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<refentry xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="glGetActiveUniformBlock">
    <info>
        <copyright>
            <year>2010-2013</year>
            <holder>Khronos Group</holder>
        </copyright>
    </info>
    <refmeta>
        <refentrytitle>glGetActiveUniformBlock</refentrytitle>
        <manvolnum>3G</manvolnum>
    </refmeta>
    <refnamediv>
        <refname>glGetActiveUniformBlock</refname>
        <refpurpose>query information about an active uniform block</refpurpose>
    </refnamediv>
    <refsynopsisdiv><title>C Specification</title>
        <funcsynopsis>
            <funcprototype>
                <funcdef>void <function>glGetActiveUniformBlockiv</function></funcdef>
                <paramdef>GLuint <parameter>program</parameter></paramdef>
                <paramdef>GLuint <parameter>uniformBlockIndex</parameter></paramdef>
                <paramdef>GLenum <parameter>pname</parameter></paramdef>
                <paramdef>GLint *<parameter>params</parameter></paramdef>
            </funcprototype>
        </funcsynopsis>
    </refsynopsisdiv>
    <refsect1 xml:id="parameters"><title>Parameters</title>
        <variablelist>
        <varlistentry>
            <term><parameter>program</parameter></term>
            <listitem>
                <para>
                    Specifies the name of a program containing the uniform block.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>uniformBlockIndex</parameter></term>
            <listitem>
                <para>
                    Specifies the index of the uniform block within <parameter>program</parameter>.
                </para>
            </listitem>
        </varlistentry>
                <varlistentry>
            <term><parameter>pname</parameter></term>
            <listitem>
                <para>
                    Specifies the name of the parameter to query.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>params</parameter></term>
            <listitem>
                <para>
                    Specifies the address of a variable to receive the result of the query.
                </para>
            </listitem>
        </varlistentry>
        </variablelist>
    </refsect1>
    <refsect1 xml:id="description"><title>Description</title>
        <para>
            <function>glGetActiveUniformBlockiv</function> retrieves information about an active uniform block within <parameter>program</parameter>.
        </para>
        <para>
            <parameter>program</parameter> must be the name of a program object for which the command
            <citerefentry><refentrytitle>glLinkProgram</refentrytitle></citerefentry> must have been called in the past, although it is not required that
            <citerefentry><refentrytitle>glLinkProgram</refentrytitle></citerefentry> must have succeeded. The link could have failed because the number
            of active uniforms exceeded the limit.
        </para>
        <para>
            <parameter>uniformBlockIndex</parameter> is an active uniform block index of <parameter>program</parameter>, and must be less than the value
            of <constant>GL_ACTIVE_UNIFORM_BLOCKS</constant>.
        </para>
        <para>
            Upon success, the uniform block parameter(s) specified by <parameter>pname</parameter> are returned in <parameter>params</parameter>. If an error
            occurs, nothing will be written to <parameter>params</parameter>.
        </para>
        <para>
            If <parameter>pname</parameter> is <constant>GL_UNIFORM_BLOCK_BINDING</constant>, then the index of the uniform buffer binding point
            last selected by the uniform block specified by <parameter>uniformBlockIndex</parameter> for <parameter>program</parameter> is returned. If
            no uniform block has been previously specified, zero is returned.
        </para>
        <para>
            If <parameter>pname</parameter> is <constant>GL_UNIFORM_BLOCK_DATA_SIZE</constant>, then the implementation-dependent minimum total buffer
            object size, in basic machine units, required to hold all active uniforms in the uniform block identified by <parameter>uniformBlockIndex</parameter>
            is returned. It is neither guaranteed nor expected that a given implementation will arrange uniform values as tightly packed in a buffer
            object. The exception to this is the <emphasis>std140 uniform block layout</emphasis>, which guarantees specific packing behavior and does not
            require the application to query for offsets and strides. In this case the minimum size may still be queried, even though it is determined in
            advance based only on the uniform block declaration.
        </para>
        <para>
            If <parameter>pname</parameter> is <constant>GL_UNIFORM_BLOCK_NAME_LENGTH</constant>, then the total length (including the nul terminator) of
            the name of the uniform block identified by <parameter>uniformBlockIndex</parameter> is returned.
        </para>
        <para>
            If <parameter>pname</parameter> is <constant>GL_UNIFORM_BLOCK_ACTIVE_UNIFORMS</constant>, then the number of active uniforms in the uniform
            block identified by <parameter>uniformBlockIndex</parameter> is returned.
        </para>
        <para>
            If <parameter>pname</parameter> is <constant>GL_UNIFORM_BLOCK_ACTIVE_UNIFORM_INDICES</constant>, then a list of the active uniform indices
            for the uniform block identified by <parameter>uniformBlockIndex</parameter> is returned. The number of elements that will be written to
            <parameter>params</parameter> is the value of <constant>GL_UNIFORM_BLOCK_ACTIVE_UNIFORMS</constant> for <parameter>uniformBlockIndex</parameter>.
        </para>
        <para>
            If <parameter>pname</parameter> is <constant>GL_UNIFORM_BLOCK_REFERENCED_BY_VERTEX_SHADER</constant>, <constant>GL_UNIFORM_BLOCK_REFERENCED_BY_TESS_CONTROL_SHADER</constant>,
            <constant>GL_UNIFORM_BLOCK_REFERENCED_BY_TESS_EVALUATION_SHADER</constant>, <constant>GL_UNIFORM_BLOCK_REFERENCED_BY_GEOMETRY_SHADER</constant>,
            <constant>GL_UNIFORM_BLOCK_REFERENCED_BY_FRAGMENT_SHADER</constant>, or <constant>GL_UNIFORM_BLOCK_REFERENCED_BY_COMPUTE_SHADER</constant>
            then a boolean value indicating whether the uniform block identified by
            <parameter>uniformBlockIndex</parameter> is referenced by the vertex, tessellation control, tessellation evaluation, geometry, fragment or compute
            programming stages of program, respectively, is returned.
        </para>
    </refsect1>
    <refsect1 xml:id="errors"><title>Errors</title>
        <para>
            <constant>GL_INVALID_VALUE</constant> is generated if <parameter>uniformBlockIndex</parameter> is greater than or equal to the value
            of <constant>GL_ACTIVE_UNIFORM_BLOCKS</constant> or is not the index of an active uniform block in <parameter>program</parameter>.
        </para>
        <para>
            <constant>GL_INVALID_ENUM</constant> is generated if <parameter>pname</parameter> is not one of the accepted tokens.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated if <parameter>program</parameter> is not the name of a program object for which
            <citerefentry><refentrytitle>glLinkProgram</refentrytitle></citerefentry> has been called in the past.
        </para>
    </refsect1>
    <refsect1 xml:id="notes"><title>Notes</title>
        <para>
            <function>glGetActiveUniformBlockiv</function> is available only if the GL version is 3.1 or greater.
        </para>
        <para>
            <constant>GL_UNIFORM_BLOCK_REFERENCED_BY_COMPUTE_SHADER</constant> is accepted only if the GL version is 4.3
            or greater.
        </para>
    </refsect1>
    <refsect1 xml:id="seealso"><title>See Also</title>
        <para>
            <citerefentry><refentrytitle>glGetActiveUniformBlockName</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glGetUniformBlockIndex</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glLinkProgram</refentrytitle></citerefentry>
        </para>
    </refsect1>
    <refsect1 xml:id="Copyright"><title>Copyright</title>
        <para>
            Copyright <trademark class="copyright"/> 2010-2013 Khronos Group.
            This material may be distributed subject to the terms and conditions set forth in
            the Open Publication License, v 1.0, 8 June 1999.
            <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://opencontent.org/openpub/">http://opencontent.org/openpub/</link>.
        </para>
    </refsect1>
</refentry>
