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makecontext, swapcontext - manipulate user context
void makecontext(ucontext_t *ucp, void *func(), int argc, ...);
int swapcontext (ucontext_t *oucp, ucontext_t *ucp);
In a SysV-like environment, one has the type 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.
For the type and the first two functions, see getcontext(2).
The makecontext() function modifies the context pointed to by ucp
(which was obtained from a call to getcontext()). Before calling
makecontext(), one should allocate a new stack for this context,
assigning to ucp->uc_stack, and define a successor context, assigning
to ucp->uc_link. When this context is later activated (using
setcontext() or swapcontext()) then first the function func is called,
with the arguments specified following argc (where argc must contain
the number of these arguments), and when this function returns, the
successor context is activated. When the successor context pointer is
NULL, the thread exits.
The swapcontext() function saves the current context in the structure
pointed to by oucp, and then activates the context pointed to by ucp.
When successful, swapcontext() does not return. (But we may return
later, in case oucp is activated, in which case it looks like
swapcontext() returns 0.) On error, swapcontext() returns -1 and sets
-1 errno appropriately.
ENOMEM Insufficient stack space left.
The interpretation of ucp->uc_stack is just as in sigaltstack(2),
namely, this struct contains start and length of a memory area to be
used as stack, regardless of the direction of growth of the stack. So,
it is not necessary for the user program to worry about this direction.
SUSv2, POSIX 1003.1-2001.
getcontext(2), sigaction(2), sigaltstack(2), sigprocmask(2),