Threads in C : valgrind report shows additional allocs that can't be freed

Question

I wrote a program in C. In a for loop it creates 12 threads.

   for (i = 0; i < 12; i++)
   {
        status=pthread_create(&ntid[i],NULL,th_f,NULL);
        if (status != 0)
        {
                printf("Error in Creating Thread\n");
                exit(1);
        }
   }

Then I joined them as shown:

   for (i = 0; i < 12; i++)
   {
                ret=pthread_join(ntid[i],&retval);
        if (ret)
        {
                printf("Error joining\n");
                exit(1);
        }
        printf("Thread terminated with status %d\n", *((int*)retval));
        free (retval);
   }

The function that thread is executing is th_f . It has 1 malloc and it is freed after joining (as you can see in `free (retval);

The valgrind report on this program is given below:

==27123== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 4 from 1)
==27123== malloc/free: in use at exit: 1,552 bytes in 5 blocks.
==27123== malloc/free: 29 allocs, 24 frees, 4,864 bytes allocated.
==27123== For counts of detected errors, rerun with: -v
==27123== searching for pointers to 5 not-freed blocks.
==27123== checked 89,896 bytes.

and

==27123== LEAK SUMMARY:
==27123==    definitely lost: 0 bytes in 0 blocks.
==27123==      possibly lost: 0 bytes in 0 blocks.
==27123==    still reachable: 1,552 bytes in 5 blocks.
==27123==         suppressed: 0 bytes in 0 blocks.

So I need answers to these questions:

There should be only 12 allocs and 12 frees.Why is the number double?
Irrespective of how many threads are created, 5 allocs are always extra. They are not freed. Why?

This discussion may be of use to you: [Still Reachable Leak detected by Valgrind](http://stackoverflow.com/questions/3840582/still-reachable-leak-detected-by-valgrind). — HostileFork says dont trust SE, Oct 16 '14 at 07:28
@PaulR Hey there is no `pthread_destroy` function in `pthread.h`. No man entry also exists? — 0aslam0, Oct 16 '14 at 07:41
My bad - I was thinking of `pthread_attr_destroy`, which does not apply here - ignore me - I'm going for coffee now... — Paul R, Oct 16 '14 at 07:43

score 0 · Accepted Answer · answered Oct 16 '14 at 07:24

0

The extra allocations and frees are probably thread stacks. The five extra are probably used internally by the threading library -- as it says, they are not leaked but are still reachable.

answered Oct 16 '14 at 07:24

David Schwartz

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@xachu4u There's no need. They're reachable. They're not leaked. They may even hold essential information. – David Schwartz Oct 16 '14 at 07:26

Threads in C : valgrind report shows additional allocs that can't be freed

1 Answers1