GLint glGetUniformLocation(GLuint program, const GLchar *name)
glGetUniformLocation returns an integer that represents the location of a specific uniform variable within a program object. name must be a null terminated string that contains no white space. name must be an active uniform variable name in program that is not a structure, an array of structures, or a subcomponent of a vector or a matrix. This function returns -1 if name does not correspond to an active uniform variable in program or if name starts with the reserved prefix "gl_".
Uniform variables that are structures or arrays of structures may be queried with glGetUniformLocation for each field within the structure. The array element operator "" and the structure field operator "." may be used in name to select elements within an array or fields within a structure. The result of using these operators is not allowed to be another structure, an array of structures, or a subcomponent of a vector or a matrix. Except if the last part of name indicates a uniform variable array, the location of the first element of an array can be retrieved with the name of the array or with the name appended by "".
The actual locations assigned to uniform variables are not known until the program object is linked successfully. After linking has occurred, the command glGetUniformLocation obtains the location of a uniform variable. This location value can then be passed to glUniform to set the value of the uniform variable or to glGetUniform to query the current value of the uniform variable. After a program object has been linked successfully, the index values for uniform variables remain fixed until the next link command occurs. Uniform variable locations and values can only be queried after a link if the link was successful.
glGetUniformLocation is available only if the GL version is 2.0 or greater.
GL_INVALID_VALUE is generated if program is not a value generated by OpenGL.
GL_INVALID_OPERATION is generated if program is not a program object.
GL_INVALID_OPERATION is generated if program has not been successfully linked.
GL_INVALID_OPERATION is generated if glGetUniformLocation is executed between the execution of glBegin and the corresponding execution of glEnd.
glGetActiveUniform with arguments program and the index of an active uniform variable
glGetProgram with arguments program and GL_ACTIVE_UNIFORMS or GL_ACTIVE_UNIFORM_MAX_LENGTH
glGetUniform with arguments program and the name of a uniform variable