Passing lisp strings in to c functions

gl-shader-setup.lisp

(defvar vertexSource "attribute vec4 position;                      
    varying vec3 color;                           
    void main()                                   
    {                                             
        gl_Position = vec4(position.xyz, 1.0);    
        color = gl_Position.xyz + vec3(0.5);      
    }")

(gl:shader-source vertex-shader vertexSource)

shader-source-impl.cpp

const GLchar* vertexSource =
    "attribute vec4 position;                      \n"
    "varying vec3 color;                           \n"
    "void main()                                   \n"
    "{                                             \n"
    "    gl_Position = vec4(position.xyz, 1.0);    \n"
    "    color = gl_Position.xyz + vec3(0.5);      \n"
    "}                                             \n";

// In c/c++ this call would have been:
//  glShaderSource(vertexShader, 1, &vertexSource, NULL);
// I'm not sure if the ecl_string already has a pointer inside it somewhere with the string 
// allocated, or if that needs to be done still, it doens't seem ready to be coerced, as this
// doesn't work.
// (const char *const *) ecl_foreign_data_pointer_safe(ecl_null_terminated_base_string(source))
// 
cl_object lisp_gl_shader_source(cl_object shader, cl_object source) {
    glShaderSource(ecl_fixnum(shader), 1, ??source??, NULL);
}