glGetBufferSubData — returns a subset of a buffer object's data store
| void glGetBufferSubData( | GLenum target, | 
| GLintptr offset, | |
| GLsizeiptr size, | |
| GLvoid * data ); | 
target
                    Specifies the target buffer object.
                    The symbolic constant must be
                    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_RESULT_BUFFER,
                    GL_TEXTURE_BUFFER,
                    GL_TRANSFORM_FEEDBACK_BUFFER, or
                    GL_UNIFORM_BUFFER.
                
offsetSpecifies the offset into the buffer object's data store from which data will be returned, measured in bytes.
sizeSpecifies the size in bytes of the data store region being returned.
dataSpecifies a pointer to the location where buffer object data is returned.
            glGetBufferSubData returns some or all of the data from the buffer object currently 
            bound to target. Data starting at byte offset offset and
            extending for size bytes is copied from the data store to the memory pointed to by
            data. An error is thrown if the buffer object is currently mapped, or if
            offset and size together define a range beyond the bounds 
            of the buffer object's data store.
        
            If an error is generated,
            no change is made to the contents of data.
        
            The GL_ATOMIC_COUNTER_BUFER target is available only if the GL version is 4.2 or greater.
        
            The GL_DISPATCH_INDIRECT_BUFFER target is available only if the GL version is 4.3 or greater.
        
            The GL_QUERY_RESULT_BUFFER target is available only if the GL version is 4.4 or greater.
        
            GL_INVALID_ENUM is generated if target is not 
            GL_ARRAY_BUFFER, GL_ELEMENT_ARRAY_BUFFER, 
            GL_PIXEL_PACK_BUFFER, or GL_PIXEL_UNPACK_BUFFER.
        
            GL_INVALID_VALUE is generated if offset or 
            size is negative, or if together they define a region of memory
            that extends beyond the buffer object's allocated data store.
        
            GL_INVALID_OPERATION is generated if the reserved buffer object name 0 is bound to target.
        
            GL_INVALID_OPERATION is generated if the buffer object being queried is mapped.
        
Copyright © 2005 Addison-Wesley. Copyright © 2011-2013 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/.