GLMAPFLUSHEXPLICITBIT

GLMAPFLUSHEXPLICITBIT, also known as GL_MAP_FLUSH_EXPLICIT_BIT, is a bit flag used in OpenGL to control how data written through mapped buffer ranges is flushed to the graphics subsystem. It is part of the set of map access bits used with buffer mapping functions such as glMapBufferRange.

When this flag is included in the access mask passed to glMapBufferRange, the application takes responsibility for explicitly flushing written data. In practice, this means that writes to the mapped memory are not automatically made visible to the GL by the driver; instead, the programmer must call glFlushMappedBufferRange with the appropriate offset and length to ensure those changes are visible to subsequent GL operations. This explicit flushing enables finer-grained synchronization and can reduce unwanted stalls in certain dynamic or streaming workloads.

GL_MAP_FLUSH_EXPLICIT_BIT is commonly used in conjunction with other related flags, such as GL_MAP_PERSISTENT_BIT, to support persistent mapped buffers. In persistent mapping, a buffer remains mapped for an extended period, and explicit flushing provides you with precise control over when data becomes visible to the GPU, avoiding implicit synchronization that could otherwise cause performance bottlenecks. If GL_MAP_FLUSH_EXPLICIT_BIT is not used, the driver may flush automatically at certain events (such as unmap), depending on the specific implementation and other flags chosen.

The flag is defined within the OpenGL specification as part of the map buffer range extension (ARB_map_buffer_range). It is typically encountered by developers implementing high-performance, dynamic data uploads to GPU buffers, such as streaming vertex data or dynamic uniform buffers, where predictable synchronization behavior is important.