Ucontext in linux

Question

I read that ucontext is used to save context between multiple threads in linux. Since the OS does the context switching between different threads, why does linux provide this header file (ucontext.h) for context switching?

Solution

ucontext.h defines functions (setcontext, getcontext, makecontext, and swapcontext) and structures (ucontext_t and mcontext_t) that allow the programmer to save and restore the program context.

Using these functions, a programmer can implement advanced flow control schemes, such as coroutines or lightweight user threads.

A good amount of detail (including an example of implementing coroutines) is given in the GNU C Library: Complete Context Control.

It should be noted ucontext.h was obsoleted in POSIX.1-2008 ("IEEE Std 1003.1-2001/Cor 2-2004, item XBD/TC2/D6/28 is applied, updating the getcontext(), makecontext(), setcontext(), and swapcontext() functions to be obsolescent." on the ucontext.h page ).)

Comments

[Jean-Michaël Celerier]

Most serious userland threads would use setjmp() / longjmp() I think now.

[Embedded Programmer]

does it mean that ucontext.h is provided to make own threads in program? it depends upon user whether he wants to make own threads or not. what is problem with setjmp/longjmp in program?

[user1095108]

setjmp/longjmp unwind the stack with msvc.