Access/dereference glm::mat4 contents

gistfile1.txt

#include <glm/gtc/type_ptr.hpp>
..
glm::mat4 m = glm::mat4(1.0f);
..
auto p = glm::value_ptr(m);
std::cout << sizeof(p) << std::endl;
glUniformMatrix4fv(shader_uniform_m, 1, GL_FALSE, p);