Sun Microsystems, Inc. gratefully acknowledges The Open Group for
permission to reproduce portions of its copyrighted documentation.
Original documentation from The Open Group can be obtained online at
http://www.opengroup.org/bookstore/.

The Institute of Electrical and Electronics Engineers and The Open
Group, have given us permission to reprint portions of their
documentation.

In the following statement, the phrase ``this text'' refers to portions
of the system documentation.

Portions of this text are reprinted and reproduced in electronic form
in the SunOS Reference Manual, from IEEE Std 1003.1, 2004 Edition,
Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6,
Copyright (C) 2001-2004 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy
between these versions and the original IEEE and The Open Group
Standard, the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html.

This notice shall appear on any product containing this material.

The contents of this file are subject to the terms of the
Common Development and Distribution License (the "License").
You may not use this file except in compliance with the License.

You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
or http://www.opensolaris.org/os/licensing.
See the License for the specific language governing permissions
and limitations under the License.

When distributing Covered Code, include this CDDL HEADER in each
file and include the License file at usr/src/OPENSOLARIS.LICENSE.
If applicable, add the following below this CDDL HEADER, with the
fields enclosed by brackets "[]" replaced with your own identifying
information: Portions Copyright [yyyy] [name of copyright owner]


Copyright 1989 AT&T
Portions Copyright (c) 1992, X/Open Company Limited. All Rights Reserved.
Copyright (c) 2004, Sun Microsystems, Inc. All Rights Reserved.

makecontext, swapcontext - manipulate user contexts

#include <ucontext.h>

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

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

The makecontext() function modifies the context specified by ucp, which has been initialized using getcontext(2). When this context is resumed using swapcontext() or setcontext(2), execution continues by calling the function func, passing it the arguments that follow argc in the makecontext() call. The value of argc must match the number of pointer-sized integer arguments passed to func, otherwise the behavior is undefined.

Before a call is made to makecontext(), the context being modified should have a stack allocated for it. The stack is assigned to the context by initializing the uc_stack member.

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(). If the uc_link member is initialized to NULL, the thread executing func will exit when func returns. See pthread_exit(3C).

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.

If the ucp or oucp argument points to an invalid address, the behavior is undefined and errno may be set to EFAULT.

On successful completion, swapcontext() returns 0. Otherwise, -1 is returned and errno is set to indicate the error. The swapcontext() function will fail if: ENOMEM

The swapcontext() function may fail if: EFAULT

Example 1 Alternate execution context on a stack whose memory was allocated using mmap().

#include <stdio.h>
#include <ucontext.h>
#include <sys/mman.h>

void
assign(long a, int *b)
{
 *b = (int)a;
}

int
main(int argc, char **argv)
{
 ucontext_t uc, back;
 size_t sz = 0x10000;
 int value = 0;

 getcontext(&uc);

 uc.uc_stack.ss_sp = mmap(0, sz,
 PROT_READ | PROT_WRITE | PROT_EXEC,
 MAP_PRIVATE | MAP_ANON, -1, 0);
 uc.uc_stack.ss_size = sz;
 uc.uc_stack.ss_flags = 0;

 uc.uc_link = &back;

 makecontext(&uc, assign, 2, 100L, &value);
 swapcontext(&back, &uc);

 printf("done %d\en", value);

 return (0);
}

These functions are useful for implementing user-level context switching between multiple threads of control within a process (co-processing). More effective multiple threads of control can be obtained by using native support for multithreading. See threads(7). See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE ATTRIBUTE VALUE

Interface Stability Standard

MT-Level MT-Safe

getcontext (2), mmap (2), sigaction (2), sigprocmask (2), pthread_exit (3C), ucontext.h (3HEAD), attributes (7), standards (7), threads (7) The semantics of the uc_stack member of the ucontext_t structure have changed as they apply to inputs to makecontext(). Prior to Solaris 10, the ss_sp member of the uc_stack structure represented the high memory address of the area reserved for the stack. The ss_sp member now represents the base (low memory address), in keeping with other uses of ss_sp.

This change in the meaning of ss_sp is now the default behavior. The -D__MAKECONTEXT_V2_SOURCE compilation flag used in Solaris 9 update releases to access this behavior is obsolete.

Binary compatibility has been preserved with releases prior to Solaris 10. Before recompiling, applications that use makecontext() must be updated to reflect this behavior change. The example below demonstrates a typical change that must be applied:

--- example1_s9.c Thu Oct 3 11:58:17 2002
+++ example1.c Thu Jun 27 13:28:16 2002
@@ -27,12 +27,9 @@
 uc.uc_stack.ss_sp = mmap(0, sz,
 PROT_READ | PROT_WRITE | PROT_EXEC,
 MAP_PRIVATE | MAP_ANON, -1, 0);
- uc.uc_stack.ss_sp = (char *)uc.uc_stack.ss_sp + sz - 8;
 uc.uc_stack.ss_size = sz;
 uc.uc_stack.ss_flags = 0;

 uc.uc_link = &back

 makecontext(&uc, assign, 2, 100L, &value);