The __global__ Function Qualifier

Your CUDA file holds both CPU and GPU code. A tiny keyword tells the compiler which side a function belongs to. That keyword is the function qualifier. ⚡

The __global__ qualifier marks a function as a GPU kernel: code the CPU launches but the GPU actually runs across many threads.

A __global__ function is the only kind the host can call but the device executes. It is the bridge between your two worlds.

A kernel must return void. It cannot hand a value back to the CPU, so results travel out through memory instead.

When you launch a __global__ kernel, the CPU does not wait. The call returns instantly while the GPU works in the background. 🚀

One __global__ definition runs on thousands of threads at once. You write the body once; the GPU clones the work across cores.

Calling a kernel needs special triple angle brackets that set how many threads run. Normal parentheses alone will not do.

Kernel arguments are copied to the device. Pass device pointers for arrays, since a host pointer would be meaningless on the GPU.

Do not confuse them: __global__ is a launchable entry point, while __device__ functions are helpers callable only from GPU code.

A __global__ kernel cannot call itself directly the way host functions do. Classic recursion belongs to device helper functions instead.

Mastering __global__ unlocks everything ahead: every kernel you write, launch, and tune starts with this one little keyword.

Let us check your grasp of the __global__ qualifier.

You learned that __global__ marks a GPU kernel: called by the host, run by the device, returns void, and launched with triple angle brackets. 🎉