POSIX線程的棧管理

#include <stdlib.h>
#include <pthread.h>
// 通過調節三個參數，了解當前計算機的線程棧容量
#define NTHREADS 4
#define N 1000
#define MEGEXTRA 1000000
pthread_attr_t attr;
void *dowork(void *threadid)
{
size_t mystacksize;
pthread_attr_getstacksize (&attr, &mystacksize);
printf("[WORKER THREAD %ld] Stack size = %li bytes \n", (long)threadid, mystacksize);
// 輔助線程的棧絕大部分耗在下面這個數組上了。
// 因為給每個線程多分配了MEGEXTRA字節空間，故而運行沒問題。
// 可以嘗試設置負數的MEGEXTRA，看程序如何報錯。
double A[N][N];
for (int i=0; i<N; i++)
{
for (int j=0; j<N; j++)
{
A[i][j] = ((i*j)/3.452) + (N-i);
}
}
return NULL;
}
int _tmain(int argc, _TCHAR* argv[])
{
size_t stacksize;
long t;
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
pthread_attr_getstacksize (&attr, &stacksize);
printf("[MASTER THREAD] Default stack size = %li\n", stacksize);
stacksize = sizeof(double) * N * N + MEGEXTRA;
pthread_attr_setstacksize (&attr, stacksize);
printf("[MASTER THREAD] Creating threads with stack size = %li bytes\n",stacksize);
pthread_t threads[NTHREADS];
for(t=0; t<NTHREADS; t++)
{
int rc = pthread_create(&threads[t], &attr, dowork, (void *)t);
if (rc)
{
printf("[MASTER THREAD] ERROR; return code from pthread_create() is %d\n", rc);
exit(-1);
}
}
for(t=0; t<NTHREADS; t++)
{
pthread_join(threads[t], NULL);
}
return 0;
}

我在WinXP上試驗時，得到棧的默認大小是零，這是為什麼？