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<refentry id="glGetActiveAttrib">
    <refmeta>
    <refentrytitle>glGetActiveAttrib</refentrytitle>
    <manvolnum>3G</manvolnum>
    </refmeta>
    <refnamediv>
    <refname>glGetActiveAttrib</refname>
    <refpurpose>Returns information about an active attribute variable for the specified program object</refpurpose>
    </refnamediv>
    <refsynopsisdiv><title>C Specification</title>
    <funcsynopsis>
        <funcprototype>
        <funcdef>void <function>glGetActiveAttrib</function></funcdef>
        <paramdef>GLuint <parameter>program</parameter></paramdef>
        <paramdef>GLuint <parameter>index</parameter></paramdef>
        <paramdef>GLsizei <parameter>bufSize</parameter></paramdef>
        <paramdef>GLsizei *<parameter>length</parameter></paramdef>
        <paramdef>GLint *<parameter>size</parameter></paramdef>
        <paramdef>GLenum *<parameter>type</parameter></paramdef>
        <paramdef>GLchar *<parameter>name</parameter></paramdef>
        </funcprototype>
    </funcsynopsis>
    </refsynopsisdiv>
    <refsect1 id="parameters"><title>Parameters</title>
    <variablelist>
        <varlistentry>
        <term><parameter>program</parameter></term>
        <listitem>
            <para>Specifies the program object to be
            queried.</para>
        </listitem>
        </varlistentry>
        <varlistentry>
        <term><parameter>index</parameter></term>
        <listitem>
            <para>Specifies the index of the attribute variable
            to be queried.</para>
        </listitem>
        </varlistentry>
        <varlistentry>
        <term><parameter>bufSize</parameter></term>
        <listitem>
            <para>Specifies the maximum number of characters
            OpenGL is allowed to write in the character buffer
            indicated by <parameter>name</parameter>.</para>
        </listitem>
        </varlistentry>
        <varlistentry>
        <term><parameter>length</parameter></term>
        <listitem>
            <para>Returns the number of characters actually
            written by OpenGL in the string indicated by
            <parameter>name</parameter> (excluding the null
            terminator) if a value other than
            <constant>NULL</constant> is passed.</para>
        </listitem>
        </varlistentry>
        <varlistentry>
        <term><parameter>size</parameter></term>
        <listitem>
            <para>Returns the size of the attribute
            variable.</para>
        </listitem>
        </varlistentry>
        <varlistentry>
        <term><parameter>type</parameter></term>
        <listitem>
            <para>Returns the data type of the attribute
            variable.</para>
        </listitem>
        </varlistentry>
        <varlistentry>
        <term><parameter>name</parameter></term>
        <listitem>
            <para>Returns a null terminated string containing
            the name of the attribute variable.</para>
        </listitem>
        </varlistentry>
    </variablelist>
    </refsect1>
    <refsect1 id="description"><title>Description</title>
    <para><function>glGetActiveAttrib</function> returns information
    about an active attribute variable in the program object
    specified by <parameter>program</parameter>. The number of
    active attributes can be obtained by calling
    <citerefentry><refentrytitle>glGetProgram</refentrytitle></citerefentry>
    with the value <constant>GL_ACTIVE_ATTRIBUTES</constant>. A
    value of 0 for <parameter>index</parameter> selects the first
    active attribute variable. Permissible values for
    <parameter>index</parameter> range from 0 to the number of
    active attribute variables minus 1.</para>

    <para>A vertex shader may use either built-in attribute
    variables, user-defined attribute variables, or both. Built-in
    attribute variables have a prefix of &quot;gl_&quot; and
    reference conventional OpenGL vertex attribtes (e.g.,
    <parameter>gl_Vertex</parameter>,
    <parameter>gl_Normal</parameter>, etc., see the OpenGL Shading
    Language specification for a complete list.) User-defined
    attribute variables have arbitrary names and obtain their values
    through numbered generic vertex attributes. An attribute
    variable (either built-in or user-defined) is considered active
    if it is determined during the link operation that it may be
    accessed during program execution. Therefore,
    <parameter>program</parameter> should have previously been the
    target of a call to
    <citerefentry><refentrytitle>glLinkProgram</refentrytitle></citerefentry>,
    but it is not necessary for it to have been linked
    successfully.</para>

    <para>The size of the character buffer required to store the
    longest attribute variable name in
    <parameter>program</parameter> can be obtained by calling
    <citerefentry><refentrytitle>glGetProgram</refentrytitle></citerefentry>
    with the value
    <constant>GL_ACTIVE_ATTRIBUTE_MAX_LENGTH</constant>. This value
    should be used to allocate a buffer of sufficient size to store
    the returned attribute name. The size of this character buffer
    is passed in <parameter>bufSize</parameter>, and a pointer to
    this character buffer is passed in
    <parameter>name</parameter>.</para>

    <para><function>glGetActiveAttrib</function> returns the name of
    the attribute variable indicated by
    <parameter>index</parameter>, storing it in the character buffer
    specified by <parameter>name</parameter>. The string returned
    will be null terminated. The actual number of characters written
    into this buffer is returned in <parameter>length</parameter>,
    and this count does not include the null termination character.
    If the length of the returned string is not required, a value of
    <constant>NULL</constant> can be passed in the
    <parameter>length</parameter> argument.</para>

    <para>The <parameter>type</parameter> argument specifies a
    pointer to a variable into which the attribute variable's data type
    will be written. The symbolic
    constants <constant>GL_FLOAT</constant>,
    <constant>GL_FLOAT_VEC2</constant>,
    <constant>GL_FLOAT_VEC3</constant>,
    <constant>GL_FLOAT_VEC4</constant>,
    <constant>GL_FLOAT_MAT2</constant>,
    <constant>GL_FLOAT_MAT3</constant>,
    <constant>GL_FLOAT_MAT4</constant>,
    <constant>GL_FLOAT_MAT2x3</constant>,
    <constant>GL_FLOAT_MAT2x4</constant>,
    <constant>GL_FLOAT_MAT3x2</constant>,
    <constant>GL_FLOAT_MAT3x4</constant>,
    <constant>GL_FLOAT_MAT4x2</constant>,
    <constant>GL_FLOAT_MAT4x3</constant>,
    <constant>GL_INT</constant>,
    <constant>GL_INT_VEC2</constant>,
    <constant>GL_INT_VEC3</constant>,
    <constant>GL_INT_VEC4</constant>,
    <constant>GL_UNSIGNED_INT_VEC</constant>,
    <constant>GL_UNSIGNED_INT_VEC2</constant>,
    <constant>GL_UNSIGNED_INT_VEC3</constant>,
    <constant>GL_UNSIGNED_INT_VEC4</constant>,
    <constant>DOUBLE</constant>,
    <constant>DOUBLE_VEC2</constant>,
    <constant>DOUBLE_VEC3</constant>,
    <constant>DOUBLE_VEC4</constant>,
    <constant>DOUBLE_MAT2</constant>,
    <constant>DOUBLE_MAT3</constant>,
    <constant>DOUBLE_MAT4</constant>,
    <constant>DOUBLE_MAT2x3</constant>,
    <constant>DOUBLE_MAT2x4</constant>,
    <constant>DOUBLE_MAT3x2</constant>,
    <constant>DOUBLE_MAT3x4</constant>,
    <constant>DOUBLE_MAT4x2</constant>, or
    <constant>DOUBLE_MAT4x3</constant>
     may be returned. The
    <parameter>size</parameter> argument will return the size of the
    attribute, in units of the type returned in
    <parameter>type</parameter>.</para>

    <para>The list of active attribute variables may include both
    built-in attribute variables (which begin with the prefix
    &quot;gl_&quot;) as well as user-defined attribute variable
    names.</para>

    <para>This function will return as much information as it can
    about the specified active attribute variable. If no information
    is available, <parameter>length</parameter> will be 0, and
    <parameter>name</parameter> will be an empty string. This
    situation could occur if this function is called after a link
    operation that failed. If an error occurs, the return values
    <parameter>length</parameter>, <parameter>size</parameter>,
    <parameter>type</parameter>, and <parameter>name</parameter>
    will be unmodified.</para>
    </refsect1>
    <refsect1 id="errors"><title>Errors</title>
    <para><constant>GL_INVALID_VALUE</constant> is generated if
    <parameter>program</parameter> is not a value generated by
    OpenGL.</para>

    <para><constant>GL_INVALID_OPERATION</constant> is generated if
    <parameter>program</parameter> is not a program object.</para>

    <para><constant>GL_INVALID_VALUE</constant> is generated if
    <parameter>index</parameter> is greater than or equal to the
    number of active attribute variables in
    <parameter>program</parameter>.</para>

    <para><constant>GL_INVALID_VALUE</constant> is generated if
    <parameter>bufSize</parameter> is less than 0.</para>
    </refsect1>
    <refsect1 id="associatedgets"><title>Associated Gets</title>
    <para><citerefentry><refentrytitle>glGet</refentrytitle></citerefentry>
    with argument <constant>GL_MAX_VERTEX_ATTRIBS</constant>.</para>

    <para><citerefentry><refentrytitle>glGetProgram</refentrytitle></citerefentry>
    with argument <constant>GL_ACTIVE_ATTRIBUTES</constant> or
    <constant>GL_ACTIVE_ATTRIBUTE_MAX_LENGTH</constant>.</para>

    <para><citerefentry><refentrytitle>glIsProgram</refentrytitle></citerefentry></para>
    </refsect1>
    <refsect1 id="seealso"><title>See Also</title>
    <para><citerefentry><refentrytitle>glBindAttribLocation</refentrytitle></citerefentry>,
    <citerefentry><refentrytitle>glLinkProgram</refentrytitle></citerefentry>,
    <citerefentry><refentrytitle>glVertexAttrib</refentrytitle></citerefentry>,
    <citerefentry><refentrytitle>glVertexAttribPointer</refentrytitle></citerefentry></para>
    </refsect1>
    <refsect1 id="Copyright"><title>Copyright</title>
        <para>
            Copyright <trademark class="copyright"></trademark> 2003-2005 3Dlabs Inc. Ltd. 
            Copyright <trademark class="copyright"></trademark> 2010 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.
            <ulink url="http://opencontent.org/openpub/">http://opencontent.org/openpub/</ulink>.
        </para>
    </refsect1>
</refentry>
