代码如下，为什么这样，不能实现两个视频同时播放，把在our_thread1和our_thread2中的gdk_threads_enter();gdk_threads_leave();两个去掉，两个视频就可以同时播放，这是为什么？

#include  
static GtkWidget *fixed; 
static GtkWidget *button1; 
static GtkWidget *button2; 
void our_thread1(char* t1) 
{ 
gdk_threads_enter();
system(t1);
gdk_threads_leave();
} 
void our_thread2(char* t2) 
{ 
gdk_threads_enter();
system(t2);
gdk_threads_leave();
}
void on_begin() 
{ 
g_thread_create(our_thread1,"mplayer -geometry 70:80 1.avi",FALSE,NULL); 
g_thread_create(our_thread2,"mplayer -geometry 70:80 2.avi",FALSE,NULL); 
} 
int main(int argc,char* argv[]) 
{ 
GtkWidget *window,*view; 

if (!g_thread_supported()) 
 g_thread_init(NULL); 
gdk_threads_init();
gtk_init(&argc,&argv); 
window=gtk_window_new(GTK_WINDOW_TOPLEVEL); 
gtk_window_set_title(GTK_WINDOW(window),"thread apllication"); 
g_signal_connect(G_OBJECT(window),"delete_event", 
 G_CALLBACK(gtk_main_quit),NULL); 
gtk_container_set_border_width(GTK_CONTAINER(window),10); 

button=gtk_button_new_with_label("Start"); 
gtk_box_pack_start(GTK_BOX(vbox),button,FALSE,FALSE,5); 
g_signal_connect(G_OBJECT(button),"clicked", 
G_CALLBACK(on_begin),NULL); 
gtk_widget_show_all(window); 
gdk_threads_enter(); 
gtk_main(); 
gdk_threads_leave(); 
return FALSE; 
}

gdk_threads_enter就是互斥锁，一个占有了，另一个就得等待，当然只能有一个播放了。