no more thread scheduling when waitpid in one thread

Hello,

Running ngpt 2.0.4 on redhat linux kernel 2.4.19.

A simple program with 2 threads :
- one thread print "="
- one thread, forks and runs waitpid on the child
  the child forked print "."
- the main thread wait for joining with the above 2 threads

Running this program, I have the ouput :
...............
and then
======

Should have mixed . and =

Do you know this problem and how to fix it ?
Thanks.

Below is code used to test.

#ifdef GLOBAL
#include <pthread.h>
#else
#define _PTHREAD_PRIVATE
#include "pthread.h"
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <linux/unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

#define die(str) \
    do { \
        fprintf(stderr, "**die: %s: errno=%d\n", str, errno); \
        exit(1); \
    } while (0)

static void *child_print(void *_arg)
{
    char *name = (char *)_arg;
    int i=0;
    fprintf(stderr, "child_print: startup %s \n", name);      

    for (i = 0; i < 50; i++) {
        if (i % 5 == 0)
            sleep(1);
        fprintf(stderr, "=");
    }
    fprintf(stderr,"\n");

    fprintf(stderr, "child_print: shutdown %s \n", name);
    // _exit(0);    /* the process will exit */
    return _arg;
}

static void *child_fork(void *_arg)
{
    char *name = (char *)_arg;
    int pid;

    fprintf(stderr, "child_fork: startup %s \n", name);
    
    pid = fork();
    if (pid == 0) {
      /* we are in child */
      int i=0;
     
      for (i = 0; i < 50; i++) {
        if (i % 5 == 0)
            sleep(1);
        fprintf(stderr, ".");
      }
      fprintf(stderr,"\n");
    }
    else {
      int res=0;
      int status=0;

      fprintf(stderr,"child_fork: waitpid\n");
      res = waitpid(pid, &status, 0);
      fprintf(stderr, "child_fork: shutdown %s \n", name);
    }
    // _exit(0);    /* the process will exit */
    return _arg;
}

int main(int argc, char *argv[])
{
    pthread_attr_t thread_attr;
    pthread_t thread[4];
    char *rc;

    fprintf(stderr, "main: init\n");

    fprintf(stderr, "main: initializing attribute object\n");
    if (pthread_attr_init(&thread_attr) != 0)
        die("pthread_attr_init");
    if (pthread_attr_setdetachstate(&thread_attr, 
PTHREAD_CREATE_JOINABLE) != 0)
        die("pthread_attr_setdetachstate");
    if (argc > 1) {
    /* create as bound-thread (as 1:1 mode) */
    if (pthread_attr_setscope(&thread_attr, PTHREAD_SCOPE_SYSTEM) != 0)
        die("pthread_attr_setscope");
    }

    fprintf(stderr, "main: create child_print\n");
    if (pthread_create(&thread[0], &thread_attr, child_print, (void 
*)"child_print") != 0)
        die("pthread_create");

    fprintf(stderr, "main: create child_fork\n");
    if (pthread_create(&thread[1], &thread_attr, child_fork, (void 
*)"child_fork") != 0)
        die("pthread_create");
    
     sleep(5);

    fprintf(stderr, "main: joining...\n");
    if (pthread_join(thread[0], (void **)&rc) != 0)
        die("pthread_join");
    fprintf(stderr, "main: joined thread: %s\n", rc);
    if (pthread_join(thread[1], (void **)&rc) != 0)
        die("pthread_join");
    fprintf(stderr, "main: joined thread: %s\n", rc);

    fprintf(stderr, "main: exit\n");
    exit(0);
}