NAME

       glMapBufferRange - map a section of a buffer object's data store

C SPECIFICATION

       void *glMapBufferRange(GLenum target, GLintptr offset, GLsizeiptr length, GLbitfield access);

PARAMETERS

       target
           Specifies a binding to which the target buffer is bound.

       offset
           Specifies a the starting offset within the buffer of the range to be mapped.

       length
           Specifies a length of the range to be mapped.

       access
           Specifies a combination of access flags indicating the desired access to the range.

DESCRIPTION

       glMapBufferRange maps all or part of the data store of a buffer object into the client's address space.
       target specifies the target to which the buffer is bound and must be one of GL_ARRAY_BUFFER,
       GL_ATOMIC_COUNTER_BUFFER, GL_COPY_READ_BUFFER, GL_COPY_WRITE_BUFFER, GL_DRAW_INDIRECT_BUFFER,
       GL_DISPATCH_INDIRECT_BUFFER, GL_ELEMENT_ARRAY_BUFFER, GL_PIXEL_PACK_BUFFER, GL_PIXEL_UNPACK_BUFFER,
       GL_QUERY_BUFFER, GL_TEXTURE_BUFFER, GL_TRANSFORM_FEEDBACK_BUFFER, GL_UNIFORM_BUFFER or
       GL_SHADER_STORAGE_BUFFER.  offset and length indicate the range of data in the buffer object htat is to
       be mapped, in terms of basic machine units.  access is a bitfield containing flags which describe the
       requested mapping. These flags are described below.

       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 access:

       •   GL_MAP_READ_BIT 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.

       •   GL_MAP_WRITE_BIT 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.

       Furthermore, the following optional flag bits in access may be used to modify the mapping:

       •   GL_MAP_INVALIDATE_RANGE_BIT 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 GL_MAP_READ_BIT.

       •   GL_MAP_INVALIDATE_BUFFER_BIT 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 GL_MAP_READ_BIT.

       •   GL_MAP_FLUSH_EXPLICIT_BIT 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
           glFlushMappedBufferRange(). 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 GL_MAP_WRITE_BIT. When this option is selected, flushing is strictly
           limited to regions that are explicitly indicated with calls to glFlushMappedBufferRange() prior to
           unmap; if this option is not selected glUnmapBuffer() will automatically flush the entire mapped
           range when called.

       •   GL_MAP_UNSYNCHRONIZED_BIT indicates that the GL should not attempt to synchronize pending operations
           on the buffer prior to returning from glMapBufferRange. 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.

       •   GL_MAP_PERSISTENT_BIT 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
           glBufferStorage() command in which the GL_MAP_PERSISTENT_BIT was also set.

       •   GL_MAP_COHERENT_BIT 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
           glFlushMappedBufferRange() or glMemoryBarrier().

       If an error occurs, glMapBufferRange returns a NULL pointer. If no error occurs, the returned pointer
       will reflect an alignment of at least GL_MIN_MAP_BUFFER_ALIGNMENT basic machine units. The value of
       GL_MIN_MAP_BUFFER_ALIGNMENT can be retrieved by calling glGet() with pname set to
       GL_MIN_MAP_BUFFER_ALIGNMENT and must be a power of two that is at least 64. Subtracting offset from this
       returned pointed will always produce a multiple of GL_MIN_MAP_BUFFER_ALINMENT.

NOTES

       Alignment of the returned pointer is guaranteed only if the version of the GL version is 4.2 or greater.
       Also, the GL_ATOMIC_COUNTER_BUFFER target is accepted only if the GL version is 4.2 or greater.

       The GL_DISPATCH_INDIRECT_BUFFER and GL_SHADER_STORAGE_BUFFER targets are accepted only if the GL version
       is 4.3 or greater.

       The GL_QUERY_BUFFER target is available only if the GL version is 4.4 or greater.

       The GL_MAP_PERSISTENT_BIT and GL_MAP_COHERENT_BIT flags are available only if the GL version is 4.4 or
       greater.

ERRORS

       GL_INVALID_VALUE is generated if either of offset or length is negative, or if offset + length is greater
       than the value of GL_BUFFER_SIZE.

       GL_INVALID_VALUE is generated if access has any bits set other than those defined above.

       GL_INVALID_OPERATION is generated for any of the following conditions:

       •   The buffer is already in a mapped state.

       •   Neither GL_MAP_READ_BIT or GL_MAP_WRITE_BIT is set.

       •   GL_MAP_READ_BIT is set and any of GL_MAP_INVALIDATE_RANGE_BIT, GL_MAP_INVALIDATE_BUFFER_BIT, or
           GL_MAP_UNSYNCHRONIZED_BIT is set.

       •   GL_MAP_FLUSH_EXPLICIT_BIT is set and GL_MAP_WRITE_BIT is not set.

       •   Any of GL_MAP_READ_BIT, GL_MAP_WRITE_BIT, GL_MAP_PERSISTENT_BIT, or GL_MAP_COHERENT_BIT are included
           in access, but the same bit is not included in the buffer's storage flags.

       GL_OUT_OF_MEMORY is generated if glMapBufferRange fails because memory for the mapping could not be
       obtained.

SEE ALSO

       glMapBuffer(), glFlushMappedBufferRange(), glBindBuffer(), glBufferStorage()

COPYRIGHT

       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.  http://opencontent.org/openpub/.

AUTHORS

       opengl.org