Defining Structs in GLSL

// Inside fragment/vertex shader
struct TheStruct // user defined structure.
{
  vec3 first;
  vec4 second;
  mat4x3 third;
};
 
uniform TheStruct aUniformOfArrayType; // instance of uniform user-defined parameter.

In application(C++) code, the name " aUniformOfArrayType" does not have a location; calling glGetUniformLocation on it will return -1. However, each of aUniformOfArrayType's fields do have a location. So you can call "glGetUniformLocation(program, " aUniformOfArrayType.first");" it will give a location and you can set valut to that parameter like a vec3 parameter in shader.

Details of uniform objects are explained in this link:http://www.opengl.org/wiki/GLSL_Uniform[^]