why pthread causes a memory leak

Question

Whenever I create a pthread, valgrind outputs a memory leak,

For example the below code:

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

void *timer1_function (void *eit){
  (void) eit;
    printf("hello world\n");
    pthread_exit(NULL);
}

int main(void){
   pthread_t timer1;
   pthread_create( &timer1, NULL, timer1_function,  NULL);  ///////line13
   int i=0;
   for(i=0;i<2;i++){usleep(1);}
   return 0;
}

valgrind outputs

==1395== HEAP SUMMARY:
==1395==     in use at exit: 136 bytes in 1 blocks
==1395==   total heap usage: 6 allocs, 5 frees, 1,134 bytes allocated
==1395== 
==1395== 136 bytes in 1 blocks are possibly lost in loss record 1 of 1
==1395==    at 0x402A629: calloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==1395==    by 0x4011304: allocate_dtv (dl-tls.c:297)
==1395==    by 0x4011AAB: _dl_allocate_tls (dl-tls.c:461)
==1395==    by 0x4052470: pthread_create@@GLIBC_2.1 (allocatestack.c:571)
==1395==    by 0x8048566: main (test.c:13)
==1395== 
==1395== LEAK SUMMARY:
==1395==    definitely lost: 0 bytes in 0 blocks
==1395==    indirectly lost: 0 bytes in 0 blocks
==1395==      possibly lost: 136 bytes in 1 blocks
==1395==    still reachable: 0 bytes in 0 blocks
==1395==         suppressed: 0 bytes in 0 blocks

why pthread_create cause a problem although I was using the man page as reference, and how can I fix it?

Answer 1

A thread is an allocated resource and you did not free it before exiting. You should call pthread_join; this would also eliminate the need for your hackish and incorrect sleep loop.

It's possible that even once you fix this, valgrind will still see a "leak", since some implementations of POSIX threads (I'm guessing you're using glibc/NPTL) cache and reuse thread resources rather than freeing them fully. I'm not sure if valgrind works around this or not.