fedora but it gives me an error for the commands being run in main saying they're undefined:
here's the code:

#include<pthread.h>
#include<stdio.h>

#include<semaphore.h>

using namespace std;
sem_t s1,s2;
void *ping(void*)
{
for(int i=0;i<5;i++)
{
sem_wait(&s1);
printf("ping ");
sem_post(&s2);
}
}
void *pong(void*)
{
for(int i=0;i<5;i++)
{
sem_wait(&s2);
printf("pong \n");
sem_post(&s1);
}
}
int main()
{
pthread_t t1,t2;
// pthread_attr_t attr;
// pthread_attr_init(&attr);
sem_init(&s1,0,1);
sem_init(&s2,0,0);
pthread_create(&t1,NULL,&pong,NULL);
pthread_create(&t2,NULL,&ping,NULL);
pthread_join(t1,NULL);
pthread_join(t2,NULL);
return 0;
}

[carrotcake1029]

Your mixing some C++ with C (using namespace std; ). Also be sure to link libraries you use (-pthread).

[bithub]

In addition to what carrotcake said, your ping() and pong() functions should return values.

Also, please use code tags when posting in the future to make your code easier to read.

[Elkvis]

In addition to what carrotcake said, your ping() and pong() functions should return values.

actually, if they're thread procedures, they should call

pthread_exit(NULL);

instead of returning. granted, they should still have a return statement for standards-compliance, but in order to behave as expected, you really need to have the pthread_exit call.

edit: when creating your threads, you should not put the '&' symbol in front of the function names.

pthread_create(&t1, NULL, pong, NULL);

is the correct usage. putting the '&' in front of the function names will likely result in compiler errors.

[bithub]

actually, if they're thread procedures, they should call

pthread_exit(NULL);

instead of returning. granted, they should still have a return statement for standards-compliance, but in order to behave as expected, you really need to have the pthread_exit call.

That's wrong. Returning from the thread procedure is the same as calling pthread_exit(). There is no need to explicitly call it.

If the start_routine returns, the effect is as if there was an implicit call to pthread_exit() using the return value of start_routine as the exit status.

[Elkvis]

That's wrong. Returning from the thread procedure is the same as calling pthread_exit(). There is no need to explicitly call it.

after looking it up myself, it seems you are correct. I guess I was thinking that it was necessary to call pthread_exit() from all thread procedures, when it turns out that the only one you MUST call it from is main(), and only then if you don't join all threads before exiting.

[Codeplug]

>> putting the '&' in front of the function names...
Is optional.

>> the only one you MUST call it from is main()
That's optional as well - and should only be done if all existing threads are in a detached state.

gg