getcontext(3) - Linux manual page (original) (raw)
getcontext(3) Library Functions Manual getcontext(3)
NAME top
getcontext, setcontext - get or set the user contextLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <ucontext.h>**
**int getcontext(ucontext_t ***_ucp_**);**
**int setcontext(const ucontext_t ***_ucp_**);**DESCRIPTION top
In a System V-like environment, one has the two types _mcontextt_
and _ucontextt_ defined in _<ucontext.h>_ and the four functions
**getcontext**(), **setcontext**(), [makecontext(3)](../man3/makecontext.3.html), and [swapcontext(3)](../man3/swapcontext.3.html)
that allow user-level context switching between multiple threads
of control within a process.
The _mcontextt_ type is machine-dependent and opaque. The
_ucontextt_ 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 _sigsett_ and _stackt_ defined in _<signal.h>_. Here _uclink_
points to the context that will be resumed when the current
context terminates (in case the current context was created using
[makecontext(3)](../man3/makecontext.3.html)), _ucsigmask_ is the set of signals blocked in this
context (see [sigprocmask(2)](../man2/sigprocmask.2.html)), _ucstack_ is the stack used by this
context (see [sigaltstack(2)](../man2/sigaltstack.2.html)), and _ucmcontext_ 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)](../man3/makecontext.3.html), or
received as the third argument to a signal handler (see the
discussion of the **SA_SIGINFO** flag in [sigaction(2)](../man2/sigaction.2.html)).
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)](../man3/makecontext.3.html), program
execution continues by a call to the function _func_ specified as
the second argument of that call to [makecontext(3)](../man3/makecontext.3.html). When the
function _func_ returns, we continue with the _uclink_ member of the
structure _ucp_ specified as the first argument of that call to
[makecontext(3)](../man3/makecontext.3.html). 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".RETURN VALUE top
When successful, **getcontext**() returns 0 and **setcontext**() does not
return. On error, both return -1 and set _[errno](../man3/errno.3.html)_ to indicate the
error.ERRORS top
None defined.ATTRIBUTES top
For an explanation of the terms used in this section, see
[attributes(7)](../man7/attributes.7.html).
┌─────────────────────────────┬───────────────┬──────────────────┐
│ **Interface** │ **Attribute** │ **Value** │
├─────────────────────────────┼───────────────┼──────────────────┤
│ **getcontext**(), **setcontext**() │ Thread safety │ MT-Safe race:ucp │
└─────────────────────────────┴───────────────┴──────────────────┘STANDARDS top
None.HISTORY top
SUSv2, POSIX.1-2001.
POSIX.1-2008 removes these functions, citing portability issues,
and recommending that applications be rewritten to use POSIX
threads instead.NOTES top
The earliest incarnation of this mechanism was the
[setjmp(3)](../man3/setjmp.3.html)/[longjmp(3)](../man3/longjmp.3.html) mechanism. Since that does not define the
handling of the signal context, the next stage was the
[sigsetjmp(3)](../man3/sigsetjmp.3.html)/[siglongjmp(3)](../man3/siglongjmp.3.html) 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)](../man3/longjmp.3.html): it is undefined what would
happen with contexts. Use [siglongjmp(3)](../man3/siglongjmp.3.html) or **setcontext**() instead.SEE ALSO top
[sigaction(2)](../man2/sigaction.2.html), [sigaltstack(2)](../man2/sigaltstack.2.html), [sigprocmask(2)](../man2/sigprocmask.2.html), [longjmp(3)](../man3/longjmp.3.html),
[makecontext(3)](../man3/makecontext.3.html), [sigsetjmp(3)](../man3/sigsetjmp.3.html), [signal(7)](../man7/signal.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 getcontext(3)
Pages that refer to this page:PR_SET_SYSCALL_USER_DISPATCH(2const), sigaction(2), sigreturn(2), makecontext(3)