1MAKECONTEXT(3) Linux Programmer's Manual MAKECONTEXT(3)
2
6 makecontext, swapcontext - manipulate user context
7
9 #include <ucontext.h>
10 void makecontext(ucontext_t *ucp, void (*func)(), int argc, ...);
12 int swapcontext(ucontext_t *restrict oucp,
13 const ucontext_t *restrict ucp);
14
16 In a System V-like environment, one has the type ucontext_t (defined in
17 <ucontext.h> and described in getcontext(3)) and the four functions
18 getcontext(3), setcontext(3), makecontext(), and swapcontext() that al‐
19 low user-level context switching between multiple threads of control
20 within a process.
21 The makecontext() function modifies the context pointed to by ucp
23 (which was obtained from a call to getcontext(3)). Before invoking
24 makecontext(), the caller must allocate a new stack for this context
25 and assign its address to ucp->uc_stack, and define a successor context
26 and assign its address to ucp->uc_link.
27 When this context is later activated (using setcontext(3) or swapcon‐
29 text()) the function func is called, and passed the series of integer
30 (int) arguments that follow argc; the caller must specify the number of
31 these arguments in argc. When this function returns, the successor
32 context is activated. If the successor context pointer is NULL, the
33 thread exits.
34 The swapcontext() function saves the current context in the structure
36 pointed to by oucp, and then activates the context pointed to by ucp.
37
39 When successful, swapcontext() does not return. (But we may return
40 later, in case oucp is activated, in which case it looks like swapcon‐
41 text() returns 0.) On error, swapcontext() returns -1 and sets errno
42 to indicate the error.
43
45 ENOMEM Insufficient stack space left.
46
48 makecontext() and swapcontext() are provided in glibc since version
49 2.1.
50
52 For an explanation of the terms used in this section, see at‐
53 tributes(7).
54 ┌──────────────┬───────────────┬───────────────────────────────────────┐
56 │Interface │ Attribute │ Value │
57 ├──────────────┼───────────────┼───────────────────────────────────────┤
58 │makecontext() │ Thread safety │ MT-Safe race:ucp │
59 ├──────────────┼───────────────┼───────────────────────────────────────┤
60 │swapcontext() │ Thread safety │ MT-Safe race:oucp race:ucp │
61 └──────────────┴───────────────┴───────────────────────────────────────┘
62
64 SUSv2, POSIX.1-2001. POSIX.1-2008 removes the specifications of make‐
65 context() and swapcontext(), citing portability issues, and recommend‐
66 ing that applications be rewritten to use POSIX threads instead.
67
69 The interpretation of ucp->uc_stack is just as in sigaltstack(2), name‐
70 ly, this struct contains the start and length of a memory area to be
71 used as the stack, regardless of the direction of growth of the stack.
72 Thus, it is not necessary for the user program to worry about this di‐
73 rection.
74 On architectures where int and pointer types are the same size (e.g.,
76 x86-32, where both types are 32 bits), you may be able to get away with
77 passing pointers as arguments to makecontext() following argc. Howev‐
78 er, doing this is not guaranteed to be portable, is undefined according
79 to the standards, and won't work on architectures where pointers are
80 larger than ints. Nevertheless, starting with version 2.8, glibc makes
81 some changes to makecontext(), to permit this on some 64-bit architec‐
82 tures (e.g., x86-64).
83
85 The example program below demonstrates the use of getcontext(3), make‐
86 context(), and swapcontext(). Running the program produces the follow‐
87 ing output:
88 $ ./a.out
90 main: swapcontext(&uctx_main, &uctx_func2)
91 func2: started
92 func2: swapcontext(&uctx_func2, &uctx_func1)
93 func1: started
94 func1: swapcontext(&uctx_func1, &uctx_func2)
95 func2: returning
96 func1: returning
97 main: exiting
98 Program source
100 #include <ucontext.h>
102 #include <stdio.h>
103 #include <stdlib.h>
104 static ucontext_t uctx_main, uctx_func1, uctx_func2;
106 #define handle_error(msg) \
108 do { perror(msg); exit(EXIT_FAILURE); } while (0)
109 static void
111 func1(void)
112 {
113 printf("func1: started\n");
114 printf("func1: swapcontext(&uctx_func1, &uctx_func2)\n");
115 if (swapcontext(&uctx_func1, &uctx_func2) == -1)
116 handle_error("swapcontext");
117 printf("func1: returning\n");
118 }
119 static void
121 func2(void)
122 {
123 printf("func2: started\n");
124 printf("func2: swapcontext(&uctx_func2, &uctx_func1)\n");
125 if (swapcontext(&uctx_func2, &uctx_func1) == -1)
126 handle_error("swapcontext");
127 printf("func2: returning\n");
128 }
129 int
131 main(int argc, char *argv[])
132 {
133 char func1_stack[16384];
134 char func2_stack[16384];
135 if (getcontext(&uctx_func1) == -1)
137 handle_error("getcontext");
138 uctx_func1.uc_stack.ss_sp = func1_stack;
139 uctx_func1.uc_stack.ss_size = sizeof(func1_stack);
140 uctx_func1.uc_link = &uctx_main;
141 makecontext(&uctx_func1, func1, 0);
142 if (getcontext(&uctx_func2) == -1)
144 handle_error("getcontext");
145 uctx_func2.uc_stack.ss_sp = func2_stack;
146 uctx_func2.uc_stack.ss_size = sizeof(func2_stack);
147 /* Successor context is f1(), unless argc > 1 */
148 uctx_func2.uc_link = (argc > 1) ? NULL : &uctx_func1;
149 makecontext(&uctx_func2, func2, 0);
150 printf("main: swapcontext(&uctx_main, &uctx_func2)\n");
152 if (swapcontext(&uctx_main, &uctx_func2) == -1)
153 handle_error("swapcontext");
154 printf("main: exiting\n");
156 exit(EXIT_SUCCESS);
157 }
158
160 sigaction(2), sigaltstack(2), sigprocmask(2), getcontext(3),
161 sigsetjmp(3)
162
164 This page is part of release 5.12 of the Linux man-pages project. A
165 description of the project, information about reporting bugs, and the
166 latest version of this page, can be found at
167 https://www.kernel.org/doc/man-pages/.
168GNU 2021-03-22 MAKECONTEXT(3)