ucontext.h(3HEAD) Headers ucontext.h(3HEAD)NAME
ucontext.h, ucontext - user context
SYNOPSIS
#include <ucontext.h>
DESCRIPTION
The <ucontext.h> header defines the ucontext_t type as a structure that includes at least the following members:
ucontext_t uc_link
sigset_t uc_sigmask
stack_t uc_stack
mcontext_t uc_mcontext
The uc_link member is a pointer to the context that to be resumed when this context returns. If uc_link is equal to 0, this context is the
main context and the process exits when this context returns.
The uc_sigmask member defines the set of signals that are blocked when this context is active. See sigprocmask(2).
The uc_stack member defines the stack used by this context. See sigaltstack(2).
The uc_mcontext member contains the saved set of machine registers and any implementation-specific context data. Portable applications
should not modify or access uc_mcontext.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|Interface Stability |Standard |
+-----------------------------+-----------------------------+
SEE ALSO getcontext(2), sigaction(2), sigaltstack(2), sigprocmask(2), makecontext(3C), attributes(5), standards(5)SunOS 5.10 30 Aug 2002 ucontext.h(3HEAD)
Check Out this Related Man Page
GETCONTEXT(3) Linux Programmer's Manual GETCONTEXT(3)NAME
getcontext, setcontext - get or set the user context
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 getcon-
text(), 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 {
struct ucontext *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 sig-
procmask(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 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 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 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". However, 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 return. On error, both return -1 and set errno appropriately.
ERRORS
None defined.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
+---------------------------+---------------+------------------+
|Interface | Attribute | Value |
+---------------------------+---------------+------------------+
|getcontext(), setcontext() | Thread safety | MT-Safe race:ucp |
+---------------------------+---------------+------------------+
CONFORMING TO
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.
NOTES
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 her
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.
SEE ALSO sigaction(2), sigaltstack(2), sigprocmask(2), longjmp(3), makecontext(3), sigsetjmp(3)Linux 2015-03-02 GETCONTEXT(3)