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, ...);

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.

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.