I am seeing different behaviors when variable used to get return values using pthread_join is defined gloabal vs static scope. I have included code_snippet here.
Static variables
int main()
{
static int r1,r2;
pthread_t t1, t2;
int i1[] = {1,2};
int i2[] = {3,4};
r1 = pthread_create( &t1, NULL, myfn, (void*)i1);
r2 = pthread_create( &t2, NULL, myfn, (void*)i2);
pthread_join( t1, (void *)&r1 );
pthread_join( t2, (void *)&r2 );
printf("Thread 1 returns: %d\n",r1);
printf("Thread 2 returns: %d\n",r2);
return 0;
}
void *myfn( void *intarray )
{
pthread_t t=pthread_self();
int *g = (int *) intarray;
int i=0;
int d=1;
for (i=g[0];i<=g[1];i++)
d*=i;
fprintf(stderr, "TID=%u %d\n",t, d);
pthread_exit((void *)d);
}
Return value
TID=3425117952 12
TID=3433510656 2
Thread 1 returns: 2
Thread 2 returns: 12
Global variables
int r1,r2;
int main()
{
same as above
}
void *myfn( void *intarray )
{
same as above
}
Return value
TID=3425117952 12
TID=3433510656 2
Thread 1 returns: 0 <<<<< it returns 0
Thread 2 returns: 12
Could someone please explain why it behaves differently ?
Almost certainly it's because the size of
int
andvoid *
differ on your platform, so whenpthread_join()
writes avoid *
value through theint *
pointer you gave it, it overwrites adjacent memory.The different declaration of
r1
andr2
changes the layout of the variables enough to change the effect you see.Casting an
int
tovoid *
in order to return it is messy; you're better off either allocating space for a result in the main thread and passing that to the thread when it starts, or have the thread allocate the result and return a pointer to it when it finishes.However, if you insist on the cast to void method, you can fix it by passing the address of an actual
void *
object topthread_join
and then casting from that toint
: