1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
|
'\" t
.\" Copyright (C) 2001 Andries Brouwer (aeb@cwi.nl)
.\" and Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 2006-08-02, mtk, Added example program
.\"
.TH makecontext 3 2023-10-31 "Linux man-pages 6.7"
.SH NAME
makecontext, swapcontext \- manipulate user context
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <ucontext.h>
.P
.BI "void makecontext(ucontext_t *" ucp ", void (*" func ")(), int " argc \
", ...);"
.BI "int swapcontext(ucontext_t *restrict " oucp ,
.BI " const ucontext_t *restrict " ucp );
.fi
.SH DESCRIPTION
In a System V-like environment, one has the type
.I ucontext_t
(defined in
.I <ucontext.h>
and described in
.BR getcontext (3))
and the four functions
.BR getcontext (3),
.BR setcontext (3),
.BR makecontext (),
and
.BR swapcontext ()
that allow user-level context switching
between multiple threads of control within a process.
.P
The
.BR makecontext ()
function modifies the context pointed to
by \fIucp\fP (which was obtained from a call to
.BR getcontext (3)).
Before invoking
.BR makecontext (),
the caller must allocate a new stack
for this context and assign its address to \fIucp\->uc_stack\fP,
and define a successor context and
assign its address to \fIucp\->uc_link\fP.
.P
When this context is later activated (using
.BR setcontext (3)
or
.BR swapcontext ())
the function \fIfunc\fP is called,
and passed the series of integer
.RI ( int )
arguments that follow
.IR argc ;
the caller must specify the number of these arguments in
.IR argc .
When this function returns, the successor context is activated.
If the successor context pointer is NULL, the thread exits.
.P
The
.BR swapcontext ()
function saves the current context in
the structure pointed to by \fIoucp\fP, and then activates the
context pointed to by \fIucp\fP.
.SH RETURN VALUE
When successful,
.BR swapcontext ()
does not return.
(But we may return later, in case \fIoucp\fP is
activated, in which case it looks like
.BR swapcontext ()
returns 0.)
On error,
.BR swapcontext ()
returns \-1 and sets
.I errno
to indicate the error.
.SH ERRORS
.TP
.B ENOMEM
Insufficient stack space left.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lbx
l l l.
Interface Attribute Value
T{
.na
.nh
.BR makecontext ()
T} Thread safety T{
.na
.nh
MT-Safe race:ucp
T}
T{
.na
.nh
.BR swapcontext ()
T} Thread safety T{
.na
.nh
MT-Safe race:oucp race:ucp
T}
.TE
.SH STANDARDS
None.
.SH HISTORY
glibc 2.1.
SUSv2, POSIX.1-2001.
Removed in POSIX.1-2008,
citing portability issues, and
recommending that applications be rewritten to use POSIX threads instead.
.SH NOTES
The interpretation of \fIucp\->uc_stack\fP is just as in
.BR sigaltstack (2),
namely, this struct contains the start and length of a memory area
to be used as the stack, regardless of the direction of growth of
the stack.
Thus, it is not necessary for the user program to
worry about this direction.
.P
On architectures where
.I int
and pointer types are the same size
(e.g., x86-32, where both types are 32 bits),
you may be able to get away with passing pointers as arguments to
.BR makecontext ()
following
.IR argc .
However, doing this is not guaranteed to be portable,
is undefined according to the standards,
and won't work on architectures where pointers are larger than
.IR int s.
Nevertheless, starting with glibc 2.8, glibc makes some changes to
.BR makecontext (),
to permit this on some 64-bit architectures (e.g., x86-64).
.SH EXAMPLES
The example program below demonstrates the use of
.BR getcontext (3),
.BR makecontext (),
and
.BR swapcontext ().
Running the program produces the following output:
.P
.in +4n
.EX
.RB "$" " ./a.out"
main: swapcontext(&uctx_main, &uctx_func2)
func2: started
func2: swapcontext(&uctx_func2, &uctx_func1)
func1: started
func1: swapcontext(&uctx_func1, &uctx_func2)
func2: returning
func1: returning
main: exiting
.EE
.in
.SS Program source
\&
.\" SRC BEGIN (makecontext.c)
.EX
#include <stdio.h>
#include <stdlib.h>
#include <ucontext.h>
\&
static ucontext_t uctx_main, uctx_func1, uctx_func2;
\&
#define handle_error(msg) \e
do { perror(msg); exit(EXIT_FAILURE); } while (0)
\&
static void
func1(void)
{
printf("%s: started\en", __func__);
printf("%s: swapcontext(&uctx_func1, &uctx_func2)\en", __func__);
if (swapcontext(&uctx_func1, &uctx_func2) == \-1)
handle_error("swapcontext");
printf("%s: returning\en", __func__);
}
\&
static void
func2(void)
{
printf("%s: started\en", __func__);
printf("%s: swapcontext(&uctx_func2, &uctx_func1)\en", __func__);
if (swapcontext(&uctx_func2, &uctx_func1) == \-1)
handle_error("swapcontext");
printf("%s: returning\en", __func__);
}
\&
int
main(int argc, char *argv[])
{
char func1_stack[16384];
char func2_stack[16384];
\&
if (getcontext(&uctx_func1) == \-1)
handle_error("getcontext");
uctx_func1.uc_stack.ss_sp = func1_stack;
uctx_func1.uc_stack.ss_size = sizeof(func1_stack);
uctx_func1.uc_link = &uctx_main;
makecontext(&uctx_func1, func1, 0);
\&
if (getcontext(&uctx_func2) == \-1)
handle_error("getcontext");
uctx_func2.uc_stack.ss_sp = func2_stack;
uctx_func2.uc_stack.ss_size = sizeof(func2_stack);
/* Successor context is f1(), unless argc > 1 */
uctx_func2.uc_link = (argc > 1) ? NULL : &uctx_func1;
makecontext(&uctx_func2, func2, 0);
\&
printf("%s: swapcontext(&uctx_main, &uctx_func2)\en", __func__);
if (swapcontext(&uctx_main, &uctx_func2) == \-1)
handle_error("swapcontext");
\&
printf("%s: exiting\en", __func__);
exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR sigaction (2),
.BR sigaltstack (2),
.BR sigprocmask (2),
.BR getcontext (3),
.BR sigsetjmp (3)
|
