---

makecontext(3) — Linux manual page

makecontext(3) Library Functions Manual makecontext(3)

NAME top

 makecontext, swapcontext - manipulate user context

LIBRARY top

 Standard C library (libc, -lc)

SYNOPSIS top

 #include <ucontext.h>
 void makecontext(ucontext_t *ucp, typeof(void (int arg0, ...)) *func,
 int argc, ...);
 int swapcontext(ucontext_t *restrict oucp,
 const ucontext_t *restrict ucp);

DESCRIPTION top

 In a System V-like environment, one has the type ucontext_t
 (defined in <ucontext.h> and described in getcontext(3)) and the
 four functions getcontext(3), setcontext(3), makecontext(), and
 swapcontext() that allow user-level context switching between
 multiple threads of control within a process.
 The makecontext() function modifies the context pointed to by ucp
 (which was obtained from a call to getcontext(3)). Before
 invoking makecontext(), the caller must allocate a new stack for
 this context and assign its address to ucp->uc_stack, and define a
 successor context and assign its address to ucp->uc_link.
 When this context is later activated (using setcontext(3) or
 swapcontext()) the function func is called, and passed the series
 of integer (int) arguments that follow argc; the caller must
 specify the number of these arguments in argc. When this function
 returns, the successor context is activated. If the successor
 context pointer is NULL, the thread exits.
 The swapcontext() function saves the current context in the
 structure pointed to by oucp, and then activates the context
 pointed to by ucp.

RETURN VALUE top

 When successful, swapcontext() does not return. (But we may
 return later, in case oucp is activated, in which case it looks
 like swapcontext() returns 0.) On error, swapcontext() returns -1
 and sets errno  to indicate the error.

ERRORS top

 ENOMEM Insufficient stack space left.

ATTRIBUTES top

 For an explanation of the terms used in this section, see
 attributes(7).
 ┌───────────────┬───────────────┬────────────────────────────────┐
 │ Interface │ Attribute │ Value │
 ├───────────────┼───────────────┼────────────────────────────────┤
 │ makecontext() │ Thread safety │ MT-Safe race:ucp │
 ├───────────────┼───────────────┼────────────────────────────────┤
 │ swapcontext() │ Thread safety │ MT-Safe race:oucp race:ucp │
 └───────────────┴───────────────┴────────────────────────────────┘

STANDARDS top

 None.

HISTORY top

 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.

NOTES top

 The interpretation of ucp->uc_stack is just as in 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.
 On architectures where 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 makecontext()
 following 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 ints.
 Nevertheless, starting with glibc 2.8, glibc makes some changes to
 makecontext(), to permit this on some 64-bit architectures (e.g.,
 x86-64).

EXAMPLES top

 The example program below demonstrates the use of getcontext(3),
 makecontext(), and swapcontext(). Running the program produces
 the following output:
 $ ./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
 Program source
 #include <stdio.h>
 #include <stdlib.h>
 #include <ucontext.h>
 static ucontext_t uctx_main, uctx_func1, uctx_func2;
 #define handle_error(msg) \
 do { perror(msg); exit(EXIT_FAILURE); } while (0)
 static void
 func1(void)
 {
 printf("%s: started\n", __func__);
 printf("%s: swapcontext(&uctx_func1, &uctx_func2)\n", __func__);
 if (swapcontext(&uctx_func1, &uctx_func2) == -1)
 handle_error("swapcontext");
 printf("%s: returning\n", __func__);
 }
 static void
 func2(void)
 {
 printf("%s: started\n", __func__);
 printf("%s: swapcontext(&uctx_func2, &uctx_func1)\n", __func__);
 if (swapcontext(&uctx_func2, &uctx_func1) == -1)
 handle_error("swapcontext");
 printf("%s: returning\n", __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)\n", __func__);
 if (swapcontext(&uctx_main, &uctx_func2) == -1)
 handle_error("swapcontext");
 printf("%s: exiting\n", __func__);
 exit(EXIT_SUCCESS);
 }

SEE ALSO top

 sigaction(2), sigaltstack(2), sigprocmask(2), getcontext(3),
 sigsetjmp(3)

COLOPHON top

 This page is part of the man-pages (Linux kernel and C library
 user-space interface documentation) project. Information about
 the project can be found at 
 ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
 for this manual page, see
 ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
 This page was obtained from the tarball man-pages-6.10.tar.gz
 fetched from
 ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
 2025年02月02日. If you discover any rendering problems in this HTML
 version of the page, or you believe there is a better or more up-
 to-date source for the page, or you have corrections or
 improvements to the information in this COLOPHON (which is not
 part of the original manual page), send a mail to
 man-pages@man7.org
Linux man-pages 6.10 2024年12月13日 makecontext(3)

---

Pages that refer to this page: sigaltstack(2), getcontext(3), signal(7)

---

---

---