How To Use VBO in QT with opengl es?

[halftone]

when I was using qt with opengl es, in my render code, I used VBO in the way just like in win32 project, the code is the following:

@
void DrawMesh()

{
#if 1
glDisableClientState(GL_NORMAL_ARRAY);
glDisableClientState(GL_TEXTURE_COORD_ARRAY);
GLuint buf1,buf2;
glGenBuffers(1,&buf1);
glBindBuffer(GL_ARRAY_BUFFER,buf1);
GLfloat a[]={
0.0f, 0.0f, 0.0f, 0.0f,
1.0f, 0.0f, 0.0f, 1.0f,
0.0f, 1.0f, 0.0f, 2.0f
};

GLushort index[] = {
0,1,2
};
glBufferData(GL_ARRAY_BUFFER,12sizeof(GLfloat),a,GL_STATIC_DRAW);
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(3,GL_FLOAT,4sizeof(GL_FLOAT),0);
glGenBuffers(1,&buf2);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,buf2);
glBufferData(GL_ELEMENT_ARRAY_BUFFER,3*sizeof(GLushort),index,GL_STATIC_DRAW);
glDrawElements(GL_TRIANGLES,3,GL_UNSIGNED_SHORT,0);
glDisableClientState(GL_VERTEX_ARRAY);
#endif
}
@

when run to “glDrawElements(GL_TRIANGLES,3,GL_UNSIGNED_SHORT,0);” error occured “Unhandled exception at 0×0239faec in hellogl_es2.exe: 0xC0000005: Access violation reading location 0×00000000. ”
I want to know why this happened?
here if I remove VBO parts, no error occured, how to use VBO in QT?