Segmentation Fault at pthread_join

Question

So when I run my code, I'm getting a segmentation fault right at the pthread_join. There is a print statement after my pthread_join that doesn't run. Does anyone have any idea why? Could you give me some hints or ideas as to how to figure this out??

the output prints out all of row numbers for my matrix until the end, then it leaves matrixCalc function and prints "after threads are created". This happens when I put in an argument for 1 thread.

I've included a small section of my code here:

int main(int argc, char*argv[]) 
{
  //takes in number of threads as 1st arg
  pthread_attr_init(&attr);
  //initialize matrix here

  //passes num of threads through matrixcalc
  for(i = 0; i < numberOfThreads; i++)
    {
      threadCount++;
      pthread_create(&tid, &attr, matrixCalc(threadCount), NULL);  
    }
  printf("after threads are created\n");
  pthread_join(tid, NULL);  
  printf("after join\n");
  exit(0);
  return 0;
}

Here is matrix calc function:

    void *matrixCalc(threadCount) 
{
  int i, j, sum, tempNum, currentRow;
  currentRow = threadCount;
  sum=0;

  while(currentRow < 1200)
    {
      //cycles through the column j for matrix B
      for(j=0; j<500; j++)
        {
          //cycles through the diff i values for the set row in matrix A and column in matrix B
          for(i=0; i<1000; i++)
            {
              //Matrix A set i value is at threadcount-1
              //Matrix B i value = j
              //Matrix B j value = i
              //Multiply together and add to sum
              tempNum = (matrixA[currentRow-1][i])*(matrixB[i][j]);
              sum = sum+tempNum;
            }
          //Set Matrix C at i value = currentRow and jvalue = i to sum
          matrixC[currentRow-1][j] = sum;
          //printf("%d\n", matrixC[currentRow-1][i]);
        }
        //increase threadcount by number of threads 
        //until you hit max/past max val
        currentRow = currentRow + nThreads;
        //printf("%d\n", currentRow);
    }
    return NULL;

}

@BitFiddlingCodeMonkey, I set x = pthread_create and checked if x !0, but it is =0. — angyxpoo, Nov 25 '13 at 18:47
@BitFiddlingCodeMonkey i have it set up so x = pthread_create(&tid, &attr, matrixCalc(threadCount), NULL); if(x != 0) { printf("error creating threadn\n"); } — angyxpoo, Nov 25 '13 at 18:51
@angyxpoo thanks. In matrixCalc, could you use "pthread_exit(NULL)" instead of "return NULL" - at the last line -. I am not sure but you can try. — taneryilmaz, Nov 25 '13 at 18:57
@tyilmaz, thanks. the pthread_exit(NULL) got rid of the seg fault, but it just exits the program prematurely now, without doing any of my other print statements. — angyxpoo, Nov 25 '13 at 19:02

alk · Accepted Answer · 2018-08-22T09:53:56.357

When calling pthread_create() you need to pass the address of a function of type void *(*)(void *). What the code does is calling a function there so its result is getting passed to pthread_create().

Change this line

pthread_create(&tid, &attr, matrixCalc(threadCount), NULL);

to become

pthread_create(&tid, &attr, matrixCalc, NULL);

or

pthread_create(&tid, &attr, &matrixCalc, NULL);

which in fact is the same.

As already mentioned above the thread function needs to be declared as void *(*)(void *).

So change this

 void *matrixCalc(threadCount)

will will become this

 void * matrixCalc(void *)

As the code seems to try to spawn off multiple threads and all should be joined perpare room to store the several pthread-ids.

This could for example be done using an array like this:

pthread_t tid[numberOfThreads] = {0};

Then create the thread like this:

pthread_create(&tid[i], &attr, matrixCalc, NULL);

To passed the thread number (counter i) down to the thread also give it room by defining

int thread_counts[numberOfThreads] = {0};

assign it and pass it as 4^th parameter on the thread's creation:

 thread_counts[i] = i;
 pthread_create(&tid[i], &attr, matrixCalc, &thread_Counts[i]);

Down in the thread function then get it by modifying

void *matrixCalc(threadCount) 
{
  int i, j, sum, tempNum, currentRow;
  currentRow = threadCount;
  ...

like this:

void * matrixCalc(void * pv) 
{
  int i, j, sum, tempNum, currentRow;
  currentRow = *((int*) pv);
  ...

Finally to join all thread replace the single call to pthread_join() by a loop:

for (i = 0; i < numberOfThreads; ++i)
{
  pthread_join(tid[i], NULL);  
}

score 1 · Answer 2 · edited Nov 25 '13 at 19:22

1

int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void * (*start_routine) (void *), void *arg);

The third parameter is a start function taking a void ptr and returning a void ptr.

The fourth parameter takes a void ptr pointing to the data you want to pass, in this case threadcnt.

edited Nov 25 '13 at 19:22

alk

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answered Nov 25 '13 at 18:48

Duck

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I'm trying to figure out how to change my line of code to match this. For 1st param, I would use &tid, for second param, I would use &attr, for 3rd param would I then use void (*matrixCalc)(void *) ? Or &matrixCalc? I'm getting errors with what I'm trying to use. and I tried to create a pointer to point to my threadCount but am getting an error. would I just do &threadCount – angyxpoo Nov 25 '13 at 19:04
Sorry, it's a busy day. See alk's answer. He seems to have it pretty much covered. FWIW we need to have an FAQ on this. At least a 3rd of the `pthread` questions seem to be about exactly this. – Duck Nov 25 '13 at 19:24

Segmentation Fault at pthread_join

2 Answers2