.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "MAKECONTEXT" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" makecontext 
.SH NAME
makecontext, swapcontext \- manipulate user contexts
.SH SYNOPSIS
.LP
\fB#include <ucontext.h>
.br
.sp
void makecontext(ucontext_t *\fP\fIucp\fP\fB, void (*\fP\fIfunc\fP\fB)(void),
.br
\ \ \ \ \ \  int\fP \fIargc\fP\fB, ...);
.br
int swapcontext(ucontext_t *restrict\fP \fIoucp\fP\fB,
.br
\ \ \ \ \ \  const ucontext_t *restrict\fP \fIucp\fP\fB); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fImakecontext\fP() function shall modify the context specified
by \fIucp\fP, which has been initialized using \fIgetcontext\fP().
When this context is resumed using \fIswapcontext\fP() or \fIsetcontext\fP(),
program execution shall continue by calling \fIfunc\fP, passing it
the
arguments that follow \fIargc\fP in the \fImakecontext\fP() call.
.LP
Before a call is made to \fImakecontext\fP(), the application shall
ensure that the context being modified has a stack
allocated for it. The application shall ensure that the value of \fIargc\fP
matches the number of arguments of type \fBint\fP
passed to \fIfunc\fP; otherwise, the behavior is undefined.
.LP
The \fIuc_link\fP member is used to determine the context that shall
be resumed when the context being modified by
\fImakecontext\fP() returns. The application shall ensure that the
\fIuc_link\fP member is initialized prior to the call to
\fImakecontext\fP().
.LP
The \fIswapcontext\fP() function shall save the current context in
the context structure pointed to by \fIoucp\fP and shall
set the context to the context structure pointed to by \fIucp\fP.
.SH RETURN VALUE
.LP
Upon successful completion, \fIswapcontext\fP() shall return 0. Otherwise,
-1 shall be returned and \fIerrno\fP set to
indicate the error.
.SH ERRORS
.LP
The \fIswapcontext\fP() function shall fail if:
.TP 7
.B ENOMEM
The \fIucp\fP argument does not have enough stack left to complete
the operation.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
The following example illustrates the use of \fImakecontext\fP():
.sp
.RS
.nf

\fB#include <stdio.h>
#include <ucontext.h>
.sp

static ucontext_t ctx[3];
.sp

static void
f1 (void)
{
    puts("start f1");
    swapcontext(&ctx[1], &ctx[2]);
    puts("finish f1");
}
.sp

static void
f2 (void)
{
    puts("start f2");
    swapcontext(&ctx[2], &ctx[1]);
    puts("finish f2");
}
.sp

int
main (void)
{
    char st1[8192];
    char st2[8192];
.sp

    getcontext(&ctx[1]);
    ctx[1].uc_stack.ss_sp = st1;
    ctx[1].uc_stack.ss_size = sizeof st1;
    ctx[1].uc_link = &ctx[0];
    makecontext(&ctx[1], f1, 0);
.sp

    getcontext(&ctx[2]);
    ctx[2].uc_stack.ss_sp = st2;
    ctx[2].uc_stack.ss_size = sizeof st2;
    ctx[2].uc_link = &ctx[1];
    makecontext(&ctx[2], f2, 0);
.sp

    swapcontext(&ctx[0], &ctx[2]);
    return 0;
}
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
None.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIexit\fP() , \fIgetcontext\fP() , \fIsigaction\fP() , \fIsigprocmask\fP()
, the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, \fI<ucontext.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version 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 .