C++ segmentation fault when creating pthreads

Question

void createThreads(int k){
struct threadData threadData[k];

int numThreads = k;
int i = 0;
int err = 0;

pthread_t *threads = static_cast<pthread_t*>(malloc(sizeof(pthread_t) * numThreads));
for(i = 0;i<numThreads;i++){

    threadData[i].thread_id = i;
    threadData[i].startIndex = ((N/k)*i);
    threadData[i].stopIndex = ((N/k)*(i+1));

    err = pthread_create(&threads[i], NULL, foo, (void *)&threadData[i]);


    if(err != 0){
        printf("error creating thread\n");
    }
}
}

Here, N and k are integers where the remainder of N/k is guaranteed to be 0. including createThreads(numThreads); in main will cause my program to seg fault and commenting it out will take care of this however any printf debug statements I put into createThreads (even on the first line inside the function) will not show so I am pretty confused as to how to debug this. All help is appreciated.

It's not the cause of your problem, but why are you using malloc in C++? — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Mar 15, 2015 at 22:58
struct threadData threadData[k]; This is not legal C++. You can't create an array using a variable as the number of entries. — PaulMcKenzie
– PaulMcKenzie, Commented Mar 15, 2015 at 23:03

marcinj · Accepted Answer · 2015-03-15 23:00:53Z

2

I suppose the problem is that your arg parameter is on stack of your createThreads function:

struct threadData threadData[k];

so once your thread gets created and run, and createThreads returns, threadData is no longer valid, so your thread function should not touch argument data. Otherwise its undefinded behaviour and crash.

so to fix it, you should either, make threadData global (outside createThreads), or malloc it.

answered Mar 15, 2015 at 23:00

marcinj

50.3k10 gold badges85 silver badges103 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

user0815 Over a year ago

This is definitely a problem. But there can also be further problems in foo itself.

Collectives™ on Stack Overflow

C++ segmentation fault when creating pthreads

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related