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<refentry id="glGetActiveUniform">
    <refmeta>
	<refentrytitle>glGetActiveUniform</refentrytitle>
	<manvolnum>3G</manvolnum>
    </refmeta>
    <refnamediv>
	<refname>glGetActiveUniform</refname>
	<refpurpose>return information about an active uniform variable</refpurpose>
    </refnamediv>
    <refsynopsisdiv><title>C Specification</title>
	<funcsynopsis>
	    <funcprototype>
		<funcdef>void <function>glGetActiveUniform</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 uniform 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 uniform
		    variable.</para>
		</listitem>
	    </varlistentry>
	    <varlistentry>
		<term><parameter>type</parameter></term>
		<listitem>
		    <para>Returns the data type of the uniform
		    variable.</para>
		</listitem>
	    </varlistentry>
	    <varlistentry>
		<term><parameter>name</parameter></term>
		<listitem>
		    <para>Returns a null terminated string containing
		    the name of the uniform variable.</para>
		</listitem>
	    </varlistentry>
	</variablelist>
    </refsect1>
    <refsect1 id="description"><title>Description</title>
	<para><function>glGetActiveUniform</function> returns
	information about an active uniform variable in the program
	object specified by <parameter>program</parameter>. The number
	of active uniform variables can be obtained by calling
	<citerefentry><refentrytitle>glGetProgramiv</refentrytitle></citerefentry>
	with the value <constant>GL_ACTIVE_UNIFORMS</constant>. A value
	of 0 for <parameter>index</parameter> selects the first active
	uniform variable. Permissible values for
	<parameter>index</parameter> range from 0 to the number of
	active uniform variables minus 1.</para>

	<para>Shaders may use either built-in uniform variables,
	user-defined uniform variables, or both. Built-in uniform
	variables have a prefix of &quot;gl_&quot; and reference
	existing OpenGL state or values derived from such state (e.g.,
	<parameter>gl_DepthRange</parameter>).
	User-defined uniform variables have arbitrary names and obtain
	their values from the application through calls to
	<citerefentry><refentrytitle>glUniform</refentrytitle></citerefentry>.
	A uniform variable (either built-in or user-defined) is
	considered active if it is determined during the link operation
	that it may be accessed during program execution. Therefore,
	<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 uniform variable name in <parameter>program</parameter>
	can be obtained by calling
	<citerefentry><refentrytitle>glGetProgramiv</refentrytitle></citerefentry>
	with the value
	<constant>GL_ACTIVE_UNIFORM_MAX_LENGTH</constant>. This value
	should be used to allocate a buffer of sufficient size to store
	the returned uniform variable name. The size of this character
	buffer is passed in <parameter>bufSize</parameter>, and a
	pointer to this character buffer is passed in
	<parameter>name.</parameter></para>

	<para><function>glGetActiveUniform</function> returns the name
	of the uniform 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 uniform 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_INT</constant>,
	<constant>GL_INT_VEC2</constant>,
	<constant>GL_INT_VEC3</constant>,
	<constant>GL_INT_VEC4</constant>,
	<constant>GL_BOOL</constant>,
	<constant>GL_BOOL_VEC2</constant>,
	<constant>GL_BOOL_VEC3</constant>,
	<constant>GL_BOOL_VEC4</constant>,
	<constant>GL_FLOAT_MAT2</constant>,
	<constant>GL_FLOAT_MAT3</constant>,
	<constant>GL_FLOAT_MAT4</constant>,
	<constant>GL_SAMPLER_2D</constant>, or
	<constant>GL_SAMPLER_CUBE</constant>
	may be returned.</para>

	<para>If one or more elements of an array are active, the name
	of the array is returned in <parameter>name</parameter>, the
	type is returned in <parameter>type</parameter>, and the
	<parameter>size</parameter> parameter returns the highest array
	element index used, plus one, as determined by the compiler
	and/or linker. Only one active uniform variable will be reported
	for a uniform array.</para>

	<para>Uniform variables that are declared as structures or
	arrays of structures will not be returned directly by this
	function. Instead, each of these uniform variables will be
	reduced to its fundamental components containing the
	&quot;.&quot; and &quot;[]&quot; operators such that each of the
	names is valid as an argument to
	<citerefentry><refentrytitle>glGetUniformLocation</refentrytitle></citerefentry>.
	Each of these reduced uniform variables is counted as one active
	uniform variable and is assigned an index. A valid name cannot
	be a structure, an array of structures, or a subcomponent of a
	vector or matrix.</para>

	<para>The size of the uniform variable will be returned in
	<parameter>size</parameter>. Uniform variables other than arrays
	will have a size of 1. Structures and arrays of structures will
	be reduced as described earlier, such that each of the names
	returned will be a data type in the earlier list. If this
	reduction results in an array, the size returned will be as
	described for uniform arrays; otherwise, the size returned will
	be 1.</para>

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

	<para>This function will return as much information as it can
	about the specified active uniform 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 uniform 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_UNIFORM_VECTORS
	</constant> or
	<constant>GL_MAX_FRAGMENT_UNIFORM_VECTORS</constant>.</para>

	<para><citerefentry><refentrytitle>glGetProgramiv</refentrytitle></citerefentry>
	with argument <constant>GL_ACTIVE_UNIFORMS</constant> or
	<constant>GL_ACTIVE_UNIFORM_MAX_LENGTH</constant>.</para>

	<para><citerefentry><refentrytitle>glIsProgram</refentrytitle></citerefentry></para>
    </refsect1>
    <refsect1 id="seealso"><title>See Also</title>
	<para><citerefentry><refentrytitle>glGetActiveAttrib</refentrytitle></citerefentry>,
	<citerefentry><refentrytitle>glGetUniform</refentrytitle></citerefentry>,
	<citerefentry><refentrytitle>glGetUniformLocation</refentrytitle></citerefentry>,
	<citerefentry><refentrytitle>glLinkProgram</refentrytitle></citerefentry>,
	<citerefentry><refentrytitle>glUniform</refentrytitle></citerefentry>,
	<citerefentry><refentrytitle>glUseProgram</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>
