makecontext(3) - Linux manual page (original) (raw)
makecontext(3) Library Functions Manual makecontext(3)
NAME top
makecontext, swapcontext - manipulate user contextLIBRARY 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 _ucontextt_
(defined in _<ucontext.h>_ and described in [getcontext(3)](../man3/getcontext.3.html)) and the
four functions [getcontext(3)](../man3/getcontext.3.html), [setcontext(3)](../man3/setcontext.3.html), **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)](../man3/getcontext.3.html)). Before
invoking **makecontext**(), the caller must allocate a new stack for
this context and assign its address to _ucp->ucstack_, and define a
successor context and assign its address to _ucp->uclink_.
When this context is later activated (using [setcontext(3)](../man3/setcontext.3.html) 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](../man3/errno.3.html)_ 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)](../man7/attributes.7.html).
┌───────────────┬───────────────┬────────────────────────────────┐
│ **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->ucstack_ is just as in [sigaltstack(2)](../man2/sigaltstack.2.html),
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 _int_s.
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)](../man3/getcontext.3.html),
**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: exitingProgram 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)](../man2/sigaction.2.html), [sigaltstack(2)](../man2/sigaltstack.2.html), [sigprocmask(2)](../man2/sigprocmask.2.html), [getcontext(3)](../man3/getcontext.3.html),
[sigsetjmp(3)](../man3/sigsetjmp.3.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-12-13 makecontext(3)
Pages that refer to this page:sigaltstack(2), getcontext(3), signal(7)