setcontext is one of a family of C library functions (the others being getcontext, makecontext and swapcontext) used for context control. The setcontext family allows the implementation in C of advanced control flow patterns such as iterators, fibers, and coroutines. They may be viewed as an advanced version of setjmp/longjmp; whereas the latter allows only a single non-local jump up the stack, setcontext allows the creation of multiple cooperative threads of control, each with its own stack. Specification setcontext was specified in POSIX.1-2001 and the Single Unix Specification, version 2, but not all Unix-like operating systems provide them. POSIX.1-2004 obsoleted these functions, and in POSIX.1-2008 they were removed, with POSIX Threads indicated as a possible replacement. Citing IEEE Std 1003.1, 2004 Edition: With the incorporation of the ISO/IEC 9899:1999 standard into this specification it was found that the ISO C standard (Subclause 6.11.6) specifies that the use of function declarators with empty parentheses is an obsolescent feature. Therefore, using the function prototype: void makecontext(ucontext_t *ucp, void (*func)(), int argc, ...); is making use of an obsolescent feature of the ISO C standard. Therefore, a strictly conforming POSIX application cannot use this form. Therefore, use of getcontext(), makecontext(), and swapcontext() is marked obsolescent. There is no way in the ISO C standard to specify a non-obsolescent function prototype indicating that a function will be called with an arbitrary number (including zero) of arguments of arbitrary types (including integers, pointers to data, pointers to functions, and composite types). Definitions The functions and associated types are defined in the ucontext.h system header file. This includes the ucontext_t type, with which all four functions operate: typedef struct { ucontext_t *uc_link; sigset_t uc_sigmask; stack_t uc_stack; mcontext_t uc_mcontext; ... } ucontext_t; uc_link points to the context which will be resumed when the current context exits, if the context was created with makecontext (a secondary context). uc_sigmask is used to store the set of signals blocked in the context, and uc_stack is the stack used by the context. uc_mcontext stores execution state, including all registers and CPU flags, the instruction pointer, and the stack pointer; mcontext_t is an opaque type. The functions are: int setcontext(const ucontext_t *ucp) This function transfers control to the context in ucp. Execution continues from the point at which the context was stored in ucp. setcontext does not return. int getcontext(ucontext_t *ucp) Saves current context into ucp. This function returns in two possible