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<refentry xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="glMapBufferRange">
    <info>
        <copyright>
            <year>2013</year>
            <holder>Khronos Group</holder>
        </copyright>
    </info>
    <refmeta>
        <refentrytitle>glMapBufferRange</refentrytitle>
        <manvolnum>3G</manvolnum>
    </refmeta>
    <refnamediv>
        <refname>glMapBufferRange</refname>
        <refpurpose>map a section of a buffer object's data store</refpurpose>
    </refnamediv>
    <refsynopsisdiv><title>C Specification</title>
        <funcsynopsis>
            <funcprototype>
                <funcdef>void *<function>glMapBufferRange</function></funcdef>
                <paramdef>GLenum <parameter>target</parameter></paramdef>
                <paramdef>GLintptr <parameter>offset</parameter></paramdef>
                <paramdef>GLsizeiptr <parameter>length</parameter></paramdef>
                <paramdef>GLbitfield <parameter>access</parameter></paramdef>
            </funcprototype>
        </funcsynopsis>
    </refsynopsisdiv>
    <refsect1 xml:id="parameters"><title>Parameters</title>
        <variablelist>
        <varlistentry>
            <term><parameter>target</parameter></term>
            <listitem>
                <para>
                    Specifies a binding to which the target buffer is bound.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>offset</parameter></term>
            <listitem>
                <para>
                    Specifies a the starting offset within the buffer of the range to be mapped.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>length</parameter></term>
            <listitem>
                <para>
                    Specifies a length of the range to be mapped.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>access</parameter></term>
            <listitem>
                <para>
                    Specifies a combination of access flags indicating the desired access to the range.
                </para>
            </listitem>
        </varlistentry>
        </variablelist>
    </refsect1>
    <refsect1 xml:id="description"><title>Description</title>
        <para>
            <function>glMapBufferRange</function> maps all or part of the data store of a buffer object into the client's address
            space. <parameter>target</parameter> specifies the target to which the buffer is bound and must be one of <constant>GL_ARRAY_BUFFER</constant>,
            <constant>GL_ATOMIC_COUNTER_BUFFER</constant>,
            <constant>GL_COPY_READ_BUFFER</constant>, <constant>GL_COPY_WRITE_BUFFER</constant>, <constant>GL_DRAW_INDIRECT_BUFFER</constant>,
            <constant>GL_DISPATCH_INDIRECT_BUFFER</constant>, <constant>GL_ELEMENT_ARRAY_BUFFER</constant>,
            <constant>GL_PIXEL_PACK_BUFFER</constant>, <constant>GL_PIXEL_UNPACK_BUFFER</constant>, <constant>GL_QUERY_BUFFER</constant>, <constant>GL_TEXTURE_BUFFER</constant>,
            <constant>GL_TRANSFORM_FEEDBACK_BUFFER</constant>, <constant>GL_UNIFORM_BUFFER</constant> or <constant>GL_SHADER_STORAGE_BUFFER</constant>. <parameter>offset</parameter> and
            <parameter>length</parameter> indicate the range of data in the buffer object htat is to be mapped, in terms of basic machine units.
            <parameter>access</parameter> is a bitfield containing flags which describe the requested mapping. These flags are described below.
        </para>
        <para>
            If no error occurs, a pointer to the beginning of the mapped range is returned once all pending operations on that buffer have
            completed, and may be used to modify and/or query the corresponding range of the buffer, according to the following flag bits set
            in <parameter>access</parameter>:
            <itemizedlist>
                <listitem>
                    <para>
                        <constant>GL_MAP_READ_BIT</constant> indicates that the returned pointer may be used to read
                        buffer object data. No GL error is generated if the pointer is used to query
                        a mapping which excludes this flag, but the result is undefined and system
                        errors (possibly including program termination) may occur.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <constant>GL_MAP_WRITE_BIT</constant> indicates that the returned pointer may be used to modify
                        buffer object data. No GL error is generated if the pointer is used to modify
                        a mapping which excludes this flag, but the result is undefined and system
                        errors (possibly including program termination) may occur.
                    </para>
                </listitem>
            </itemizedlist>
        </para>
        <para>
            Furthermore, the following <emphasis>optional</emphasis> flag bits in <parameter>access</parameter> may be used to modify the mapping:
            <itemizedlist>
                <listitem>
                    <para>
                        <constant>GL_MAP_INVALIDATE_RANGE_BIT</constant> indicates that the previous contents of the
                        specified range may be discarded. Data within this range are undefined with
                        the exception of subsequently written data. No GL error is generated if sub-
                        sequent GL operations access unwritten data, but the result is undefined and
                        system errors (possibly including program termination) may occur. This flag
                        may not be used in combination with <constant>GL_MAP_READ_BIT</constant>.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <constant>GL_MAP_INVALIDATE_BUFFER_BIT</constant> indicates that the previous contents of the
                        entire buffer may be discarded. Data within the entire buffer are undefined
                        with the exception of subsequently written data. No GL error is generated if
                        subsequent GL operations access unwritten data, but the result is undefined
                        and system errors (possibly including program termination) may occur. This
                        flag may not be used in combination with <constant>GL_MAP_READ_BIT</constant>.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <constant>GL_MAP_FLUSH_EXPLICIT_BIT</constant> indicates that one or more discrete subranges
                        of the mapping may be modified.  When this flag is set, modifications to
                        each subrange must be explicitly flushed by calling <citerefentry><refentrytitle>glFlushMappedBufferRange</refentrytitle></citerefentry>.
                        No GL error is set if a subrange of the mapping is modified and
                        not flushed, but data within the corresponding subrange of the buffer are undefined.
                        This flag may only be used in conjunction with <constant>GL_MAP_WRITE_BIT</constant>.
                        When this option is selected, flushing is strictly limited to regions that are
                        explicitly indicated with calls to <citerefentry><refentrytitle>glFlushMappedBufferRange</refentrytitle></citerefentry>
                        prior to unmap; if this option is not selected <citerefentry><refentrytitle>glUnmapBuffer</refentrytitle></citerefentry>
                        will automatically flush the entire mapped range when called.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <constant>GL_MAP_UNSYNCHRONIZED_BIT</constant> indicates that the GL should not attempt to
                        synchronize pending operations on the buffer prior to returning from <function>glMapBufferRange</function>.
                        No GL error is generated if pending operations which source or modify the buffer overlap the mapped region,
                        but the result of such previous and any subsequent operations is undefined.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <constant>GL_MAP_PERSISTENT_BIT</constant> indicates that the mapping is to be made in a persistent
                        fassion and that the client intends to hold and use the returned pointer during subsequent GL operation. It is not
                        an error to call drawing commands (render) while buffers are mapped using this flag. It is an error to
                        specify this flag if the buffer's data store was not allocated through a call to the <citerefentry><refentrytitle>glBufferStorage</refentrytitle></citerefentry>
                        command in which the <constant>GL_MAP_PERSISTENT_BIT</constant> was also set.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <constant>GL_MAP_COHERENT_BIT</constant> indicates that a persistent mapping is also to be coherent.
                        Coherent maps guarantee that the effect of writes to a buffer's data store by either the client or server will
                        eventually become visible to the other without further intervention from the application. In the absence
                        of this bit, persistent mappings are not coherent and modified ranges of the buffer store must be explicitly
                        communicated to the GL, either by unmapping the buffer, or through a call to
                        <citerefentry><refentrytitle>glFlushMappedBufferRange</refentrytitle></citerefentry> or <citerefentry><refentrytitle>glMemoryBarrier</refentrytitle></citerefentry>.
                    </para>
                </listitem>
            </itemizedlist>
        </para>
        <para>
            If an error occurs, <function>glMapBufferRange</function> returns a <code>NULL</code> pointer.
            If no error occurs, the returned pointer will reflect an alignment of at least <constant>GL_MIN_MAP_BUFFER_ALIGNMENT</constant>
            basic machine units. The value of <constant>GL_MIN_MAP_BUFFER_ALIGNMENT</constant> can be retrieved by calling
            <citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with <parameter>pname</parameter> set to
            <constant>GL_MIN_MAP_BUFFER_ALIGNMENT</constant> and must be a power of two that is at least 64. Subtracting <parameter>offset</parameter>
            from this returned pointed will always produce a multiple of <constant>GL_MIN_MAP_BUFFER_ALINMENT</constant>.
        </para>
    </refsect1>
    <refsect1 xml:id="notes"><title>Notes</title>
        <para>
            Alignment of the returned pointer is guaranteed only if the version
            of the GL version is 4.2 or greater. Also, the <constant>GL_ATOMIC_COUNTER_BUFFER</constant>
            target is accepted only if the GL version is 4.2 or greater.
        </para>
        <para>
            The <constant>GL_DISPATCH_INDIRECT_BUFFER</constant> and <constant>GL_SHADER_STORAGE_BUFFER</constant> targets are accepted only if the
            GL version is 4.3 or greater.
        </para>
        <para>
            The <constant>GL_QUERY_BUFFER</constant> target is available only if the GL version is 4.4 or greater.
        </para>
        <para>
            The <constant>GL_MAP_PERSISTENT_BIT</constant> and <constant>GL_MAP_COHERENT_BIT</constant> flags are available
            only if the GL version is 4.4 or greater.
        </para>
    </refsect1>
    <refsect1 xml:id="errors"><title>Errors</title>
        <para>
            <constant>GL_INVALID_VALUE</constant> is generated if either of <parameter>offset</parameter> or <parameter>length</parameter> is negative,
            or if <parameter>offset</parameter> + <parameter>length</parameter> is greater than the value of <constant>GL_BUFFER_SIZE</constant>.
        </para>
        <para>
            <constant>GL_INVALID_VALUE</constant> is generated if <parameter>access</parameter> has any bits set other than those defined above.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated for any of the following conditions:
            <itemizedlist>
                <listitem>
                    <para>
                        The buffer is already in a mapped state.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        Neither <constant>GL_MAP_READ_BIT</constant> or <constant>GL_MAP_WRITE_BIT</constant> is set.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <constant>GL_MAP_READ_BIT</constant> is set and any of <constant>GL_MAP_INVALIDATE_RANGE_BIT</constant>,
                        <constant>GL_MAP_INVALIDATE_BUFFER_BIT</constant>, or <constant>GL_MAP_UNSYNCHRONIZED_BIT</constant> is set.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        <constant>GL_MAP_FLUSH_EXPLICIT_BIT</constant> is set and <constant>GL_MAP_WRITE_BIT</constant> is not set.
                    </para>
                </listitem>
                <listitem>
                    <para>
                        Any of <constant>GL_MAP_READ_BIT</constant>, <constant>GL_MAP_WRITE_BIT</constant>, <constant>GL_MAP_PERSISTENT_BIT</constant>,
                        or <constant>GL_MAP_COHERENT_BIT</constant> are included in <parameter>access</parameter>, but the same bit
                        is not included in the buffer's storage flags.
                    </para>
                </listitem>
            </itemizedlist>
        </para>
        <para>
            <constant>GL_OUT_OF_MEMORY</constant> is generated if <function>glMapBufferRange</function> fails because memory for the
            mapping could not be obtained.
        </para>
    </refsect1>
    <refsect1 xml:id="seealso"><title>See Also</title>
        <para>
            <citerefentry><refentrytitle>glMapBuffer</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glFlushMappedBufferRange</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glBindBuffer</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glBufferStorage</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>
