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glGetActiveUniform

Name

glGetActiveUniform — Returns information about an active uniform variable for the specified program object

C Specification

void glGetActiveUniform( GLuint program,
GLuint index,
GLsizei bufSize,
GLsizei *length,
GLint *size,
GLenum *type,
GLchar *name);

Parameters

program

Specifies the program object to be queried.

index

Specifies the index of the uniform variable to be queried.

bufSize

Specifies the maximum number of characters OpenGL is allowed to write in the character buffer indicated by name.

length

Returns the number of characters actually written by OpenGL in the string indicated by name (excluding the null terminator) if a value other than NULL is passed.

size

Returns the size of the uniform variable.

type

Returns the data type of the uniform variable.

name

Returns a null terminated string containing the name of the uniform variable.

Description

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 glGetProgramiv with the value GL_ACTIVE_UNIFORMS. A value of zero for index selects the first active uniform variable. Permissible values for index range from zero to the number of active uniform variables minus one.

Shaders may use either 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_DepthRange, 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 it is determined during the link operation that it may 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 glGetProgramiv 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 will be 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 will return a pointer to the uniform variable's data type. The symbolic constants returned for uniform types are shown in the table below.

Returned Symbolic Contant Shader Uniform Type
GL_FLOAT float
GL_FLOAT_VEC2 vec2
GL_FLOAT_VEC3 vec3
GL_FLOAT_VEC4 vec4
GL_INT int
GL_INT_VEC2 ivec2
GL_INT_VEC3 ivec3
GL_INT_VEC4 ivec4
GL_UNSIGNED_INT unsigned int
GL_UNSIGNED_INT_VEC2 uvec2
GL_UNSIGNED_INT_VEC3 uvec3
GL_UNSIGNED_INT_VEC4 uvec4
GL_BOOL bool
GL_BOOL_VEC2 bvec2
GL_BOOL_VEC3 bvec3
GL_BOOL_VEC4 bvec4
GL_FLOAT_MAT2 mat2
GL_FLOAT_MAT3 mat3
GL_FLOAT_MAT4 mat4
GL_FLOAT_MAT2x3 mat2x3
GL_FLOAT_MAT2x4 mat2x4
GL_FLOAT_MAT3x2 mat3x2
GL_FLOAT_MAT3x4 mat3x4
GL_FLOAT_MAT4x2 mat4x2
GL_FLOAT_MAT4x3 mat4x3
GL_SAMPLER_2D sampler2D
GL_SAMPLER_3D sampler3D
GL_SAMPLER_CUBE samplerCube
GL_SAMPLER_2D_SHADOW sampler2DShadow
GL_SAMPLER_2D_ARRAY sampler2DArray
GL_SAMPLER_2D_ARRAY_SHADOW sampler2DArrayShadow
GL_SAMPLER_CUBE_SHADOW samplerCubeShadow
GL_INT_SAMPLER_2D isampler2D
GL_INT_SAMPLER_3D isampler3D
GL_INT_SAMPLER_CUBE isamplerCube
GL_INT_SAMPLER_2D_ARRAY isampler2DArray
GL_UNSIGNED_INT_SAMPLER_2D usampler2D
GL_UNSIGNED_INT_SAMPLER_3D usampler3D
GL_UNSIGNED_INT_SAMPLER_CUBE usamplerCube
GL_UNSIGNED_INT_SAMPLER_2D_ARRAY usampler2DArray

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 will be reported for a uniform array. If the active uniform is an array, the uniform name returned in name will always be the name of the uniform array appended with "[0]".

Uniform variables that are declared as structures or arrays of structures will not be returned directly by this function. Instead, each of these uniform variables will be 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 will be returned in size. Uniform variables other than arrays will have a size of 1. Structures and arrays of structures will be reduced as described earlier, such that each of the names returned will be a data type in the earlier list. If this reduction results in an array, the size returned will be as described for uniform arrays; otherwise, the size returned will be 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 will return as much information as it can about the specified active uniform variable. If no information is available, length will be 0, and name will be 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 will be unmodified.

Errors

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_VALUE is generated if bufSize is less than 0.

Associated Gets

glGet with argument GL_MAX_VERTEX_UNIFORM_COMPONENTS, GL_MAX_FRAGMENT_UNIFORM_COMPONENTS, GL_MAX_COMBINED_VERTEX_UNIFORM_COMPONENTS, or GL_MAX_COMBINED_FRAGMENT_UNIFORM_COMPONENTS.

glGetProgramiv with argument GL_ACTIVE_UNIFORMS or GL_ACTIVE_UNIFORM_MAX_LENGTH.

glIsProgram

Examples

Retrieve uniform data after the shader has been compiled
// The index returned from these GLint functions gets passed to glEnableVertexAttribArray during rendering.
GLint position_attrib_index = glGetAttribLocation(program, "position"); // program is what is returned by glCreateProgram.
GLint texcoord_attrib_index = glGetAttribLocation(program, "texcoord");
GLint normal_attrib_index = glGetAttribLocation(program, "normal");
GLint color_attrib_index = glGetAttribLocation(program, "color");

glBindFragDataLocation(program, 0, "output_color"); // "output_color" is an output value in your vertex shader and an input value in your fragment shader.

GLint num_uniforms;
glGetProgramiv(program, GL_ACTIVE_UNIFORMS, &num_uniforms);
GLchar uniform_name[256];
GLsizei length;
GLint size;
GLenum type;
for (int i = 0; i < num_uniforms; i++)
{
    glGetActiveUniform(program, i, sizeof(uniform_name), &length, &size, &type, uniform_name);
    // ... save this uniform data so it can be used during rendering
}

Tutorials

API Version Support

OpenGL ES API Version
Function Name 2.0 3.0 3.1
glGetActiveUniform
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