Name

    ARB_program_interface_query

Name Strings

    GL_ARB_program_interface_query

Contact

    Pat Brown, NVIDIA (pbrown 'at' nvidia.com)

Contributors

    Members of the Khronos OpenGL ARB TSG

Notice

    Copyright (c) 2012-2013 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

Status

    Complete.
    Approved by the ARB on 2012/06/12.

Version

    Last Modified Date: August 19, 2013
    Revision:           20

Number

    ARB Extension #134

Dependencies

    OpenGL 2.0 is required.

    This extension is written against the OpenGL 4.2 Compatibility Profile
    Specification (January 19, 2012).

    OpenGL 3.0, ARB_transform_feedback, and EXT_transform_feedback affect the
    definition of this extension.

    OpenGL 3.1 and ARB_uniform_buffer_object affect the definition of this
    extension.

    OpenGL 4.0 and ARB_shader_subroutine affect the definition of this
    extension.

    OpenGL 4.2 and ARB_shader_atomic_counters affect the definition of this
    extension.

    OpenGL 4.3 and ARB_shader_storage_buffer_object affect the definition of
    this extension.

    OpenGL 4.3 and ARB_arrays_of_arrays affect the definition of this
    extension.

    OpenGL 4.3 and ARB_compute_shader affect the definition of this extension.

    OpenGL 4.3 and ARB_explicit_uniform_location affect the definition of this
    extension.

Overview

    This extension provides a single unified set of query commands that can be
    used by applications to determine properties of various interfaces and
    resources used by program objects to communicate with application code,
    fixed-function OpenGL pipeline stages, and other programs.  In unextended
    OpenGL 4.2, there is a separate set of query commands for each different
    type of interface or resource used by the program.  These different sets
    of queries are structured nearly identically, but the queries for some
    interfaces have limited capability (e.g., there is no ability to enumerate
    fragment shader outputs).

    With the single set of query commands provided by this extension, a
    consistent set of queries is available for all interfaces, and a new
    interface can be added without having to introduce a completely new set of
    query commands.  These queries are intended to provide a superset of the
    capabilities provided by similar queries in OpenGL 4.2, and should allow
    for the deprecation of the existing queries.

    This extension defines two terms:  interfaces and active resources.  Each
    interface of a program object provides a way for the program to
    communicate with application code, fixed-function OpenGL pipeline stages,
    and other programs.  Examples of interfaces for a program object include
    inputs (receiving values from vertex attributes or outputs of other
    programs), outputs (sending values to other programs or per-fragment
    operations), uniforms (receiving values from API calls), uniform blocks
    (receiving values from bound buffer objects), subroutines and subroutine
    uniforms (receiving API calls to indicate functions to call during program
    execution), and atomic counter buffers (holding values to be manipulated
    by atomic counter shader functions).  Each interface of a program has a
    set of active resources used by the program.  For example, the resources
    of a program's input interface includes all active input variables used by
    the first stage of the program.  The resources of a program's uniform
    block interface consists of the set of uniform blocks with at least one
    member used by any shader in the program.

IP Status

    No known IP claims.

New Procedures and Functions

      void GetProgramInterfaceiv(uint program, enum programInterface,
                                 enum pname, int *params);
      uint GetProgramResourceIndex(uint program, enum programInterface,
                                   const char *name);
      void GetProgramResourceName(uint program, enum programInterface,
                                  uint index, sizei bufSize, sizei *length,
                                  char *name);
      void GetProgramResourceiv(uint program, enum programInterface,
                                uint index, sizei propCount,
                                const enum *props, sizei bufSize,
                                sizei *length, int *params);
      int GetProgramResourceLocation(uint program, enum programInterface,
                                     const char *name);
      int GetProgramResourceLocationIndex(uint program, enum programInterface,
                                          const char *name);

New Tokens

    Accepted by the <programInterface> parameter of GetProgramInterfaceiv,
    GetProgramResourceIndex, GetProgramResourceName, GetProgramResourceiv,
    GetProgramResourceLocation, and GetProgramResourceLocationIndex:

        UNIFORM                                         0x92E1
        UNIFORM_BLOCK                                   0x92E2
        PROGRAM_INPUT                                   0x92E3
        PROGRAM_OUTPUT                                  0x92E4
        BUFFER_VARIABLE                                 0x92E5
        SHADER_STORAGE_BLOCK                            0x92E6
        ATOMIC_COUNTER_BUFFER                           (already in 4.2)
        VERTEX_SUBROUTINE                               0x92E8
        TESS_CONTROL_SUBROUTINE                         0x92E9
        TESS_EVALUATION_SUBROUTINE                      0x92EA
        GEOMETRY_SUBROUTINE                             0x92EB
        FRAGMENT_SUBROUTINE                             0x92EC
        COMPUTE_SUBROUTINE                              0x92ED
        VERTEX_SUBROUTINE_UNIFORM                       0x92EE
        TESS_CONTROL_SUBROUTINE_UNIFORM                 0x92EF
        TESS_EVALUATION_SUBROUTINE_UNIFORM              0x92F0
        GEOMETRY_SUBROUTINE_UNIFORM                     0x92F1
        FRAGMENT_SUBROUTINE_UNIFORM                     0x92F2
        COMPUTE_SUBROUTINE_UNIFORM                      0x92F3
        TRANSFORM_FEEDBACK_VARYING                      0x92F4

    Accepted by the <pname> parameter of GetProgramInterfaceiv:

        ACTIVE_RESOURCES                                0x92F5
        MAX_NAME_LENGTH                                 0x92F6
        MAX_NUM_ACTIVE_VARIABLES                        0x92F7
        MAX_NUM_COMPATIBLE_SUBROUTINES                  0x92F8

    Accepted in the <props> array of GetProgramResourceiv:

        NAME_LENGTH                                     0x92F9
        TYPE                                            0x92FA
        ARRAY_SIZE                                      0x92FB
        OFFSET                                          0x92FC
        BLOCK_INDEX                                     0x92FD
        ARRAY_STRIDE                                    0x92FE
        MATRIX_STRIDE                                   0x92FF
        IS_ROW_MAJOR                                    0x9300
        ATOMIC_COUNTER_BUFFER_INDEX                     0x9301
        BUFFER_BINDING                                  0x9302
        BUFFER_DATA_SIZE                                0x9303
        NUM_ACTIVE_VARIABLES                            0x9304
        ACTIVE_VARIABLES                                0x9305
        REFERENCED_BY_VERTEX_SHADER                     0x9306
        REFERENCED_BY_TESS_CONTROL_SHADER               0x9307
        REFERENCED_BY_TESS_EVALUATION_SHADER            0x9308
        REFERENCED_BY_GEOMETRY_SHADER                   0x9309
        REFERENCED_BY_FRAGMENT_SHADER                   0x930A
        REFERENCED_BY_COMPUTE_SHADER                    0x930B
        TOP_LEVEL_ARRAY_SIZE                            0x930C
        TOP_LEVEL_ARRAY_STRIDE                          0x930D
        LOCATION                                        0x930E
        LOCATION_INDEX                                  0x930F
        IS_PER_PATCH                                    0x92E7

        NUM_COMPATIBLE_SUBROUTINES                      (already in 4.2)
        COMPATIBLE_SUBROUTINES                          (already in 4.2)


Additions to Chapter 2 of the OpenGL 4.2 (Compatibility Profile) Specification
(OpenGL Operation)

    Modify Section 2.14.3, Program Objects (p. 97)

    (insert at the end of the section, p. 101)

    Program Interfaces

    When a program object is made part of the current rendering state, its
    executable code may communicate with other GL pipeline stages or
    application code through a variety of /interfaces/.  When a program is
    linked, the GL builds a list of /active resources/ for each interface.
    Examples of active resources include variables, interface blocks, and
    subroutines used by shader code.  Resources referenced in shader code are
    considered /active/ unless the compiler and linker can conclusively
    determine that they have no observable effect on the results produced by
    the executable code of the program.  For example, variables might be
    considered inactive if they are declared but not used in executable code,
    used only in a clause of an "if" statement that would never be executed,
    used only in functions that are never called, or used only in computations
    of temporary variables having no effect on any shader output.  In cases
    where the compiler or linker cannot make a conclusive determination, any
    resource referenced by shader code will be considered active.  The set of
    active resources on for any interface is implementation-dependent because
    it depends on various analysis and optimizations performed by the compiler
    and linker.

    If a program is linked successfully, the GL will generate lists of active
    resources based on the executable code produced by the link.  If a program
    is linked unsuccessfully, the link may have failed for a number of
    reasons, including cases where the program required more resources than
    supported by the implementation.  Implementations are permitted, but not
    required, to record lists of resources that would have been considered
    active had the program linked successfully.  If an implementation does not
    record information for any given interface, the corresponding list of
    active resources is considered empty.  If a program has never been linked,
    all lists of active resources are considered empty.

    The GL provides a number of commands to query properties of the interfaces
    of a program object.  Each such command accepts a <programInterface>
    token, identifying a specific interface.  The supported values for
    <programInterface> are as follows:

      * UNIFORM corresponds to the set of active uniform variables (section
        2.14.7) used by <program>.

      * UNIFORM_BLOCK corresponds to the set of active uniform blocks (section
        2.14.7) used by <program>.

      * ATOMIC_COUNTER_BUFFER corresponds to the set of active atomic counter
        buffer binding points (section 2.14.7) used by <program>.

      * PROGRAM_INPUT corresponds to the set of active input variables used by
        the first shader stage of <program>.  If <program> includes multiple
        shader stages, input variables from any shader stage other than the
        first will not be enumerated.

      * PROGRAM_OUTPUT corresponds to the set of active output variables
        (section 2.14.11) used by the last shader stage of <program>.  If
        <program> includes multiple shader stages, output variables from any
        shader stage other than the last will not be enumerated.

      * VERTEX_SUBROUTINE, TESS_CONTROL_SUBROUTINE,
        TESS_EVALUATION_SUBROUTINE, GEOMETRY_SUBROUTINE, FRAGMENT_SUBROUTINE,
        and COMPUTE_SUBROUTINE correspond to the set of active subroutines for
        the vertex, tessellation control, tessellation evaluation, geometry,
        fragment, and compute shader stages of <program>, respectively
        (section 2.14.8).

      * VERTEX_SUBROUTINE_UNIFORM, TESS_CONTROL_SUBROUTINE_UNIFORM,
        TESS_EVALUATION_SUBROUTINE_UNIFORM, GEOMETRY_SUBROUTINE_UNIFORM,
        FRAGMENT_SUBROUTINE_UNIFORM, and COMPUTE_SUBROUTINE_UNIFORM correspond
        to the set of active subroutine uniform variables used by the vertex,
        tessellation control, tessellation evaluation, geometry, fragment, and
        compute shader stages of <program>, respectively (section 2.14.8).

      * TRANSFORM_FEEDBACK_VARYING corresponds to the set of output variables
        in the last non-fragment stage of <program> that would be captured
        when transform feedback is active (section 2.20.2).

      * BUFFER_VARIABLE corresponds to the set of active buffer variables (see
        the ARB_shader_storage_buffer_object extension) used by <program>.

      * SHADER_STORAGE_BLOCK corresponds to the set of active shader storage
        blocks (see the ARB_shader_storage_buffer_object extension) used by
        <program>.

    When building a list of active variable or interface blocks, resources
    with aggregate types (such as arrays or structures) may produce multiple
    entries in the active resource list for the corresponding interface.
    Additionally, each active variable, interface block, or subroutine in the
    list is assigned an associated name string that can be used by
    applications to refer to the resources.  For interfaces involving
    variables, interface blocks, or subroutines, the entries of active
    resource lists are generated as follows:

      * For an active variable declared as a single instance of a basic type,
        a single entry will be generated, using the variable name from the
        shader source.

      * For an active variable declared as an array of basic types, a single
        entry will be generated, with its name string formed by concatenating
        the name of the array and the string "[0]".

      * For an active variable declared as a structure, a separate entry will
        be generated for each active structure member.  The name of each entry
        is formed by concatenating the name of the structure, the "."
        character, and the name of the structure member.  If a structure
        member to enumerate is itself a structure or array, these enumeration
        rules are applied recursively.

      * For an active variable declared as an array of an aggregate data type
        (structures or arrays), a separate entry will be generated for each
        active array element, unless noted immediately below.  The name of
        each entry is formed by concatenating the name of the array, the "["
        character, an integer identifying the element number, and the "]"
        character.  These enumeration rules are applied recursively, treating
        each enumerated array element as a separate active variable.

      * For an active shader storage block member declared as an array, an
        entry will be generated only for the first array element, regardless
        of its type.  For arrays of aggregate types, the enumeration rules are
        applied recursively for the single enumerated array element.

      * For an active interface block not declared as an array of block
        instances, a single entry will be generated, using the block name from
        the shader source.

      * For an active interface block declared as an array of instances,
        separate entries will be generated for each active instance.  The name
        of the instance is formed by concatenating the block name, the "["
        character, an integer identifying the instance number, and the "]"
        character.

      * For an active subroutine, a single entry will be generated, using the
        subroutine name from the shader source.

    When an integer array element or block instance number is part of the name
    string, it will be specified in decimal form without a "+" or "-" sign or
    any extra leading zeroes.  Additionally, the name string will not include
    white space anywhere in the string.

    The order of the active resource list is implementation-dependent for all
    interfaces except for TRANSFORM_FEEDBACK_VARYING.  For
    TRANSFORM_FEEDBACK_VARYING, the active resource list will use the variable
    order specified in the the most recent call to TransformFeedbackVaryings
    before the last call to LinkProgram.

    For the ATOMIC_COUNTER_BUFFER interface, the list of active buffer binding
    points is built by identifying each unique binding point associated with
    one or more active atomic counter uniform variables.  Active atomic
    counter buffers do not have an associated name string.

    For the UNIFORM, PROGRAM_INPUT, PROGRAM_OUTPUT, and
    TRANSFORM_FEEDBACK_VARYING interfaces, the active resource list will
    include all active variables for the interface, including any active
    built-in variables.

    For PROGRAM_INPUT and PROGRAM_OUTPUT interfaces for shaders that recieve
    or produce patch primitves, the active resource list will include both
    per-vertex and per-patch inputs and outputs.

    For the TRANSFORM_FEEDBACK_VARYING interface, the active resource list
    will entries for the special varying names gl_NextBuffer,
    gl_SkipComponents1, gl_SkipComponents2, gl_SkipComponents3, and
    gl_SkipComponents4 (section 2.14.11).  These variables are used to control
    how varying values are written to transform feedback buffers.  When
    enumerating the properties of such resources, these variables are
    considered to have a TYPE of NONE and an ARRAY_SIZE of 0 (gl_NextBuffer),
    1, 2, 3, and 4, respectively.

    When a program is linked successfully, active variables in the UNIFORM,
    PROGRAM_INPUT, PROGRAM_OUTPUT interface, or in any of the subroutine
    uniform interfaces, are assigned one or more signed integer /locations/.
    These locations can be used by commands to assign values to uniforms and
    subroutine uniforms, to identify generic vertex attributes associated with
    vertex shader inputs, or to identify fragment color output numbers and
    indices associated with fragment shader outputs.  For such variables
    declared as arrays, separate locations will be assigned to each active
    array element.  Not all active variables are assigned valid locations; the
    following variables will have an effective location of -1:

      * uniforms declared as atomic counters;

      * members of a uniform block;

      * built-in inputs, outputs, and uniforms (starting with "gl_"); and

      * inputs or outputs not declared with a "location" layout qualifier,
        except for vertex shader inputs and fragment shader outputs.

    If a program has not been linked or has was last linked unsuccessfully, no
    locations will be assigned.

    The command

      void GetProgramInterfaceiv(uint program, enum programInterface,
                                 enum pname, int *params);

    queries a property of the interface <programInterface> in program
    <program>, returning its value in <params>.  The property to return is
    specified by <pname>.

    If <pname> is ACTIVE_RESOURCES, the value returned is the number of
    resources in the active resource list for <programInterface>.  If the list
    of active resources for <programInterface> is empty, zero is returned.

    If <pname> is MAX_NAME_LENGTH, the value returned is the length of the
    longest active name string for an active resource in <programInterface>.
    This length includes an extra character for the null terminator.  If the
    list of active resources for <programInterface> is empty, zero is
    returned.  The error INVALID_OPERATION is generated if <programInterface>
    is ATOMIC_COUNTER_BUFFER, since active atomic counter buffer resources are
    not assigned name strings.

    If <pname> is MAX_NUM_ACTIVE_VARIABLES, the value returned is the number
    of active variables belonging to the interface block or atomic counter
    buffer resource in <programInterface> with the most active variables.  If
    the list of active resources for <programInterface> is empty, zero is
    returned.  The error INVALID_OPERATION is generated if <programInterface>
    is not UNIFORM_BLOCK, ATOMIC_COUNTER_BUFFER, or SHADER_STORAGE_BLOCK.

    If <pname> is MAX_NUM_COMPATIBLE_SUBROUTINES, the value returned is the
    number of compatible subroutines belonging to the active subroutine
    uniform in <programInterface> with the most compatible subroutines.  If
    the list of active resources for <programInterface> is empty, zero is
    returned.  The error INVALID_OPERATION is generated unless
    <programInterface> is VERTEX_SUBROUTINE_UNIFORM,
    TESS_CONTROL_SUBROUTINE_UNIFORM, TESS_EVALUATION_SUBROUTINE_UNIFORM,
    GEOMETRY_SUBROUTINE_UNIFORM, FRAGMENT_SUBROUTINE_UNIFORM, or
    COMPUTE_SUBROUTINE_UNIFORM.

    Each entry in the active resource list for an interface is assigned a
    unique unsigned integer index in the range 0..<N>-1, where <N> is the
    number of entries in the active resource list.  The command

      uint GetProgramResourceIndex(uint program, enum programInterface,
                                   const char *name);

    returns the unsigned integer index assigned to a resource named <name> in
    the interface type <programInterface> of program object <program>.  The
    error INVALID_ENUM is generated if <programInterface> is
    ATOMIC_COUNTER_BUFFER, since active atomic counter buffer resources are
    not assigned name strings.

    If <name> exactly matches the name string of one of the active resources
    for <programInterface>, the index of the matched resource is returned.
    Additionally, if <name> would exactly match the name string of an active
    resource if "[0]" were appended to <name>, the index of the matched
    resource is returned.  Otherwise, <name> is considered not to be the name
    of an active resource, and INVALID_INDEX is returned.  Note that if an
    interface enumerates a single active resource list entry for an array
    variable (e.g., "a[0]"), a <name> identifying any array element other than
    the first (e.g., "a[1]") is not considered to match.

    For the interface TRANSFORM_FEEDBACK_VARYING, the value INVALID_INDEX
    should be returned when querying the index assigned to the special names
    "gl_NextBuffer", "gl_SkipComponents1", "gl_SkipComponents2",
    "gl_SkipComponents3", and "gl_SkipComponents4".

    The command

      void GetProgramResourceName(uint program, enum programInterface,
                                  uint index, sizei bufSize, sizei *length,
                                  char *name);

    returns the name string assigned to the single active resource with an
    index of <index> in the interface <programInterface> of program object
    <program>.  The error INVALID_VALUE is generated if <index> is greater
    than or equal to the number of entries in the active resource list for
    <programInterface>.  The error INVALID_ENUM is generated if
    <programInterface> is ATOMIC_COUNTER_BUFFER, since active atomic counter
    buffer resources are not assigned name strings.

    The name string assigned to the active resource identified by <index> is
    returned as a null-terminated string in <name>.  The actual number of
    characters written into <name>, excluding the null terminator, is returned
    in <length>.  If <length> is NULL, no length is returned.  The maximum
    number of characters that may be written into <name>, including the null
    terminator, is specified by <bufSize>.  If the length of the name string
    (including the null terminator) is greater than <bufSize>, the first
    <bufSize>-1 characters of the name string will be written to <name>,
    followed by a null terminator.  If <bufSize> is zero, no error will be
    generated but no characters will be written to <name>.  The length of the
    longest name string for <programInterface>, including a null terminator,
    can be queried by calling GetProgramInterfaceiv with a <pname> of
    MAX_NAME_LENGTH.

    The command

      void GetProgramResourceiv(uint program, enum programInterface,
                                uint index, sizei propCount,
                                const enum *props, sizei bufSize,
                                sizei *length, int *params);

    returns values for multiple properties of a single active resource with an
    index of <index> in the interface <programInterface> of program object
    <program>.  For each resource, values for <propCount> properties specified
    by the array <props> are returned.  The error INVALID_VALUE is generated
    if <propCount> is zero.  The error INVALID_ENUM is generated if any value
    in <props> is not one of the properties described immediately below.  The
    error INVALID_OPERATION is generated if any value in <props> is not
    allowed for <programInterface>.  The set of allowed <programInterface>
    values for each property can be found in Table X.1.

    The values associated with the properties of the active resource are
    written to consecutive entries in <params>, in increasing order according
    to position in <props>.  If no error is generated, only the first
    <bufSize> integer values will be written to <params>; any extra values
    will not be returned.  If <length> is not NULL, the actual number of
    integer values written to <params> will be written to <length>.

      Property                     Supported Interfaces
      ---------------------------  ----------------------------------------
      NAME_LENGTH                  all but ATOMIC_COUNTER_BUFFER

      TYPE                         UNIFORM, PROGRAM_INPUT, PROGRAM_OUTPUT,
                                     TRANSFORM_FEEDBACK_VARYING,
                                     BUFFER_VARIABLE

      ARRAY_SIZE                   UNIFORM, BUFFER_VARIABLE, PROGRAM_INPUT,
                                     PROGRAM_OUTPUT, VERTEX_SUBROUTINE_
                                     UNIFORM, TESS_CONTROL_SUBROUTINE_UNIFORM,
                                     TESS_EVALUATION_SUBROUTINE_UNIFORM,
                                     GEOMETRY_SUBROUTINE_UNIFORM, FRAGMENT_
                                     SUBROUTINE_UNIFORM, COMPUTE_SUBROUTINE_
                                     UNIFORM, TRANSFORM_FEEDBACK_VARYING

      OFFSET                       UNIFORM, BUFFER_VARIABLE
      BLOCK_INDEX                  UNIFORM, BUFFER_VARIABLE
      ARRAY_STRIDE                 UNIFORM, BUFFER_VARIABLE
      MATRIX_STRIDE                UNIFORM, BUFFER_VARIABLE
      IS_ROW_MAJOR                 UNIFORM, BUFFER_VARIABLE

      ATOMIC_COUNTER_BUFFER_INDEX  UNIFORM

      BUFFER_BINDING               UNIFORM_BLOCK, ATOMIC_COUNTER_BUFFER,
                                     SHADER_STORAGE_BLOCK
      BUFFER_DATA_SIZE             UNIFORM_BLOCK, ATOMIC_COUNTER_BUFFER
                                     SHADER_STORAGE_BLOCK
      NUM_ACTIVE_VARIABLES         UNIFORM_BLOCK, ATOMIC_COUNTER_BUFFER
                                     SHADER_STORAGE_BLOCK
      ACTIVE_VARIABLES             UNIFORM_BLOCK, ATOMIC_COUNTER_BUFFER
                                     SHADER_STORAGE_BLOCK

      REFERENCED_BY_VERTEX_        UNIFORM, UNIFORM_BLOCK, ATOMIC_COUNTER_
        SHADER                       BUFFER, SHADER_STORAGE_BLOCK,
                                     BUFFER_VARIABLE, PROGRAM_INPUT,
                                     PROGRAM_OUTPUT

      REFERENCED_BY_TESS_          UNIFORM, UNIFORM_BLOCK, ATOMIC_COUNTER_
        CONTROL_SHADER               BUFFER, SHADER_STORAGE_BLOCK,
                                     BUFFER_VARIABLE, PROGRAM_INPUT,
                                     PROGRAM_OUTPUT

      REFERENCED_BY_TESS_          UNIFORM, UNIFORM_BLOCK, ATOMIC_COUNTER_
        EVALUATION_SHADER            BUFFER, SHADER_STORAGE_BLOCK,
                                     BUFFER_VARIABLE, PROGRAM_INPUT,
                                     PROGRAM_OUTPUT

      REFERENCED_BY_GEOMETRY_      UNIFORM, UNIFORM_BLOCK, ATOMIC_COUNTER_
        SHADER                       BUFFER, SHADER_STORAGE_BLOCK,
                                     BUFFER_VARIABLE, PROGRAM_INPUT,
                                     PROGRAM_OUTPUT

      REFERENCED_BY_FRAGMENT_      UNIFORM, UNIFORM_BLOCK, ATOMIC_COUNTER_
        SHADER                       BUFFER, SHADER_STORAGE_BLOCK,
                                     BUFFER_VARIABLE, PROGRAM_INPUT,
                                     PROGRAM_OUTPUT

      REFERENCED_BY_COMPUTE_       UNIFORM, UNIFORM_BLOCK, ATOMIC_COUNTER_
        SHADER                       BUFFER, SHADER_STORAGE_BLOCK,
                                     BUFFER_VARIABLE, PROGRAM_INPUT,
                                     PROGRAM_OUTPUT

      NUM_COMPATIBLE_SUBROUTINES   VERTEX_SUBROUTINE_UNIFORM, TESS_CONTROL_
                                     SUBROUTINE_UNIFORM, TESS_EVALUATION_
                                     SUBROUTINE_UNIFORM, GEOMETRY_SUBROUTINE_
                                     UNIFORM, FRAGMENT_SUBROUTINE_UNIFORM,
                                     COMPUTE_SUBROUTINE_UNIFORM

      COMPATIBLE_SUBROUTINES       VERTEX_SUBROUTINE_UNIFORM, TESS_CONTROL_
                                     SUBROUTINE_UNIFORM, TESS_EVALUATION_
                                     SUBROUTINE_UNIFORM, GEOMETRY_SUBROUTINE_
                                     UNIFORM, FRAGMENT_SUBROUTINE_UNIFORM,
                                     COMPUTE_SUBROUTINE_UNIFORM

      TOP_LEVEL_ARRAY_SIZE         BUFFER_VARIABLE

      TOP_LEVEL_ARRAY_STRIDE       BUFFER_VARIABLE

      LOCATION                     UNIFORM, PROGRAM_INPUT, PROGRAM_OUTPUT,
                                     VERTEX_SUBROUTINE_UNIFORM, TESS_CONTROL_
                                     SUBROUTINE_UNIFORM, TESS_EVALUATION_
                                     SUBROUTINE_UNIFORM, GEOMETRY_SUBROUTINE_
                                     UNIFORM, FRAGMENT_SUBROUTINE_UNIFORM,
                                     COMPUTE_SUBROUTINE_UNIFORM

      LOCATION_INDEX               PROGRAM_OUTPUT

      IS_PER_PATCH                 PROGRAM_INPUT, PROGRAM_OUTPUT

      Table X.1, GetProgramResourceiv properties and supported interfaces

    For the property NAME_LENGTH, a single integer identifying the length of
    the name string associated with an active variable, interface block, or
    subroutine is written to <params>.  The name length includes a terminating
    null character.

    For the property TYPE, a single integer identifying the type of an active
    variable is written to <params>.  The integer returned is one of the
    values found in table 2.16.

    For the property ARRAY_SIZE, a single integer identifying the number of
    active array elements of an active variable is written to <params>. The
    array size returned is in units of the type associated with the property
    TYPE. For active variables not corresponding to an array of basic types,
    the value one is written to <params>. If the variable is a shader
    storage block member in an array with no declared size, the value zero
    is written to <params>.

    For the property BLOCK_INDEX, a single integer identifying the index of
    the active interface block containing an active variable is written to
    <params>.  If the variable is not the member of an interface block, the
    value -1 is written to <params>.

    For the property OFFSET, a single integer identifying the offset of an
    active variable is written to <params>.  For active variables backed by a
    buffer object, the value written is the offset, in basic machine units,
    relative to the base of buffer range holding the values of the variable.
    For active variables not backed by a buffer object, an offset of -1 is
    written to <params>.

    For the property ARRAY_STRIDE, a single integer identifying the stride
    between array elements in an active variable is written to <params>.  For
    active variables declared as an array of basic types, the value written is
    the difference, in basic machine units, between the offsets of consecutive
    elements in an array.  For active variables not declared as an array of
    basic types, zero is written to <params>.  For active variables not backed
    by a buffer object, -1 is written to <params>, regardless of the variable
    type.

    For the property MATRIX_STRIDE, a single integer identifying the stride
    between columns of a column-major matrix or rows of a row-major matrix is
    written to <params>.  For active variables declared a single matrix or
    array of matrices, the value written is the difference, in basic machine
    units, between the offsets of consecutive columns or rows in each matrix.
    For active variables not declared as a matrix or array of matrices, zero
    is written to <params>.  For active variables not backed by a buffer
    object, -1 is written to <params>, regardless of the variable type.

    For the property IS_ROW_MAJOR, a single integer identifying whether an
    active variable is a row-major matrix is written to <params>.  For active
    variables backed by a buffer object, declared as a single matrix or array
    of matrices, and stored in row-major order, one is written to <params>.
    For all other active variables, zero is written to <params>.

    For the property ATOMIC_COUNTER_BUFFER_INDEX, a single integer identifying
    the index of the active atomic counter buffer containing an active
    variable is written to <params>.  If the variable is not an atomic counter
    uniform, the value -1 is written to <params>.

    For the property of BUFFER_BINDING, to index of the buffer binding point
    associated with the active uniform block, shader storage block, or atomic
    counter buffer is written to <params>.

    For the property of BUFFER_DATA_SIZE, then the implementation-dependent
    minimum total buffer object size, in basic machine units, required to hold
    all active variables associated with an active uniform block, shader
    storage block, or atomic counter buffer is written to <params>.  If the
    final member of an active shader storage block is array with no declared
    size, the minimum buffer size is computed assuming the array was declared
    as an array with one element.

    For the property of NUM_ACTIVE_VARIABLES, the number of active variables
    associated with an active uniform block, shader storage block, or atomic
    counter buffer is written to <params>.

    For the property of ACTIVE_VARIABLES, an array of active variable indices
    associated with an active uniform block, shader storage block, or atomic
    counter buffer is written to <params>.  The number of values written to
    <params> for an active resource is given by the value of the property
    NUM_ACTIVE_VARIABLES for the resource.

    For the properties REFERENCED_BY_VERTEX_SHADER,
    REFERENCED_BY_TESS_CONTROL_SHADER, REFERENCED_BY_TESS_EVALUATION_SHADER,
    REFERENCED_BY_GEOMETRY_SHADER, REFERENCED_BY_FRAGMENT_SHADER, and
    REFERENCED_BY_COMPUTE_SHADER, a single integer is written to <params>,
    identifying whether the active resource is referenced by the vertex,
    tessellation control, tessellation evaluation, geometry, or fragment
    shaders, respectively, in the program object.  The value one is written to
    <params> if an active variable is referenced by the corresponding shader,
    or if an active uniform block, shader storage block, or atomic counter
    buffer contains at least one variable referenced by the corresponding
    shader.  Otherwise, the value zero is written to <params>.

    For the property TOP_LEVEL_ARRAY_SIZE, a single integer identifying the
    number of active array elements of the top-level shader storage block
    member containing to the active variable is written to <params>.  If the
    top-level block member is not declared as an array, the value one is
    written to <params>.  If the top-level block member is an array with no
    declared size, the value zero is written to <params>.

    For the property TOP_LEVEL_ARRAY_STRIDE, a single integer identifying the
    stride between array elements of the top-level shader storage block member
    containing the active variable is written to <params>.  For top-level
    block members declared as arrays, the value written is the difference, in
    basic machine units, between the offsets of the active variable for
    consecutive elements in the top-level array.  For top-level block members
    not declared as an array, zero is written to <params>.

    For the property LOCATION, a single integer identifying the assigned
    location for an active uniform, input, output, or subroutine uniform
    variable is written to <params>.  For input, output, or uniform variables
    with locations specified by a layout qualifier, the specified location is
    used.  For vertex shader input or fragment shader output variables without
    a layout qualifier, the location assigned when a program is linked is
    written to <params>.  For all other input and output variables, the value
    -1 is written to <params>.  For uniforms in uniform blocks, the value -1
    is written to <params>.

    For the property LOCATION_INDEX, a single integer identifying the fragment
    color index of an active fragment shader output variable is written to
    <params>.  If the active variable is an output for a non-fragment shader,
    the value -1 will be written to <params>.

    For the property IS_PER_PATCH, a single integer identifying whether the
    input or output is a per-patch attribute.  If the active variable is a
    per-patch attribute (declared with the "patch" qualifier), the value one
    is written to <params>; otherwise, the value zero is written to <params>.

    The commands

      int GetProgramResourceLocation(uint program, enum programInterface,
                                     const char *name);
      int GetProgramResourceLocationIndex(uint program, enum programInterface,
                                          const char *name);

    returns the location or the fragment color index, respectively, assigned
    to the variable named <name> in interface <programInterface> of program
    object <program>.  For both commands, the error INVALID_OPERATION is
    generated if <program> has not been linked or was last linked
    unsuccessfully.  For GetProgramResourceLocation, <programInterface> must
    be one of UNIFORM, PROGRAM_INPUT, PROGRAM_OUTPUT,
    VERTEX_SUBROUTINE_UNIFORM, TESS_CONTROL_SUBROUTINE_UNIFORM,
    TESS_EVALUATION_SUBROUTINE_UNIFORM, GEOMETRY_SUBROUTINE_UNIFORM,
    FRAGMENT_SUBROUTINE_UNIFORM, or COMPUTE_SUBROUTINE_UNIFORM.  For
    GetProgramResourceLocationIndex, <programInterface> must be
    PROGRAM_OUTPUT.  The value -1 will be returned by either command if an
    error occurs, if <name> does not identify an active variable on
    <programInterface>, or if <name> identifies an active variable that does
    not have a valid location assigned, as described above.  The locations
    returned by these commands are the same locations returned when querying
    the LOCATION and LOCATION_INDEX resource properties.

    A string provided to GetProgramResourceLocation or
    GetProgramResourceLocationIndex is considered to match an active variable
    if:

      * the string exactly matches the name of the active variable;

      * if the string identifies the base name of an active array, where the
        string would exactly match the name of the variable if the suffix
        "[0]" were appended to the string; or

      * if the string identifies an active element of the array, where the
        string ends with the concatenation of the "[" character, an integer
        (with no "+" sign, extra leading zeroes, or whitespace) identifying an
        array element, and the "]" character, the integer is less than the
        number of active elements of the array variable, and where the string
        would exactly match the enumerated name of the array if the decimal
        integer were replaced with zero.

    Any other string is considered not to identify an active variable.  If the
    string specifies an element of an array variable,
    GetProgramResourceLocation and GetProgramResourceLocationIndex return the
    location or fragment color index assigned to that element.  If it
    specifies the base name of an array, it identifies the resources
    associated with the first element of the array.


    Modify Section 2.14.6, Vertex Attributes, p. 109

    (delete text beginning with the second paragraph of p. 110, through the
     second paragraph of p. 112)

    (modify the third paragraph, p. 112, removing the first sentence)  The
    command

      void BindAttribLocation(uint program, uint index, const char *name);

    ...

    (insert after second paragraph, p. 113)

    To determine the set of active vertex attribute variables used by a
    program, applications can query the properties and active resources of the
    PROGRAM_INPUT interface of a program including a vertex shader.

    Additionally, the command

      void GetActiveAttrib(uint program, uint index,
                           sizei bufSize, sizei *length, int *size,
                           enum *type, char *name);

    can be used to determine properties of the active input variable assigned
    the index <index> in program object <program>.  If no error occurs, the
    command is equivalent to calling:

      const enum props[] = { ARRAY_SIZE, TYPE };
      GetProgramResourceName(program, PROGRAM_INPUT, index, bufSize, length,
                             name);
      GetProgramResourceiv(program, PROGRAM_INPUT, index, 1, &props[0],
                           1, NULL, size);
      GetProgramResourceiv(program, PROGRAM_INPUT, index, 1, &props[1],
                           1, NULL, (int *) type);

    If <index> is not the index of an active input variable in <program>, the
    error INVALID_VALUE is generated.  If <program> does not include a vertex
    shader, it has no active vertex attributes and the error INVALID_VALUE is
    generated for all values of <index>.  For GetActiveAttrib, all active
    vertex shader input variables are enumerated, including the special
    built-in inputs gl_VertexID and gl_InstanceID.

    The command

      int GetAttribLocation(uint program, const char *name);

    can be used to determine the location assigned to the active input
    variable named <name> in program object <program>.  The error
    INVALID_OPERATION is generated and -1 is returned if <program> has not
    been linked or was last linked unsuccessfully.  If <program> has been
    successfully linked but contains no vertex shader, no error will be
    generated but -1 will be returned.  Otherwise, the command is equivalent
    to calling:

      GetProgramResourceLocation(program, PROGRAM_INPUT, name);

    There is an implementation-dependent limit on the number of active
    attribute variables [[compatibility profile only:  (either conventional or
    generic)]] in a vertex shader.  A program with more than
    MAX_VERTEX_ATTRIBS active attribute variables may fail to link, unless
    device-dependent optimizations are able to make the program fit within
    available hardware resources.  For the purposes of this test, attribute
    variables of the type dvec3, dvec4, dmat2x3, dmat2x4, dmat3, dmat3x4,
    dmat4x3, and dmat4 may count as consuming twice as many attributes as
    equivalent single-precision types.  While these types use the same number
    of generic attributes as their single-precision equivalents,
    implementations are permitted to consume two single-precision vectors of
    internal storage for each three- or four-component double-precision
    vector.


    Modify Section 2.14.7, Uniform Variables, p. 113

    (replace text starting with the 3rd paragraph on p. 115 through the 3rd
    paragraph on p. 127)

    To determine the set of active uniform variables used by a program,
    applications can query the properties and active resources of the UNIFORM
    interface of a program.

    Additionally, several dedicated commands are provided to query properties
    of active uniforms.  The commands

      int GetUniformLocation(uint program, const char *name)

      void GetActiveUniformName(uint program, uint uniformIndex,
                                sizei bufSize, sizei *length,
                                char *uniformName);

    are equivalent to calling

      GetProgramResourceLocation(program, UNIFORM, name);

      GetProgramResourceName(program, UNIFORM, uniformIndex, bufSize, length,
                             uniformName);

    The command

      void GetUniformIndices(uint program, sizei uniformCount,
                             const char * const *uniformNames, 
                             uint *uniformIndices);

    is equivalent to

      for (int i = 0; i < uniformCount; i++) {
        uniformIndices[i] =
          GetProgramResourceIndex(program, UNIFORM, uniformNames[i];
      }

    The command

      void GetActiveUniform(uint program, uint index, sizei bufSize,
                            sizei *length, int *size, enum *type,
                            char *name);

    is equivalent to

      const enum props[] = { ARRAY_SIZE, TYPE };
      GetProgramResourceName(program, UNIFORM, index, bufSize, length, name);
      GetProgramResourceiv(program, UNIFORM, index,
                           1, &props[0], 1, NULL, size);
      GetProgramResourceiv(program, UNIFORM, index,
                           1, &props[1], 1, NULL, (int *) type);

    The command

      void GetActiveUniformsiv(uint program, sizei uniformCount,
                               const uint *uniformIndices, enum pname,
                               int *params);

    is equivalent to

      GLenum prop;
      for (int i = 0; i < uniformCount; i++) {
        GetProgramResourceiv(program, UNIFORM, uniformIndices[i], 1, &prop,
                             1, NULL, &params[i]);
      }

    where the value of <prop> is taken from Table X.2, based on the value of
    <pname>.

        <pname>                         <prop>
        ------------------------------  ------------------------------
        UNIFORM_TYPE                    TYPE
        UNIFORM_SIZE                    ARRAY_SIZE
        UNIFORM_NAME_LENGTH             NAME_LENGTH
        UNIFORM_BLOCK_INDEX             BLOCK_INDEX
        UNIFORM_OFFSET                  OFFSET
        UNIFORM_ARRAY_STRIDE            ARRAY_STRIDE
        UNIFORM_MATRIX_STRIDE           MATRIX_STRIDE
        UNIFORM_IS_ROW_MAJOR            IS_ROW_MAJOR
        UNIFORM_ATOMIC_COUNTER_         ATOMIC_COUNTER_BUFFER_INDEX
          BUFFER_INDEX

        Table X.2, GetProgramResourceiv properties used by
        GetActiveUniformsiv.

    To determine the set of active uniform blocks used by a program,
    applications can query the properties and active resources of the
    UNIFORM_BLOCK interface of a program.

    Additionally, several dedicated commands are provided to query properties
    of active uniform blocks.  The commands

      uint GetUniformBlockIndex(uint program, const char *uniformBlockName);

      void GetActiveUniformBlockName(uint program, uint uniformBlockIndex,
                                     sizei bufSize, sizei length,
                                     char *uniformBlockName);

    are equivalent to calling

      GetProgramResourceIndex(program, UNIFORM_BLOCK, uniformBlockName);

    and

      GetProgramResourceName(program, UNIFORM_BLOCK, uniformBlockIndex,
                             bufSize, length, uniformBlockName);

    The command

      void GetActiveUniformBlockiv(uint program, uint uniformBlockIndex,
                                   enum pname, int *params);

    is equivalent to calling

      GLenum prop;
      GetProgramResourceiv(program, UNIFORM_BLOCK, uniformBlockIndex,
                           1, &prop, maxSize, NULL, params);

    where the value of <prop> is taken from Table X.3, based on the value of
    <pname>, and <maxSize> is taken to specify a sufficiently large buffer to
    receive all values that would be written to <params>.

        <pname>                         <prop>
        ------------------------------  ------------------------------
        UNIFORM_BLOCK_BINDING           BUFFER_BINDING
        UNIFORM_BLOCK_DATA_SIZE         BUFFER_DATA_SIZE
        UNIFORM_BLOCK_NAME_LENGTH       NAME_LENGTH
        UNIFORM_BLOCK_ACTIVE_UNIFORMS   NUM_ACTIVE_VARIABLES
        UNIFORM_BLOCK_ACTIVE_           ACTIVE_VARIABLES
          UNIFORM_INDICES
        UNIFORM_BLOCK_REFERENCED_BY_    REFERENCED_BY_VERTEX_SHADER
          VERTEX_SHADER
        UNIFORM_BLOCK_REFERENCED_BY_    REFERENCED_BY_TESS_CONTROL_SHADER
          TESS_CONTROL_SHADER
        UNIFORM_BLOCK_REFERENCED_BY_    REFERENCED_BY_TESS_EVALUATION_SHADER
          TESS_EVALUATION_SHADER
        UNIFORM_BLOCK_REFERENCED_BY_    REFERENCED_BY_GEOMETRY_SHADER
          GEOMETRY_SHADER
        UNIFORM_BLOCK_REFERENCED_BY_    REFERENCED_BY_FRAGMENT_SHADER
          FRAGMENT_SHADER
        UNIFORM_BLOCK_REFERENCED_BY_    REFERENCED_BY_COMPUTE_SHADER
          COMPUTE_SHADER

        Table X.3, GetProgramResourceiv properties used by
        GetActiveUniformBlockiv.

    To determine the set of active atomic counter buffer binding points used
    by a program, applications can query the properties and active resources
    of the ATOMIC_COUNTER_BUFFER interface of a program.

    Additionally, the command

      void GetActiveAtomicCounterBufferiv(uint program, uint bufferIndex,
                                          enum pname, int *params);

    can be used to determine properties of active atomic counter buffer
    bindings used by <program> and is equivalent to calling

      GLenum prop;
      GetProgramResourceiv(program, ATOMIC_COUNTER_BUFFER, bufferIndex,
                           1, &prop, maxSize, NULL, params);

    where the value of <prop> is taken from Table X.4, based on the value of
    <pname>, and <maxSize> is taken to specify a sufficiently large buffer to
    receive all values that would be written to <params>.

        <pname>                         <prop>
        ------------------------------  ------------------------------
        ATOMIC_COUNTER_BUFFER_BINDING   BUFFER_BINDING
        ATOMIC_COUNTER_BUFFER_          BUFFER_DATA_SIZE
          DATA_SIZE
        ATOMIC_COUNTER_BUFFER_ACTIVE_   NUM_ACTIVE_VARIABLES
          ATOMIC_COUNTERS
        ATOMIC_COUNTER_BUFFER_ACTIVE_   ACTIVE_VARIABLES
          ATOMIC_COUNTER_INDICES
        ATOMIC_COUNTER_BUFFER_          REFERENCED_BY_VERTEX_SHADER
          REFERENCED_BY_VERTEX_SHADER
        ATOMIC_COUNTER_BUFFER_          REFERENCED_BY_TESS_CONTROL_SHADER
          REFERENCED_BY_TESS_
          CONTROL_SHADER
        ATOMIC_COUNTER_BUFFER_          REFERENCED_BY_TESS_EVALUATION_SHADER
          REFERENCED_BY_TESS_
          EVALUATION_SHADER
        ATOMIC_COUNTER_BUFFER_          REFERENCED_BY_GEOMETRY_SHADER
          REFERENCED_BY_GEOMETRY_SHADER
        ATOMIC_COUNTER_BUFFER_          REFERENCED_BY_FRAGMENT_SHADER
          REFERENCED_BY_FRAGMENT_SHADER
        ATOMIC_COUNTER_BUFFER_          REFERENCED_BY_COMPUTE_SHADER
          REFERENCED_BY_COMPUTE_SHADER

        Table X.4, GetProgramResourceiv properties used by
        GetActiveAtomicCounterBufferiv.


    Modify Section 2.14.8, Subroutine Uniform Variables, p. 136

    (This section could probably use some reorganization and a high-level
     introduction to how subroutines and subroutine variables work.  As
     written now, it mostly talks about the GL APIs operating on these without
     a lot of conceptual discussion.  These edits makes this section smaller
     by redefining the old APIs in terms of the new ones.)

    (replace language in the section describing the active subroutine and
    subroutine uniform enumeration with the following)

    To determine the set of active subroutines and subroutines used by a
    particular shader stage of a program, applications can query the
    properties and active resources of the interfaces for the shader type, as
    listed in Tables X.5 and X.6.

        Interface                       Shader Type
        ------------------------------  ------------------------------
        VERTEX_SUBROUTINE               VERTEX_SHADER
        TESS_CONTROL_SUBROUTINE         TESS_CONTROL_SHADER
        TESS_EVALUATION_SUBROUTINE      TESS_EVALUATION_SHADER
        GEOMETRY_SUBROUTINE             GEOMETRY_SHADER
        FRAGMENT_SUBROUTINE             FRAGMENT_SHADER
        COMPUTE_SUBROUTINE              COMPUTE_SHADER

        Table X.5, Interfaces for active subroutines for a particular shader
        type in a program.

        Interface                       Shader Type
        ------------------------------  ------------------------------
        VERTEX_SUBROUTINE_UNIFORM       VERTEX_SHADER
        TESS_CONTROL_SUBROUTINE_        TESS_CONTROL_SHADER
          UNIFORM
        TESS_EVALUATION_SUBROUTINE      TESS_EVALUATION_SHADER
          UNIFORM
        GEOMETRY_SUBROUTINE_UNIFORM     GEOMETRY_SHADER
        FRAGMENT_SUBROUTINE_UNIFORM     FRAGMENT_SHADER
        COMPUTE_SUBROUTINE_UNIFORM      COMPUTE_SHADER

        Table X.6, Interfaces for active subroutine uniforms for a particular
        shader type in a program.

    Dedicated commands are provided to determine properties of active
    subroutines and active subroutine uniforms.  The commands

      uint GetSubroutineIndex(uint program, enum shadertype,
                              const char *name);

      void GetActiveSubroutineName(uint program, enum shadertype, uint index,
                                   sizei bufsize, sizei *length, char *name);

    are equivalent to

      GetProgramResourceIndex(program, programInterface, name);

      GetProgramResourceName(program, programInterface, index, bufsize,
                             length, name);

    where <programInterface> is taken from Table X.5 according to the value of
    <shadertype>.

    The commands

      void GetSubroutineUniformLocation(uint program, enum shadertype,
                                        const char *name);

      void GetActiveSubroutineUniformName(uint program, enum shadertype,
                                          uint index, sizei bufsize,
                                          sizei *length, char *name);

      void GetActiveSubroutineUniformiv(uint program, enum shadertype,
                                        uint index, enum pname, int *values);

    are equivalent to

      GetProgramResourceLocation(program, programInterface, name);

      GetProgramResourceName(program, programInterface, index, bufsize, length,
                             name);

      GetProgramResourceiv(program, programInterface, index, 1, &pname,
                           maxSize, NULL, values);

    where <programInterface> is taken from Table X.6 according to the value of
    <shadertype>.  For GetActiveSubroutineUniformiv, <pname> must be one of
    NUM_COMPATIBLE_SUBROUTINES or COMPATIBLE_SUBROUTINES, and <maxSize> is
    taken to specify a sufficiently large buffer to receive all values that
    would be written to <params>.


    Modify Section 2.14.11, Output Variables, p. 140

    (replace last paragraph, p. 143 through next-to-last paragraph, p. 144
    with the following)

    To determine the set of output variables in a linked program object that
    will be captured in transform feedback mode, applications can query the
    properties and active resources of the TRANSFORM_FEEDBACK_VARYING
    interface.

    Additionally, the dedicated command

      void GetTransformFeedbackVarying(uint program, uint index,
                                       sizei bufSize, sizei *length,
                                       sizei *size, enum *type, char *name);

    can be used to enumerate properties of a single output variable captured
    in transform feedback mode, and is equivalent to

      const enum props[] = { ARRAY_SIZE, TYPE };
      GetProgramResourceName(program, TRANSFORM_FEEDBACK_VARYING, index,
                             bufSize, length, name);
      GetProgramResourceiv(program, TRANSFORM_FEEDBACK_VARYING, index,
                           1, &props[0], 1, NULL, size);
      GetProgramResourceiv(program, TRANSFORM_FEEDBACK_VARYING, index,
                           1, &props[1], 1, NULL, (int *) type);


    GetTransformFeedbackVarying may be used to query any transform
    feedback varying variable, not just those specified with
    TransformFeedbackVarying.

Additions to Chapter 3 of the OpenGL 4.2 (Compatibility Profile) Specification
(Rasterization)

    Modify Section 3.13.2, Shader Execution, p. 371

    (replace next-to-last paragraph, p. 378 through the first paragraph,
    p. 379 with the following)

    To determine the set of fragment shader output attribute variables used by
    a program, applications can query the properties and active resources of
    the PROGRAM_OUTPUT interface of a program including a fragment shader.

    Additionally, dedicated commands are provided to query the location and
    fragment color index assigned to a fragment shader output variable.  For
    the commands

      int GetFragDataLocation(uint program, const char *name);

      int GetFragDataIndex(uint program, const char *name);

    the error INVALID_OPERATION is generated and -1 is returned if <program>
    has not been linked or was last linked unsuccessfully.  If <program> has
    been successfully linked but contains no fragment shader, no error will be
    generated but -1 will be returned.  Otherwise, the commands are equivalent
    to calling:

      GetProgramResourceLocation(program, PROGRAM_OUTPUT, name);

    or

      GetProgramResourceLocationIndex(program, PROGRAM_OUTPUT, name);


Additions to Chapter 4 of the OpenGL 4.2 (Compatibility Profile) Specification
(Per-Fragment Operations and the Frame Buffer)

    None.

Additions to Chapter 5 of the OpenGL 4.2 (Compatibility Profile) Specification
(Special Functions)

    None.  Note that all the commands added here are not included in display
    lists, but no specification edits are required since existing blanket
    language covers all Get* commands.


Additions to Chapter 6 of the OpenGL 4.2 (Compatibility Profile) Specification
(State and State Requests)

    None.

Additions to Appendix A of the OpenGL 4.2 (Compatibility Profile)
Specification (Invariance)

    None.

Additions to Appendix D of the OpenGL 4.2 (Compatibility Profile)
Specification (Shared Objects and Multiple Contexts)

    None.

Additions to the AGL/EGL/GLX/WGL Specifications

    None

GLX Protocol

    TBD

Dependencies on OpenGL 3.0, ARB_transform_feedback, and EXT_transform_feedback

    If OpenGL 3.0, ARB_transform_feedback, and EXT_transform_feedback are not
    supported, references to the TRANSFORM_FEEDBACK_VARYING interface should
    be removed.

Dependencies on OpenGL 3.1 and ARB_uniform_buffer_object

    If OpenGL 3.1 and ARB_uniform_buffer_object are not supported, references
    to the UNIFORM_BLOCK interface should be removed.

Dependencies on OpenGL 4.0 and ARB_shader_subroutine

    If OpenGL 4.0 and ARB_shader_subroutine are not supported, references to
    the VERTEX_SUBROUTINE, TESS_CONTROL_SUBROUTINE,
    TESS_EVALUATION_SUBROUTINE, GEOMETRY_SUBROUTINE, FRAGMENT_SUBROUTINE,
    COMPUTE_SUBROUTINE, VERTEX_SUBROUTINE_UNIFORM,
    TESS_CONTROL_SUBROUTINE_UNIFORM, TESS_EVALUATION_SUBROUTINE_UNIFORM,
    GEOMETRY_SUBROUTINE_UNIFORM, FRAGMENT_SUBROUTINE_UNIFORM, and
    COMPUTE_SUBROUTINE_UNIFORM interfaces should be removed.

Dependencies on OpenGL 4.2 and ARB_shader_atomic_counters

    If OpenGL 4.2 and ARB_shader_atomic_counters are not supported, references
    to the ATOMIC_COUNTER_BUFFER interfaces should be removed.

Dependencies on OpenGL 4.3 and ARB_shader_storage_buffer_object

    If OpenGL 4.3 and ARB_shader_storage_buffer_object are not supported,
    references to the BUFFER_VARIABLE and SHADER_STORAGE_BLOCK interfaces
    should be removed.

Dependencies on OpenGL 4.3 and ARB_arrays_of_arrays

    If OpenGL 4.3 and ARB_arrays_of_arrays are not supported, language
    describing the enumeration of multi-dimensional arrays (i.e., arrays of
    arrays) should be removed.

Dependencies on OpenGL 4.3 and ARB_compute_shader

    If OpenGL 4.3 and ARB_compute_shader are not supported, references to the
    COMPUTE_SUBROUTINE and COMPUTE_SUBROUTINE_UNIFORM interfaces should be
    removed.

Dependencies on OpenGL 4.3 and ARB_explicit_uniform_location

    If OpenGL 4.3 and ARB_explicit_uniform_location are not supported,
    references to uniform variables being declared with a layout qualifier
    should be removed.

Errors

    An INVALID_VALUE error is generated by GetProgramInterfaceiv,
    GetProgramResourceIndex, GetProgramResourceName, GetProgramResourceiv,
    GetProgramResourceLocation, and GetProgramResourceLocationIndex if
    <program> is not the name of either a shader or program object.

    An INVALID_OPERATION error is generated by GetProgramInterfaceiv,
    GetProgramResourceIndex, GetProgramResourceName, GetProgramResourceiv,
    GetProgramResourceLocation, and GetProgramResourceLocationIndex if
    <program> is the name of a shader object.

    INVALID_OPERATION is generated by GetProgramInterfaceiv if the parameter
    <pname> is not supported for the interface <programInterface>.

    INVALID_ENUM is generated by GetProgramResourceIndex if <programInterface>
    is ATOMIC_COUNTER_BUFFER.

    INVALID_VALUE is generated by GetProgramResourceName if <index> is greater
    than or equal to the number of entries in the active resource list for
    <programInterface>.

    INVALID_ENUM is generated by GetProgramResourceName if <programInterface>
    is ATOMIC_COUNTER_BUFFER.

    INVALID_VALUE is generated by GetProgramResourceiv if <propCount> is zero.

    INVALID_ENUM is generated by GetProgramResourceiv if any value in <props>
    is not a property name supported by the command.

    INVALID_OPERATION is generated by GetProgramResourceiv if any value in
    <props> is not supported for the interface <programInterface>.

    INVALID_OPERATION is generated by GetProgramResourceLocation and
    GetProgramResourceLocationIndex if <program> has not been linked or was
    last linked unsuccessfully.

New State

    Insert new tables after Table 6.53, Program Object State (cont.), p. 557:

    Add table, labeled "Program Interface State"

                                                                   Initial
    Get Value                         Type  Get Command            Value    Description                 Sec.
    -----------------------           ----  -----------            -------  ------------------------    -----
    ACTIVE_RESOURCES                  nxZ+  GetProgramInterfaceiv     0     number of active resources  2.14.3
                                                                            on an interface
    MAX_NAME_LENGTH                   nxZ+  GetProgramInterfaceiv     0     maximum name length for     2.14.3
                                                                            active resources
    MAX_NUM_ACTIVE_VARIABLES          nxZ+  GetProgramInterfaceiv     0     maximum number of active    2.14.3
                                                                            variables for active
                                                                            resources
    MAX_NUM_COMPATIBLE_SUBROUTINES    nxZ+  GetProgramInterfaceiv     0     maximum number of compat-   2.14.3
                                                                            ible subroutines for
                                                                            subroutine uniforms

    Add table, labeled "Program Object Resource State"

                                                             Initial
    Get Value                         Type  Get Command      Value    Description                Sec.
    -----------------------           ----  -----------      -------  ------------------------   -----
    NAME_LENGTH                       Z+    GetProgram-         -     length of active resource  2.14.3
                                              Resourceiv              name
    TYPE                              Z+    GetProgram-         -     active resource data type  2.14.3
                                              Resourceiv
    ARRAY_SIZE                        Z+    GetProgram-         -     active resource array size 2.14.3
                                              Resourceiv
    OFFSET                            Z+    GetProgram-         -     active resource offset in  2.14.3
                                              Resourceiv              memory
    BLOCK_INDEX                       Z+    GetProgram-         -     index of interface block   2.14.3
                                              Resourceiv              owning resource
    ARRAY_STRIDE                      Z+    GetProgram-         -     active resource array      2.14.3
                                              Resourceiv              stride in memory
    MATRIX_STRIDE                     Z+    GetProgram-         -     active resource matrix     2.14.3
                                              Resourceiv              stride in memory
    IS_ROW_MAJOR                      Z+    GetProgram-         -     active resource stored as  2.14.3
                                              Resourceiv              a row major matrix?
    ATOMIC_COUNTER_BUFFER_INDEX       Z+    GetProgram-         -     index of atomic counter    2.14.3
                                              Resourceiv              buffer owning resource
    BUFFER_BINDING                    Z+    GetProgram-         -     buffer binding assigned    2.14.3
                                              Resourceiv              to active resource
    BUFFER_DATA_SIZE                  Z+    GetProgram-         -     minimum buffer data size   2.14.3
                                              Resourceiv              required for resource
    NUM_ACTIVE_VARIABLES              Z+    GetProgram-         -     number of active variables 2.14.3
                                              Resourceiv              owned by active resource
    ACTIVE_VARIABLES                  Z+    GetProgram-         -     list of active variables   2.14.3
                                              Resourceiv              owned by active resource
    REFERENCED_BY_VERTEX_SHADER       Z+    GetProgram-         -     active resource used by    2.14.3
                                              Resourceiv              vertex shader?
    REFERENCED_BY_TESS_CONTROL_       Z+    GetProgram-         -     active resource used by    2.14.3
      SHADER                                  Resourceiv              tess. control shader?
    REFERENCED_BY_TESS_EVALUATION_    Z+    GetProgram-         -     active resource used by    2.14.3
      SHADER                                  Resourceiv              tess evaluation shader?
    REFERENCED_BY_GEOMETRY_SHADER     Z+    GetProgram-         -     active resource used by    2.14.3
                                              Resourceiv              geometry shader?
    REFERENCED_BY_FRAGMENT_SHADER     Z+    GetProgram-         -     active resource used by    2.14.3
                                              Resourceiv              fragment shader?
    REFERENCED_BY_COMPUTE_SHADER      Z+    GetProgram-         -     active resource used by    2.14.3
                                              Resourceiv              compute shader?
    TOP_LEVEL_ARRAY_SIZE              Z+    GetProgram-         -     array size of top level    2.14.3
                                              Resourceiv              shd. storage block member
    TOP_LEVEL_ARRAY_STRIDE            Z+    GetProgram-         -     array stride of top level  2.14.3
                                              Resourceiv              shd. storage block member
    LOCATION                          Z+    GetProgram-         -     location assigned to       2.14.3
                                              Resourceiv              active resource
    LOCATION_INDEX                    Z+    GetProgram-         -     location index assigned    2.14.3
                                              Resourceiv              to active resource
    IS_PER_PATCH                      Z+    GetProgram-         -     is active input/output     2.14.3
                                              Resourceiv              a per-patch attribute?
    NUM_COMPATIBLE_SUBROUTINES        Z+    GetProgram-         -     number of compatible       2.14.3
                                              Resourceiv              subroutines for active
                                                                      subroutine uniform
    COMPATIBLE_SUBROUTINES            Z+    GetProgram-         -     list of compatible         2.14.3
                                              Resourceiv              subroutines for active
                                                                      subroutine uniform

New Implementation Dependent State

    None.

Sample Code

    !!! TBD !!!

Conformance Tests

    !!! TBD !!!

Issues

    (1) Does the list of active resources in a program include built-ins where
        applicable?

      RESOLVED:  Yes; built-ins should be enumerated when present.

    (2) Should this extension fully support all program interfaces and
        resources, even those that have limited or no query capabilities
        today?  In particular, there is very little enumeration support for
        fragment outputs and transform feedback varyings.  OpenGL 4.1 added
        ARB_separate_shader_objects that allowed two independent programs to
        form an interface, but provided no mechanism for querying the active
        variables on either side of that interface.

      RESOLVED:  Let's support as much as we can with a single set of
      functions.

    (3) How should we support enumeration of variables and interface blocks
        provided by the ARB_shader_storage_buffer_object extension?

      RESOLVED:  The enumeration mechanisms provided by this extension will be
      the only way to enumerate buffer variables and shader storage blocks.
      ARB_shader_storage_buffer_object will require this extension.

    (4) The commands provided this extension are intended to supersede
        existing enumeration APIs (e.g., GetActiveUniform, GetActiveAttrib).
        The old APIs can eventually be deprecated.  Both new and old APIs
        generate a list of active resources and assign indices to each entry
        in the list.  Should assigned indices match between the new and old
        APIs?

      RESOLVED:  Yes.  While there is no strong reason why the new APIs
      couldn't adopt new enumeration rules for existing resource types, it's
      easier to enumerate the same way that we've done for the older API
      calls, for several reasons:

        * Minimizes changes for existing applications using the old
          enumeration APIs or for applications needing to support both the new
          and old APIs.  All they need to do is map their existing API usage
          to the new APIs without having to reconsider whether any other
          assumption of their old logic is still valid.

        * Minimizes complexity for implementations needing to support both new
          and old APIs, at least until the old APIs are deprecated/removed.

        * Makes it easier to redefine the existing functions in terms of the
          new ones.

    (5) Existing enumeration APIs handle entities like arrays of structures by
        unrolling them.  For example, a uniform array of structures with 8
        elements and 3 structure members will result in 24 entries enumerated
        by GetActiveUniform.  Should we re-work the API to enumerate fewer
        entries?

      RESOLVED:  For existing resources, no, for the reasons discussed above.
      The main problem with that approach is if a shader uses a uniform block
      with a large array of structures.  For newer resource types, we can and
      will adopt new rules.

    (6) How should enumeration rules work for complex arrays/structures in
        shader storage blocks (ARB_shader_storage_buffer_object)?

      RESOLVED:  One of the intended use cases is to allow for shader storage
      blocks consisting of little more than a large (or unsized) array of
      structures.  For example, we might have shader code like:

        struct VertexInfo {
          float position[3];
          float normal[3];
        };
        buffer VertexCollection {
          VertexInfo v[10000];
        };

      If existing uniform enumeration API rules were applied, this would
      enumerate 20000 separate active resources (10000 array elements with two
      members each).  That seems crazy.  This specification optimizes for this
      case by flattening arrays at the top of the block.  For that shader, we
      would enumerate only the first element of each array ("v[0].position[0]"
      and "v[0].normal[0]").  The properties TOP_LEVEL_ARRAY_SIZE and
      TOP_LEVEL_ARRAY_STRIDE allow applications to determine the size (if
      declared) and stride in buffer object memory.

      For more complex hierarchies, we chose to continue unrolling arrays in
      the middle of the hierarchy to avoid the need to specify a more complex
      API.  For example, in this code

        struct VertexInfo {
          float position[3];
          float normal[3];
        };
        struct TriangleInfo {
          VertexInfo v[3];
        };
        buffer VertexCollection {
          TriangleInfo t[10000];
        };

      we would unroll "TriangleInfo" and enumerate six resources:

        t[0].v[0].position[0]
        t[0].v[0].normal[0]
        t[0].v[1].position[0]
        t[0].v[1].normal[0]
        t[0].v[2].position[0]
        t[0].v[2].normal[0]

      Flattening "v[]" to a single entry would require either some sort of
      recursive enumeration (e.g., enumerate "t" as a structure and provide
      "pointers" to a collection of members), or some sort of new property
      that returns a variable-length array of strides.

    (7) The ARB_array_of_arrays extension allows shaders to declare
        multi-dimensional arrays that could be enumerated by this API?  How
        should arrays of arrays be enumerated?

      RESOLVED:  We define rules consistent with our enumeration rules for
      other complex types.  For existing one-dimensional arrays, we enumerate
      a single entry if the array is an array of basic types, or separate
      entries for each array element if the array is an array of structures.
      We follow similar rules here.  For a uniform array such as:

        uniform vec4 a[5][4][3];

     we enumerate twenty different entries ("a[0][0][0]" through
     "a[4][3][0]"), each of which is treated as an array with three elements.
     This is morally equivalent to what you'd get if you worked around the
     limitation in current GLSL via:

        struct ArrayBottom { vec4 c[3]; };
        struct ArrayMid    { ArrayBottom b[3]; };
        uniform ArrayMid   a[5];

     which would enumerate "a[0].b[0].c[0]" through "a[4].b[3].c[0]".

     We also apply the top-level array flattening for shader storage block
     members.  For example:

       buffer Block {
         vec4 a[5][4][3];
       };

     would enumerate only four entries -- "a[0][0][0]" through "a[0][3][0]",
     where each enumerated entry corresponds to an array of size 3, has a
     top-level size of 5, and has a top-level stride appropriate for its
     layout in memory.

    (8) For GetProgramResourceIndex, what sort of strings can be used to match
        the resource names for arrays of arrays?  For example, let's say a
        shader declares a uniform such as:

          uniform vec4 a[3][4][5];

       Which one of the following names are accepted?  "a[2][1][0]" to
       identify the base of the bottom-level array?  "a[2][1]", to identify
       the same without the final "[0]"?  "a[2]", equivalent to "a[2][0][0]"?
       Just "a", equivalent to "a[0][0][0]"?

       RESOLVED:  We only accept entries of the form "a[2][1][0]" or
       "a[2][1]", which is consistent with the existing rules that only allow
       applications to omit the last index of a bottom-level array that has
       been rolled up.

    (9) Should be consolidate the commands GetAttribLocation,
        GetUniformLocation, GetSubroutineUniformLocation, GetFragDataLocation
        (and GetFragDataIndex) into a single generic API?

        RESOLVED:  Yes.  This spec provides GetProgramResourceLocation and
        GetProgramResourceLocationIndex commands to consolidate the existing
        set of commands.  Additionally, GetProgramResourceiv can be used to
        query locations via the LOCATION and LOCATION_INDEX properties.

        There are a number of irregularities in the assignment of locations as
        compared to other uniform types:

          * Atomic counter uniforms, uniforms in blocks are not assigned
            locations.

          * Built in inputs and outputs are not assigned locations.

          * The locations of inputs other than vertex shader inputs and
            fragment shader outputs don't interact with any other GL state;
            they are used only for interfacing with other programs.  For such
            variables, we choose the return the specified location number if a
            variable is declared with a layout qualifier, and -1 (no valid
            location) otherwise.  Variables declared without location layout
            qualifiers will be assigned an internal location by the
            compiler/linker, but those locations do not necessarily correspond
            to the canonical integer locations supported in layout qualifiers.

          * For fragment shader outputs, not only do we have a location, but
            we also have an associated index that can be queried with
            GetFragDataIndex.

    (10) What should we do about tokens like ACTIVE_UNIFORMS and
         ACTIVE_UNIFORM_MAX_LENGTH that are specific to an interface, but are
         queried today by GetProgramiv?

      RESOLVED:  We provide a new GetProgramInterfaceiv query to return
      properties of an interface.  The value returned today by ACTIVE_UNIFORMS
      can be determined by calling GetProgramInterfaceiv with a
      <programInterface> of UNIFORM and a <pname> of ACTIVE_RESOURCES.
      ACTIVE_RESOURCES can also be used for other interfaces to get values
      returned today with other query tokens (e.g., ACTIVE_ATTRIBUTES).

    (11) What sort of interface should we use for querying properties of
         active resources?  We have two different styles of query today.
         Functions like GetActiveUniform() allow applications to query
         multiple (fixed) properties of a single active uniform.  Functions
         like GetActiveUniforms() allow applications to query a single
         property of multiple active uniforms?

      RESOLVED:  GetProgramResourceiv allows applications to query multiple
      properties of a single resource in a single call; applications pass in
      an array of properties to query.

      We considered providing a command GetProgramResourcesiv that would have
      allowed an application to query multiple properties of multiple
      resources in a single call, by passing in an array of resource indices.
      We decided not to include this for simplicity.  Also note that the most
      common use case we envisioned would be to query all properties of all
      active resources, with code like the following:

        struct VariableInfo {
          GLenum type;
          GLint location;
        };
        GLenum inputProps[] = { GL_TYPE, GL_LOCATION };
        GLuint nInputProps = sizeof(inputProps) / sizeof(GLenum);
        GLint nActive;
        glGetProgramInterface(program, GL_PROGRAM_INPUT, GL_ACTIVE_RESOURCES,
                              &nActive);
        VariableInfo *varInfo = malloc(nActive*sizeof(VariableInfo));
        GLuint *indices = malloc(nActive*sizeof(GLuint));
        for (GLuint i = 0; i < nActive; i++) { indices[i] = i; }
        glGetProgramResourcesiv(program, GL_PROGRAM_INPUT,
                                nActive, indices, nInputProps, inputProps,
                                nActive*nInputProps, NULL, (GLint*)varInfo);

      Such an API would require applications to fill in a dummy array of
      indices to express "all indices".  With the simplified API, more GL
      calls are required, but the code is overall simpler:

        glGetProgramInterface(program, GL_PROGRAM_INPUT, GL_ACTIVE_RESOURCES,
                              &nActive);
        VariableInfo *varInfo = malloc(nActive*sizeof(VariableInfo));
        for (GLuint i = 0; i < nActive; i++) {
          glGetProgramResourceiv(program, GL_PROGRAM_INPUT, i,
                                 nInputProps, inputProps,
                                 nActive*nInputProps, NULL,
                                 (GLint*) (&varInfo[i]));
        }

    (12) For GetProgramResourceiv, how should we handle queries of properties
         that return a variable number of values (e.g.,
         COMPATIBLE_SUBROUTINES)?  Should we support these queries in the
         general "multiple properties of multiple resources" query, or provide
         a separate "single property of single resource query"?  Should we
         just write out a variable number of values to <params>?  If we allow
         for a variable number of values to be written to <params>, should we
         provide some sort of <bufSize> parameter to guard against overflows?

      RESOLVED:  We will support these properties in the general "multiple
      properties of a single resource" query and write out the variable number
      of values to consecutive locations in the array.  If an application
      wants to query such properties for multiple resources at once, it can
      parse the values returned by the GL.  If NUM_COMPATIBLE_SUBROUTINES is
      immediately before COMPATIBLE_SUBROUTINES in the property list, the
      number of compatible subroutines written to the buffer (N) will be found
      immediately immediately prior to the N compatible subroutine numbers.
      We will provide a <bufSize> parameter to guard against overflow.
      Applications can ensure they have an adequately-sized buffer by calling
      GetProgramInterfaceiv with a <pname> of MAX_NUM_COMPATIBLE_SUBROUTINES
      to determine the maximum number of subroutines that will be written to
      any one buffer.

      It's not clear how valuable the variable-length enumeration queries
      actually are.  Unextended OpenGL 4.2 has two such queries today:
      COMPATIBLE_SUBROUTINES and UNIFORM_BLOCK_ACTIVE_UNIFORMS.
      UNIFORM_BLOCK_ACTIVE_UNIFORMS isn't strictly needed; an application can
      determine this list themselves by querying the UNIFORM_BLOCK_INDEX
      property of each active uniform.  We could do something similar for
      compatible subroutines.

    (13) Should we add a mechanism for enumerating subroutine types supported
         by a program and querying the subroutine type for subroutines and
         subroutine uniforms?

      RESOLVED:  No.  Such a mechanism could have been used to reduce the need
      for variable-length return values for GetProgramResourcesiv in some
      cases.  Instead of supporting COMPATIBLE_SUBROUTINES (list all
      subroutines compatible with a uniform type), we could have allowed
      applications to query the subroutine type for both subroutines and
      subroutine uniforms and match them in application code.  The problem
      with that approach is that subroutines can have multiple associated
      subroutine types, so a variable-length return value would still be
      needed to query subroutine types.

    (14) Should we support "REFERENCED_BY_*" for uniforms and buffer
         variables?

      RESOLVED:  Yes.  It isn't supported in unextended OpenGL 4.2, but the
      functionality seems to make sense.

    (15) Since this extension is being developed at the same time as compute
         shaders, should we add compute shader-specific enums for the old
         queries that would be superseded by the the queries added here?  Or
         should we rely solely on the new queries?  For example, should we add
         a new token "UNIFORM_BLOCK_REFERENCED_BY_COMPUTE_SHADER"?

      RESOLVED:  Yes, we will add old-style enums for the compute shader stage
      for all previously existing APIs.

    (16) How are inputs and outputs in the built-in interface block
         "gl_PerVertex" enumerated?

      RESOLVED:  We will follow the default rule for enumerating block members
      in the OpenGL API, which is:

        * If a variable is a member of an interface block without an instance
          name, it is enumerated using just the variable name.

        * If a variable is a member of an interface block with an instance
          name, it is enumerated as "BlockName.Member", where "BlockName" is
          the name of the interface block (not the instance name) and "Member"
          is the name of the variable.

      For example, in the following code:

        uniform Block1 {
          int member1;
        };
        uniform Block2 {
          int member2;
        } instance2;
        uniform Block3 {
          int member3;
        } instance3[2];  // uses two separate buffer bindings

      the three uniforms (if active) are enumerated as "member1",
      "Block2.member2", and "Block3.member3".

      For tessellation control, tessellation evaluation, and geometry shaders,
      per-vertex built-in inputs like gl_Position are in a block with an
      instance name of "gl_in[]".  Additionally, for tessellation control
      shaders, per-vertex built-in outputs like gl_Position are in a block
      with an instance name of "gl_out[]".  In these cases, applying the
      standard rules would result in an enumerated name of
      "gl_PerVertex.gl_Position".

    (17) Should we add support for enumerating per-patch shader
         inputs/outputs?  If so, how to distinguish between per-vertex and
         per-patch?

      RESOLVED:  Yes.  When program inputs or outputs include per-patch
      attributes, those variables will be enumerated along with per-vertex
      attributes.  To determine if an input or output is per-patch, use the
      resource property IS_PER_PATCH.

    (18) Do we enumerate the special "marker" variables for transform feedback
         varyings (gl_NextBuffer, gl_SkipComponents[1234])?  If so, what are
         their properties?

      RESOLVED:  Yes, we should definitely enumerate them so applications can
      determine programmatically what would be written to transform feedback
      buffers.  This capability is already supported by the existing
      GetTransformFeedback query.  We follow the existing behavior -- all of
      these entries are enumerated with a type of NONE.  gl_NextBuffer returns
      an array size of 0; gl_SkipComponents[1234] returns an array size equal
      to the specified number of components.

    (19) Should we support a <bufsize> arguments to prevent buffer overflows
         on the GetProgramInterfaceiv query?

      RESOLVED:  Yes, to provide a "safe" mechanism since the query might
      return an unpredictable and program-dependent amount of data.  We also
      provide a <length> parameter that can be used to return the number of
      words written by the query to be consistent with other similar APIs.

    (20) Are "old" token names (e.g., UNIFORM_TYPE) accepted by "new"
         commmands (GetProgramResourceiv)?  Are "new" token names (TYPE)
         accepted by "old" commands (GetActiveUniforms)?

      RESOLVED:  No.  New queries support only the new enums; old queries
      support only the old ones.

    (21) While programs may include shader code for multiple stages, shader
         subroutines and subroutine uniforms are specific to a single stage in
         the program.  This means that there is a separate interface for each
         program stage.  Should we specify <programInterface> tokens for a
         cross-product of interfaces and stages (e.g., VERTEX_SUBROUTINE,
         FRAGMENT_SUBROUTINE_UNIFORM)?  Or should we provide generic
         <programInterface> tokens (SUBROUTINE, SUBROUTINE_UNIFORM) and accept
         a second <stage> parameter?

      RESOLVED:  Use concatenated tokens to avoid creating adding a separate
      set of per-stage APIs.  With this solution, we have means 12
      concatenated tokens (6 stages crossed with SUBROUTINE and
      SUBROUTINE_UNIFORM); we don't expect a massive number of new tokens in
      the future.

    (22) What data type should be used for the <params> (return data) argument
         to GetProgramResourceiv()?

      RESOLVED:  Use an "int *" to be consistent with all the other
      "*iv"-style queries.

      We considered using a "void *" so applications could avoid manual casts
      for cases where the API would be used to fill a structure with
      properties of a resource, as in code like the following:

        struct VariableInfo {
          GLenum type;
          GLint location;
        } varInfo;
        GLenum inputProps[] = { GL_TYPE, GL_LOCATION };
        GLuint nInputProps = sizeof(inputProps) / sizeof(GLenum);
        ...
        glGetProgramResourceiv(program, GL_PROGRAM_INPUT, index,
                               nInputProps, inputProps, nInputProps, NULL,
                               (GLint*)varInfo);

    (23) The special "marker" names "gl_NextBuffer" and "gl_SkipComponents*"
         may appear multiple times in the array of strings passed to
         TransformFeedbackVaryings.  How is this condition handled in the new
         enumeration APIs?

      RESOLVED:  For the purposes of enumerating active resources in the
      TRANSFORM_FEEDBACK_VARYINGS interface, each instance of the marker is
      assigned its own index, and the name and properties associated with that
      index can be queried.  [See also issue (18).]  This is consistent with
      how GetTransformFeedbackVaryings works in unextended OpenGL 4.2.

      The problem is that since a marker name can have multiple associated
      indices, it's not clear what value should be returned when passing such
      a string to GetProgramResourceIndex().  We specify that passing any of
      these marker strings should return the value INVALID_INDEX, regardless
      of the number of times the marker appears in the inputs to
      TransformFeedbackVaryings.  Unextended OpenGL 4.2 does not provide the
      ability to query the index associated with an active transform feedback
      varying name, so this is a new issue in this extension.

    (24) The PROGRAM_INPUT and PROGRAM_OUTPUT interfaces are provided to allow
         applications to query active inputs and outputs in a program.  Do we
         enumerate all variables qualified with "in" and "out", or should
         special "system" built-ins such as gl_VertexID, gl_InstanceID, and
         gl_FrontFacing be excluded?

      RESOLVED:  We enumerate all active variables qualified with "in" or
      "out" in the PROGRAM_INPUT and PROGRAM_OUTPUT interfaces, including
      special variables.  Additionally, because we redefine the function
      glGetActiveAttrib in terms of this in terms of the PROGRAM_INPUT
      interface, we should enumerate the specials gl_VertexID and
      gl_InstanceID there as well.  Previously, there was no explicit spec
      language whether these variables should be enumerated by
      GetActiveAttrib, and various OpenGL implementations behaved differently.
      This approach allows applications to enumerate all inputs used by a
      vertex shader, whether or not they correspond to attributes in
      conventional or generic vertex attribute arrays.

    (25) What should the value of ARRAY_SIZE be for non-array and unsized
         array variables?

      RESOLVED: For compatibility with GetActiveUniform, ARRAY_SIZE of a
      non-array variable is one (initially this was specified as zero, an
      unintentional incompatibility). For unsized array variables, the size
      is zero.

Revision History

    Revision 20, August 19, 2013 (Jon Leech)
      - Change ARRAY_SIZE query to return one for active variables not
        corresponding to an array of basic types, rather than zero, for
        compatibility with GetActiveUniform (Bug 10647). Return zero for
        explicitly unsized arrays (Bug 10641). Add extra 'const' qualifier
        for GetUniformIndices <uniformNames> argument (Bug 10703).
    Revision 19, July 11, 2013 (Jon Leech)
      - Clarify that GetTransformFeedbackVarying can be used to query
        any transform feedback varying variable (Bug 10472).
    Revision 18, July 10, 2012 (pbrown)
      - Specify an INVALID_ENUM error when ATOMIC_COUNTER_BUFFER is passed
        to GetProgramResourceIndex or GetProgramResourceName.  Clarify that
        the array element portion of a string passed to
        GetProgramResourceLocation or GetProgramResourceLocationIndex
        must not have, a "+" sign, extra leading zeroes, or whitespace (bug
        9254).
    Revision 17, July 7, 2012 (pbrown)
      - Specify that "special" built-ins inputs and outputs such as
        "gl_VertexID" should be enumerated in the PROGRAM_INPUT and
        PROGRAM_OUTPUT interfaces, as well as the legacy function
        GetActiveAttrib.  Add spec language counting the built-ins gl_VertexID
        and gl_InstanceID against the active attribute limit (bug 9201).
    Revision 16, July 2, 2012 (pbrown)
      - Fix typos in the language mapping OpenGL 4.2 active subroutine and
        subroutine uniform enumeration functions to the new interfaces.
    Revision 15, June 21, 2012 (Jon Leech)
      - Update errors to match the generic errors for all commands taking
        <program> names (INVALID_VALUE if neither a shader nor program
        object name, and INVALID_OPERATION if a shader object name) (Bug
        9145).
    Revision 14, June 14, 2012 (Jon Leech)
      - Add INVALID_VALUE error if <program> is not the name of a valid
        program object (Bug 9145).
    Revision 13, May 31, 2012 (pbrown)
      - Specify that passing the special "marker" strings "gl_NextBuffer"
        and "gl_SkipComponents*" to GetProgramResourceIndex() will always
        return INVALID_INDEX (bug 9071).
    Revision 12, May 31, 2012 (pbrown)
      - Allow the BUFFER_VARIABLE, PROGRAM_INPUT, and PROGRAM_OUTPUT
        interfaces to be used by the REFERENCED_BY_*_SHADER property in
        GetProgramResourceiv (bug 9007).
      - Remove duplicate reference to BUFFER_VARIABLE in the list of
        interfaces supported by the ARRAY_SIZE property in
        GetProgramResourceiv.
    Revision 11, May 18, 2012 (pbrown)
      - Clarify that the LOCATION_INDEX property is only supported for the
        PROGRAM_OUTPUT interface and not for PROGRAM_INPUT.
    Revision 10, May 10, 2012 (pbrown)
      - Rename the formal parameter <interface> used by the functions in this
        extension to <programInterface>.  Certain versions of the Microsoft
        C/C++ compiler and/or its headers cause "interface" to be treated as a
        reserved keyword.
    Revision 9, May 10, 2012 (pbrown)
      - Fix incorrect enumerant assignments.  ATOMIC_COUNTER_BUFFER has
        already been assigned for ARB_shader_atomic_counters.  Fix
        cut-and-paste error assigning a duplicate value to
        TESS_CONTROL_SUBROUTINE.  Reassign value for IS_PER_PATCH to fill
        the ATOMIC_COUNTER_BUFFER hole.
    Revision 8, May 2, 2012 (pbrown)
      - Reserve enumerants.
      - Add state table entries.
      - Mark issues as resolved per the Khronos F2F.
      - Add issue (22), where we decided to keep the "int *" argument to
        GetProgramResourceiv().
    Revision 7, April 25, 2012
      - Rename IS_PATCH_ATTRIB to IS_PER_PATCH (bug 8752).
    Revision 6, April 12, 2012
      - Add explicit language indicating that per-patch attributes can be
        enumerated in the PROGRAM_INPUT and PROGRAM_OUTPUT interfaces, plus
        the IS_PATCH_ATTRIB property.
      - Take a first cut at redefining all the existing enumeration APIs in
        terms of the new APIs.
      - Add missing OFFSET token to new token list.
      - Rename BUFFER_SIZE to BUFFER_DATA_SIZE to be consistent with older
        token names.
      - Mark various issues as resolved.
    Revision 5, April 12, 2012
      - Remove support for querying properties of multiple resources in a
        single call; rename GetProgramResourcesiv to GetProgramResourceiv
        since only a single resource is queried at a time.
      - Fold issue (22) into issue (11).
    Revision 4, April 5, 2012
      - Add issue (22) about the need to allocate/fill an array of indices even
        if you want to query properties of all active resources at once.
    Revision 3, April 5, 2012
      - Checkpoint with various edits/cleanups.  Add an introduction; clean up
        new procedures, tokens, errors, and issues sections.
      - Rename APIs to GetProgramInterface and GetProgramResource*.
      - Add GetProgramResourceLocation API as a generic location query.
      - Add interactions with ARB_compute_shader.
      - Add interactions with ARB_explicit_uniform_location.
      - Add discussion of enumeration of built-in variables and special
        transform feedback markers.
      - Add general discussion of locations assigned to active variables.
      - Add interface properties MAX_NUM_ACTIVE_VARIABLES and
        MAX_NUM_COMPATIBLE_SUBROUTINES.
      - Add resource properties LOCATION and LOCATION_INDEX.
      - Add <bufSize> and <length> parameters to GetProgramResourcesiv.
    Revision 2, March 22, 2012
      - Added new function prototypes for review purposes.
    Revision 1, March 14, 2012
      - Initial revision.