xref: /illumos-gate/usr/src/man/man3c/makecontext.3c (revision 32991bedc3a6475f1401855c2318ae5b15f8a16b)
te
Copyright 1989 AT&T. Copyright (c) 2004, Sun Microsystems, Inc. All Rights Reserved. Portions Copyright (c) 1992, X/Open Company Limited. All Rights Reserved.
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 Sun OS 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]
MAKECONTEXT 3C "Mar 8, 2004"
NAME
makecontext, swapcontext - manipulate user contexts
SYNOPSIS

#include <ucontext.h>

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

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

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.

RETURN VALUES

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

ERRORS

The swapcontext() function will fail if: ENOMEM

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

The swapcontext() function may fail if: EFAULT

The ucp or oucp argument points to an invalid address.

EXAMPLES

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);
}
USAGE

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(5).

ATTRIBUTES

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

ATTRIBUTE TYPE ATTRIBUTE VALUE
Interface Stability Standard
MT-Level MT-Safe
SEE ALSO

mmap(2), getcontext(2), sigaction(2), sigprocmask(2), pthread_exit(3C), ucontext.h(3HEAD), attributes(5), standards(5), threads(5)

NOTES

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 demonstates 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);