makecontext(3C)

makecontext -- modify user context

Synopsis

   #include <ucontext.h>
   
   void makecontext (ucontext_t ucp, (void func)(),
   	int argc, . . . );
   

   int swapcontext (ucontext_t oucp, const ucontext_t ucp);

Description

swapcontext- saves and resets current context

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

makecontext 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 integers passed to func, otherwise the behavior is undefined.

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

swapcontext saves the current context in the context structure pointed to by oucp and sets the context to the context structure pointed to by ucp.

Return values

On successful completion, swapcontext return a value of zero. Otherwise, a value of -1 is returned and errno is set to indicate the error.

Errors

These functions will fail if either of the following is true:

ENOMEM: ucp does not have enough stack left to complete the operation.
EFAULT: ucp or oucp points to an invalid address.

References

exit(2), getcontext(2), sigaction(2), sigprocmask(2), ucontext(5)

Notices

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.

Equivalent routines and an enhanced user context structure are also provided in libc to handling floating point context changes for Pentium III processors. See ``Pentium III extended floating point support'' in New features for more information.