MAKECONTEXT(3)         FreeBSD Library Functions Manual         MAKECONTEXT(3)

NAME
     makecontext, swapcontext - modify and exchange user thread contexts

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <ucontext.h>

     void
     makecontext(ucontext_t *ucp, void (*func)(void), int argc, ...);)(void), int argc, ...);

     int
     swapcontext(ucontext_t *oucp, const ucontext_t *ucp);

DESCRIPTION
     The makecontext() function modifies the user thread context pointed to by
     ucp, which must have previously been initialized by a call to
     getcontext(3) and had a stack allocated for it.  The context is modified
     so that it will continue execution by invoking func() with the arguments
     provided.  The argc argument must be equal to the number of additional
     arguments of type int provided to makecontext() and also equal to the
     number of arguments of type int to func(); otherwise , the behavior is
     undefined.

     The ucp->uc_link argument must be initialized before calling
     makecontext() and determines the action to take when func() returns: if
     equal to NULL, the process exits; otherwise, setcontext(ucp->uc_link) is
     implicitly invoked.

     The swapcontext() function saves the current thread context in *oucp and
     makes *ucp the currently active context.

RETURN VALUES
     If successful, swapcontext() returns zero; otherwise -1 is returned and
     the global variable errno is set appropriately.

ERRORS
     The swapcontext() function will fail if:

     [ENOMEM]           There is not enough stack space in ucp to complete the
                        operation.

SEE ALSO
     setcontext(3), ucontext(3)

STANDARDS
     The makecontext() and swapcontext() functions conform to X/Open System
     Interfaces and Headers Issue 5 ("XSH5") and IEEE Std 1003.1-2001
     ("POSIX.1").

     The IEEE Std 1003.1-2004 ("POSIX.1") revision marked the functions
     makecontext() and swapcontext() as obsolete, citing portability issues
     and recommending the use of POSIX threads instead.  The IEEE Std
     1003.1-2008 ("POSIX.1") revision removed the functions from the
     specification.

     The standard does not clearly define the type of integer arguments passed
     to func via makecontext(); portable applications should not rely on the
     implementation detail that it may be possible to pass pointer arguments
     to functions.

HISTORY
     The makecontext() and swapcontext() functions first appeared in AT&T
     System V Release 4 UNIX.

FreeBSD 13.1-RELEASE-p6         March 23, 2020         FreeBSD 13.1-RELEASE-p6