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<refentry id="glGetActiveAttrib">
    <refmeta>
	<refentrytitle>glGetActiveAttrib</refentrytitle>
	<manvolnum>3G</manvolnum>
    </refmeta>
    <refnamediv>
	<refname>glGetActiveAttrib</refname>
	<refpurpose>return information about an active attribute variable</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>glGetProgramiv</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>Attribute variables have arbitrary names and obtain their values
	through numbered generic vertex attributes. An attribute
	variable 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>glGetProgramiv</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 will return a
	pointer to the attribute variable's data type. 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>, or
	<constant>GL_FLOAT_MAT4</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>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>glGetProgramiv</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>glGetActiveUniform</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. 
            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>
