Xlib seems to require a delay after GC creation.

xlib-delay-required.c

/* compile line: gcc -o test xlib-delay-required.c -lX11 */

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#include <X11/Xlib.h>

int
main(int argc, char* argv[])
{
    char * dpy_name;
    int delay_ms;

    Display * dpy;
    int scr;
    Window root;
    unsigned long black, white;
    int x, y, w, h;
    Window win;
    GC gc;
    struct timespec ts;

    dpy_name = getenv( "DISPLAY" );
    if ( ! dpy_name )
    {
        fprintf( stderr, "%s: no DISPLAY set\n", argv[0] );
        return 1;
    }

    delay_ms = ( argc > 1 ) ? atoi( argv[1] ) : 10;

    XInitThreads();

    dpy = XOpenDisplay( dpy_name );
    if ( ! dpy )
    {
        fprintf( stderr, "%s: cannot connect to X server '%s'\n",
                 argv[0], dpy_name );
        return 2;
    }

    scr = DefaultScreen( dpy );

    root = RootWindow( dpy, scr );

    black = BlackPixel( dpy, scr );
    white = WhitePixel( dpy, scr );

    x = 0;
    y = 0;

    h = 300;
    w = 300;

    win = XCreateSimpleWindow( dpy, root,
                               x, y, w, h,
                               0,        /* border */
                               black,    /* foreground */
                               white     /* background */ );

    XMapWindow(dpy, win);

    gc = XCreateGC( dpy, win, 0, 0 );
    if ( ! gc ) {
	fprintf( stderr, "%s: XCreateGC\n", argv[0] );
        return 3;
    }

    XSetForeground( dpy, gc, black );
    XSetBackground( dpy, gc, white );

    XSetLineAttributes( dpy, gc,
                        2, /* line width */
                        LineSolid,
                        CapButt,
                        JoinBevel );

    XSetFillStyle( dpy, gc, FillSolid );

    XSync( dpy, False );

    ts.tv_sec  = 0;
    ts.tv_nsec = delay_ms * 1000 * 1000;
    nanosleep(&ts, 0);

    XDrawPoint( dpy, win, gc, 5,   5);
    XDrawPoint( dpy, win, gc, 5,   h-5);
    XDrawPoint( dpy, win, gc, w-5, 5);
    XDrawPoint( dpy, win, gc, w-5, h-5);

    XDrawLine( dpy, win, gc, 50,   0,  50, 200 );
    XDrawLine( dpy, win, gc,  0, 100, 200, 100 );

    XDrawArc( dpy, win, gc, 50-(30/2), 100-(30/2), 30, 30, 0, 360*64 );

    {
        XPoint points[] = {
            {0, 0},
            {15, 15},
            {0, 15},
            {0, 0}
        };
        int npoints = sizeof(points)/sizeof(XPoint);

        XDrawLines( dpy, win, gc, points, npoints, CoordModeOrigin );
    }

    XDrawRectangle( dpy, win, gc, 120, 150, 50, 60 );

    XFillRectangle( dpy, win, gc, 60, 150, 50, 60 );

    XFlush( dpy );
    XSync( dpy, False );

    sleep( 4 );

    XCloseDisplay( dpy );
}