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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
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GETCONTEXT(3) Linux Programmer's Manual GETCONTEXT(3)
getcontext, setcontext - get or set the user context
#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 termi‐
nates (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 representa‐
tion of the saved context, that includes the calling thread's machine
registers.
The function getcontext() initializes the structure pointed at by ucp
to the currently active context.
The function setcontext() restores the user context pointed at by
ucp. A successful call does not return. The context should have
been obtained by a call of getcontext(), or makecontext(3), or passed
as third argument to a signal handler.
If the context was obtained by a call of getcontext(), program execu‐
tion continues as if this call just returned.
If the context was obtained by a call of makecontext(3), program exe‐
cution continues by a call to the function func specified as the sec‐
ond 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 appropriately.
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 │
└───────────────────────────┴───────────────┴──────────────────┘
SUSv2, POSIX.1-2001. POSIX.1-2008 removes the specification of
getcontext(), 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)
This page is part of release 5.07 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 GETCONTEXT(3)
Copyright and license for this manual page
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