GETCONTEXT(2)

NAME
getcontext, setcontext - get or set the user context

SYNOPSIS
#include <ucontext.h>

int getcontext(ucontext_t *ucp);
int setcontext(const ucontext_t *ucp);

DESCRIPTION
In a SysV-like environment, one has the two types mcontext_t and ucontext_t defined in <ucontext.h> and the four functions getcontext(), setcontext(), makecontext() and swapcontext() that allow user-level context switching between multiple threads of control within a process.

The mcontext_t type is machine-dependent and opaque. The ucontext_t type is a structure that has at least the following fields:
    typedef struct ucontext {
    	struct ucontext *uc_link;
    	sigset_t uc_sigmask;
    	stack_t uc_stack;
    	mcontext_t uc_mcontext;
    	...
    } ucontext_t;
with sigset_t and stack_t defined in <signal.h>. Here uc_link points to the context that will be resumed when the current context terminates (in case the current context was created using makecontext()), uc_sigmask is the set of signals blocked in this context (see sigprocmask(2)), uc_stack is the stack used by this context (see sigaltstack(2)), and uc_mcontext is the machine-specific representation of the saved context, that includes the calling thread's machine registers.

The function getcontext() initializes the structure pointed at by ucp to the currently active context.

The function setcontext() restores the user context pointed at by ucp. A successful call does not return. The context should have been obtained by a call of getcontext(), or makecontext(), or passed as third argument to a signal handler.

If the context was obtained by a call of getcontext(), program execution continues as if this call just returned.

If the context was obtained by a call of makecontext(), program execution continues by a call to the function func specified as the second argument of that call to makecontext(). When the function func returns, we continue with the uc_link member of the structure ucp specified as the first argument of that call to makecontext(). When this member is NULL, the thread exits.

If the context was obtained by a call to a signal handler, then old standard text says that "program execution continues with the program instruction following the instruction interrupted by the signal". However, this sentence was removed in SUSv2, and the present verdict is "the result is unspecified".

RETURN VALUE
When successful, getcontext() returns 0 and setcontext() does not return. On error, both return -1 and set errno appropriately.

ERRORS
None defined.

NOTES
The earliest incarnation of this mechanism was the setjmp()/longjmp() mechanism. Since that does not define the handling of the signal context, the next stage was the sigsetjmp()/siglongjmp() pair. The present mechanism gives much more control. On the other hand, there is no easy way to detect whether a return from getcontext() is from the first call, or via a setcontext() call. The user has to invent her own bookkeeping device, and a register variable won't do since registers are restored.

When a signal occurs, the current user context is saved and a new context is created by the kernel for the signal handler. Do not leave the handler using longjmp(): it is undefined what would happen with contexts. Use siglongjmp() or setcontext() instead.

CONFORMING TO
SUSv2

SEE ALSO
sigaction(2), sigaltstack(2), sigprocmask(2), longjmp(3), makecontext(3), sigsetjmp(3)