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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook MathML Module V1.1b1//EN"
"http://www.oasis-open.org/docbook/xml/mathml/1.1CR1/dbmathml.dtd">
<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 "gl_" 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 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>,
<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>, or
<constant>GL_FLOAT_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
"gl_") 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="notes"><title>Notes</title>
<para><function>glGetActiveAttrib</function>
is available only if the GL version is 2.0 or greater.</para>
<para><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>, and
<constant>GL_FLOAT_MAT4x3</constant>
will only be returned as a <parameter>type</parameter>
if the GL version is 2.1 or greater.</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_OPERATION</constant> is generated if
<function>glGetActiveAttrib</function> is executed between the
execution of
<citerefentry><refentrytitle>glBegin</refentrytitle></citerefentry>
and the corresponding execution of
<citerefentry><refentrytitle>glEnd</refentrytitle></citerefentry>.</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.
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>