void glGetActiveUniform(GLuint program, GLuint index, GLsizei bufSize, GLsizei *length, GLint *size, GLenum *type, GLchar *name)
glGetActiveUniform returns information about an active uniform variable in the program object specified by program. The number of active uniform variables can be obtained by calling glGetProgram with the value GL_ACTIVE_UNIFORMS. A value of 0 for index selects the first active uniform variable. Permissible values for index range from 0 to the number of active uniform variables minus 1.
Shaders may use built-in uniform variables, user-defined uniform variables, or both. Built-in uniform variables have a prefix of "gl_" and reference existing OpenGL state or values derived from such state (e.g., gl_Fog, gl_ModelViewMatrix; see the OpenGL Shading Language specification for a complete list.) User-defined uniform variables have arbitrary names and obtain their values from the application through calls to glUniform. A uniform variable (either built-in or user-defined) is considered active if during the link operation, a determination is made that the uniform variable can be accessed during program execution. Therefore, program should have previously been the target of a call to glLinkProgram, but it is not necessary for it to have been linked successfully.
The size of the character buffer required to store the longest uniform variable name in program can be obtained by calling glGetProgram with the value GL_ACTIVE_UNIFORM_MAX_LENGTH. This value should be used to allocate a buffer of sufficient size to store the returned uniform variable name. The size of this character buffer is passed in bufSize, and a pointer to this character buffer is passed in name.
glGetActiveUniform returns the name of the uniform variable indicated by index, storing it in the character buffer specified by name. The string returned is null terminated. The actual number of characters written into this buffer is returned in length, and this count does not include the null termination character. If the length of the returned string is not required, a value of NULL can be passed in the length argument.
The type argument returns a pointer to the uniform variable's data type. The symbolic constants GL_FLOAT, GL_FLOAT_VEC2, GL_FLOAT_VEC3, GL_FLOAT_VEC4, GL_INT, GL_INT_VEC2, GL_INT_VEC3, GL_INT_VEC4, GL_BOOL, GL_BOOL_VEC2, GL_BOOL_VEC3, GL_BOOL_VEC4, GL_FLOAT_MAT2, GL_FLOAT_MAT3, GL_FLOAT_MAT4, GL_SAMPLER_1D, GL_SAMPLER_2D, GL_SAMPLER_3D, GL_SAMPLER_CUBE, GL_SAMPLER_1D_SHADOW, or GL_SAMPLER_2D_SHADOW may be returned.
If one or more elements of an array are active, the name of the array is returned in name, the type is returned in type, and the size parameter returns the highest array element index used, plus one, as determined by the compiler and/or linker. Only one active uniform variable is reported for a uniform array.
Uniform variables that are declared as structures or arrays of structures are not returned directly by this function. Instead, each of these uniform variables is reduced to its fundamental components containing the "." and "" operators such that each of the names is valid as an argument to glGetUniformLocation. Each of these reduced uniform variables is counted as one active uniform variable and is assigned an index. A valid name cannot be a structure, an array of structures, or a subcomponent of a vector or matrix.
The size of the uniform variable is returned in size. Uniform variables other than arrays have a size of 1. Structures and arrays of structures are reduced as described earlier, such that each of the names returned is a data type in the earlier list. If this reduction results in an array, the size returned is as described for uniform arrays; otherwise, the size returned is 1.
The list of active uniform variables may include both built-in uniform variables (which begin with the prefix "gl_") as well as user-defined uniform variable names.
This function returns as much information as it can about the specified active uniform variable. If no information is available, length is 0, and name is an empty string. This situation could occur if this function is called after a link operation that failed. If an error occurs, the return values length, size, type, and name are unmodified.
glGetActiveUniform 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_VALUE is generated if index is greater than or equal to the number of active uniform variables in program.
GL_INVALID_OPERATION is generated if glGetActiveUniform is executed between the execution of glBegin and the corresponding execution of glEnd.
GL_INVALID_VALUE is generated if bufSize is less than 0.
glGet with argument GL_MAX_VERTEX_UNIFORM_COMPONENTS or GL_MAX_FRAGMENT_UNIFORM_COMPONENTS
glGetProgram with argument GL_ACTIVE_UNIFORMS or GL_ACTIVE_UNIFORM_MAX_LENGTH
glGetUniform, glGetUniformLocation, glLinkProgram, glUniform, glUseProgram