Bitwise flags

Bitwise flags are sets of variables, usually simple number values, which can be used to enable or disable specific usages or features of a method or other code structure. They can do this quickly and efficiently because they operate at the bit level. Related flags in the same group are generally given complementary values representing different bit positions in a single value (e.g. hexadecimal), so that multiple flag settings can be represented by a single value.

For example, in the WebGPU API, a GPUBuffer object instance is created using the GPUDevice.createBuffer() method. When invoking this method, you define a usage property in the descriptor containing one or more flags that enable different allowed usages of that buffer.

usage: GPUBufferUsage.COPY_SRC | GPUBufferUsage.MAP_WRITE;

These values are defined inside the same namespace, and each one has a hexadecimal value:

Usage flag Hexadecimal representation Decimal equivalent
GPUBufferUsage.MAP_READ 0x0001 1
GPUBufferUsage.MAP_WRITE 0x0002 2
GPUBufferUsage.COPY_SRC 0x0004 4
GPUBufferUsage.COPY_DST 0x0008 8
GPUBufferUsage.INDEX 0x0010 16
GPUBufferUsage.VERTEX 0x0020 32
GPUBufferUsage.UNIFORM 0x0040 64
GPUBufferUsage.STORAGE 0x0080 128
GPUBufferUsage.INDIRECT 0x0100 256
GPUBufferUsage.QUERY_RESOLVE 0x0200 512

When you query the GPUBuffer.usage property, you get a single decimal number returned, which is the sum of the different decimal values for the different usage flags. Returning to the above example, querying GPUBuffer.usage for the GPUBuffer created with the usage specified earlier would return the following:

  • GPUBufferUsage.COPY_SRC's decimal equivalent, 4
  • Add GPUBufferUsage.MAP_WRITE's decimal equivalent, 2
  • Equals 6.

Because of the values chosen for the different flags, each combination of values is unique, so the program can tell at a glance which flags are set from a single value. In addition, you can easily test which flags are set in the combined value using the bitwise and operator:

if (buffer.usage & GPUBufferUsage.MAP_WRITE) {
  // Buffer has MAP_WRITE usage

See also