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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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getcontext(3) Library Functions Manual getcontext(3)
getcontext, setcontext - get or set the user context
Standard C library (libc, -lc)
#include <ucontext.h>
int getcontext(ucontext_t *ucp);
int setcontext(const ucontext_t *ucp);
In a System V-like environment, one has the two types mcontext_t
and ucontext_t defined in <ucontext.h> and the four functions
getcontext(), setcontext(), makecontext(3), and swapcontext(3)
that allow user-level context switching between multiple threads
of control within a process.
The mcontext_t type is machine-dependent and opaque. The
ucontext_t type is a structure that has at least the following
fields:
typedef struct ucontext_t {
struct ucontext_t *uc_link;
sigset_t uc_sigmask;
stack_t uc_stack;
mcontext_t uc_mcontext;
...
} ucontext_t;
with sigset_t and stack_t defined in <signal.h>. Here uc_link
points to the context that will be resumed when the current
context terminates (in case the current context was created using
makecontext(3)), uc_sigmask is the set of signals blocked in this
context (see sigprocmask(2)), uc_stack is the stack used by this
context (see sigaltstack(2)), and uc_mcontext is the machine-
specific representation of the saved context, that includes the
calling thread's machine registers.
The function getcontext() initializes the structure pointed to by
ucp to the currently active context.
The function setcontext() restores the user context pointed to by
ucp. A successful call does not return. The context should have
been obtained by a call of getcontext(), or makecontext(3), or
received as the third argument to a signal handler (see the
discussion of the SA_SIGINFO flag in sigaction(2)).
If the context was obtained by a call of getcontext(), program
execution continues as if this call just returned.
If the context was obtained by a call of makecontext(3), program
execution continues by a call to the function func specified as
the second argument of that call to makecontext(3). When the
function func returns, we continue with the uc_link member of the
structure ucp specified as the first argument of that call to
makecontext(3). When this member is NULL, the thread exits.
If the context was obtained by a call to a signal handler, then
old standard text says that "program execution continues with the
program instruction following the instruction interrupted by the
signal". However, this sentence was removed in SUSv2, and the
present verdict is "the result is unspecified".
When successful, getcontext() returns 0 and setcontext() does not
return. On error, both return -1 and set errno to indicate the
error.
None defined.
For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────────────────┬───────────────┬──────────────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────┼───────────────┼──────────────────┤
│ getcontext(), setcontext() │ Thread safety │ MT-Safe race:ucp │
└─────────────────────────────┴───────────────┴──────────────────┘
None.
SUSv2, POSIX.1-2001.
POSIX.1-2008 removes these functions, citing portability issues,
and recommending that applications be rewritten to use POSIX
threads instead.
The earliest incarnation of this mechanism was the
setjmp(3)/longjmp(3) mechanism. Since that does not define the
handling of the signal context, the next stage was the
sigsetjmp(3)/siglongjmp(3) pair. The present mechanism gives much
more control. On the other hand, there is no easy way to detect
whether a return from getcontext() is from the first call, or via
a setcontext() call. The user has to invent their own bookkeeping
device, and a register variable won't do since registers are
restored.
When a signal occurs, the current user context is saved and a new
context is created by the kernel for the signal handler. Do not
leave the handler using longjmp(3): it is undefined what would
happen with contexts. Use siglongjmp(3) or setcontext() instead.
sigaction(2), sigaltstack(2), sigprocmask(2), longjmp(3),
makecontext(3), sigsetjmp(3), signal(7)
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-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. 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
[email protected]
Linux man-pages 6.15 2025-05-17 getcontext(3)
Pages that refer to this page: PR_SET_SYSCALL_USER_DISPATCH(2const), sigaction(2), sigreturn(2), makecontext(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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