These functions are useful for implementing user-level context switching between multiple threads of control within a process.

The makecontext() function modifies the context specified by ucp, which has been initialized using getcontext. When this context is resumed using swapcontext() or setcontext() (see getcontext(2)), program execution continues by calling the function func, passing it the arguments that follow argc in the makecontext() call.

Before a call is made to makecontext(), the context being modified should have a stack allocated for it. The value of argc must match the number of integer arguments passed to func, otherwise the behavior is undefined.

The uc_link member is used to determine the context that will be resumed when the context being modified by makecontext() returns. The uc_link member should be initialized prior to the call to makecontext().

The swapcontext() function saves the current context in the context structure pointed to by oucp and sets the context to the context structure pointed to by ucp.

On successful completion, swapcontext() returns 0. Otherwise, -1 is returned and errno is set to indicate the error.

The makecontext() and swapcontext() functions will fail if:

EFAULT

The ucp or oucp argument points to an invalid address.

ENOMEM

The ucp argument does not have enough stack left to complete the operation.

See attributes(5) for descriptions of the following attributes:

exit(2), getcontext(2), sigaction(2), sigprocmask(2), attributes(5), ucontext(3HEAD)

The size of the ucontext_t structure may change in future releases. To remain binary compatible, users of these features must always use makecontext() or getcontext() to create new instances of them.