fcontext

fcontext is a low-level primitive used to implement user-space context switching, enabling lightweight execution units such as fibers or coroutines without involving the operating system scheduler. It is most commonly associated with the Boost.Context project, and it has served as a building block for various coroutine libraries that need fast, cooperative switching between execution contexts.

The core concept centers on an opaque type, typically named fcontext_t, which represents a saved machine state for a thread of execution. The API commonly provides two main operations: make_fcontext, which creates a new execution context bound to a stack and an entry function to run when the context is activated; and jump_fcontext, which saves the current context and transfers control to another fcontext_t, optionally passing an argument. When the target context yields back or completes, control can return to the caller, resuming after the jump_fcontext invocation.

In practice, using fcontext involves allocating separate stacks for each context, creating contexts for the functions that will run in those stacks, and then performing explicit jumps to switch between contexts. Because the mechanism operates at the level of machine state, its implementation is platform-specific, relying on careful management of the stack pointer, instruction pointer, and callee-saved registers. As a result, it is typically non-preemptive and requires the programmer to determine when to yield control.

History and usage: fcontext has played a major role in early user-space threading and coroutine frameworks, offering a portable means to achieve cooperative multitasking across platforms. It remains relevant in projects that prioritize raw performance and fine-grained control over scheduling. Today, it is often seen as a foundation that higher-level coroutine abstractions have superseded in modern languages, with alternative approaches such as language-level coroutines and other stack-management techniques also available.