getcontext(3) Library Functions Manual getcontext(3)
NAME
getcontext, setcontext - get or set the user context
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <ucontext.h>
int getcontext(ucontext_t *ucp);
int setcontext(const ucontext_t *ucp);
DESCRIPTION
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_sig-
mask is the set of signals blocked in this context (see sigproc-
mask(2)), uc_stack is the stack used by this context (see sigalt-
stack(2)), and uc_mcontext is the machine-specific representation of
the saved context, that includes the calling thread's machine regis-
ters.
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 ob-
tained by a call of getcontext(), or makecontext(3), or received as the
third argument to a signal handler (see the discussion of the SA_SIG-
INFO flag in sigaction(2)).
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 execu-
tion continues by a call to the function func specified as the second
argument of that call to makecontext(3). When the function func re-
turns, we continue with the uc_link member of the structure ucp speci-
fied 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". How-
ever, this sentence was removed in SUSv2, and the present verdict is
"the result is unspecified".
RETURN VALUE
When successful, getcontext() returns 0 and setcontext() does not re-
turn. On error, both return -1 and set errno to indicate the error.
ERRORS
None defined.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌───────────────────────────────────┬───────────────┬──────────────────┐
│Interface │ Attribute │ Value │
├───────────────────────────────────┼───────────────┼──────────────────┤
│getcontext(), setcontext() │ Thread safety │ MT-Safe race:ucp │
└───────────────────────────────────┴───────────────┴──────────────────┘
STANDARDS
SUSv2, POSIX.1-2001. POSIX.1-2008 removes the specification of getcon-
text(), citing portability issues, and recommending that applications
be rewritten to use POSIX threads instead.
NOTES
The earliest incarnation of this mechanism was the setjmp(3)/longjmp(3)
mechanism. Since that does not define the handling of the signal con-
text, 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 regis-
ters are restored.
When a signal occurs, the current user context is saved and a new con-
text is created by the kernel for the signal handler. Do not leave the
handler using longjmp(3): it is undefined what would happen with con-
texts. Use siglongjmp(3) or setcontext() instead.
SEE ALSO
sigaction(2), sigaltstack(2), sigprocmask(2), longjmp(3), makecon-
text(3), sigsetjmp(3), signal(7)
Linux man-pages 6.03 2022-12-15 getcontext(3)
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