Xopendisplay segmentation fault - c

Question

I'm carrying out a simple X11 program in C. But I got segmentation fault(11) because of XOpenDisplay(NULL).
#include <X11/Xlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void)
{
Display *display;
Window window;
XEvent event;
char *msg = "Hello, World!";
int s;
/* /2/ some basic X11 setup */
/* open connection with the server */
display = XOpenDisplay(NULL);
if (display == NULL)
{
fprintf(stderr, "Cannot open display\n");
exit(1);
}
s = DefaultScreen(display);
/* create window */
window = XCreateSimpleWindow(display, RootWindow(display, s), 10, 10, 200, 200, 1,
BlackPixel(display, s), WhitePixel(display, s));
/* select kind of events we are interested in */
XSelectInput(display, window, ExposureMask | KeyPressMask);
/* map (show) the window */
XMapWindow(display, window);
/* /3/ event loop */
for (;;)
{
XNextEvent(display, &event);
/* /4/ draw or redraw the window */
if (event.type == Expose)
{
XFillRectangle(display, window, DefaultGC(display, s), 20, 20, 10, 10);
XDrawString(display, window, DefaultGC(display, s), 50, 50, msg, strlen(msg));
}
/* /5/ exit on key press */
if (event.type == KeyPress)
break;
}
/* /6/ close connection to server */
XCloseDisplay(display);
return 0;
}
command line:
gcc -o out test.c -I/usr/X11R6/include/ -L/usr/X11R6/lib/ -lX11
I executed it failed in Mac terminal, but it worked in Xcode. I don't know the reasons.