[SOLVED] how to pass integer value through pthread_create - C++

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I am planning to pass value "1" and "4" to the smoker_tobacco but my output shows

ubuntu@ubuntu:~/Desktop$ ./testing
140243696023296,140243670845184
Segmentation fault (core dumped)

Much appreciated for the help.

What I have tried:

C++

void *smoker_tobacco (void *id)
{
	int* tid = (int*)id;
    	cout << "Thread id:"<< *tid << endl;
}


int main(){

	pthread_t smoker1,smoker4;
	
	sem_init(&table,0,1);
	sem_init(&sem_tobacco,0,0);
	sem_init(&sem_paper,0,0);
	sem_init(&sem_match,0,0);
	
	pthread_create(&smoker1, NULL, smoker_tobacco, (void*)1);
	pthread_create(&smoker4, NULL, smoker_tobacco, (void*)4);

	cout<<smoker1<<","<<smoker4<<endl;
	
	pthread_join(smoker1,NULL);
	pthread_join(smoker4,NULL);

	
	return 0;
	
}

Posted 19-Jul-22 4:56am

cyye

Updated 19-Jul-22 5:34am

v2

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3 solutions

Solution 3

You can cast from int to void* and back again, without any problems:

C++

#include <iostream>

void f(void *p)
{
    int i = reinterpret_cast<intptr_t>(p);
    std::cout << "f: i = " << i << '\n';
}

int main()
{
    f( reinterpret_cast<void*>(1) );

    return 0;
}

Note that this when going from void* to int we have to cast the pointer to inptr_t, otherwise, in 64 bit mode we get an error message regarding the cast from void* to int looses precision.
In this case, this is reasonably safe, since we know that sizeof(void*) >= sizeof(int), so any int can be represented in a void*.

Posted 19-Jul-22 5:28am

k5054

Comments

Maciej Los 19-Jul-22 11:48am

5ed!

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CPallini · Accepted Answer · 2022-07-19T05:16:00

The correct way to retrieve the passed parameter follows

C++

void *smoker_tobacco (void *id)
{
  long tid = (long)id;
  cout << "Thread id:"<< tid << endl;
  return (void *) 0;
}

Try, for instance

C++

#include <iostream>
#include <pthread.h>
#include <semaphore.h>
using namespace std;


void *smoker_tobacco (void *id)
{
  long tid = (long)id;
  cout << "Thread id:"<< tid << endl;
  return (void *) 0;
}


int main()
{
  pthread_t smoker1,smoker4;
  sem_t table, sem_tobacco, sem_paper, sem_match;

  sem_init(&table,0,1);
  sem_init(&sem_tobacco,0,0);
  sem_init(&sem_paper,0,0);
  sem_init(&sem_match,0,0);

  pthread_create(&smoker1, NULL, smoker_tobacco, (void*)1);
  pthread_create(&smoker4, NULL, smoker_tobacco, (void*)4);

  cout<<smoker1<<","<<smoker4<<endl;

  pthread_join(smoker1,NULL);
  pthread_join(smoker4,NULL);

  return 0;
}

Richard MacCutchan · Accepted Answer · 2022-07-19T05:12:00

Solution 1

You are casting the values 1 and 4 to void*. But they are not valid addresses, hence the Segnmentation Fault. You should pass (and accept) simple values, or create proper pointers.

C++

int value1 = 1;
int value4 = 4;
	pthread_create(&smoker1, NULL, smoker_tobacco, (void*)&value1);
	pthread_create(&smoker4, NULL, smoker_tobacco, (void*)&value4);

Posted 19-Jul-22 5:12am

Richard MacCutchan

Comments

CPallini 19-Jul-22 11:18am

The OP passes the parameters correctly (yes, you can do that). The flawn is on parameter retrieval.

Richard MacCutchan 19-Jul-22 11:32am

Yes, I am well aware of that, but I decided to stick with a simpler solution.

CPallini 19-Jul-22 11:35am

Simpler?! :-)

Richard MacCutchan 19-Jul-22 11:37am

For someone who is obviously new and does not fully understand casts; yes.

CPallini 19-Jul-22 11:40am

Well, it works!
:thumbsup:

merano99 20-Jul-22 12:25pm

It could keep crashing if value1 and value4 should be on the stack. Maybe the simple cast would be safer here.

Richard MacCutchan 20-Jul-22 12:32pm

Not unless some other code destroys those fields while the thread is initialising. And in most cases the thread creator would be passing pointers to structures rather than simple variables.

cyye 19-Jul-22 11:31am

Thank you very much, it works!!