Tribblix: manual page: getcontext.2

GETCONTEXT(2) System Calls GETCONTEXT(2)

NAME

getcontext, getcontext_extd, setcontext - get and set current user
context

SYNOPSIS

#include <ucontext.h>

int
getcontext(ucontext_t *ucp);

int
getcontext_extd(ucontext_t *ucp, uint32_t flags);

int
setcontext(const ucontext_t *ucp);

DESCRIPTION

The getcontext() function initializes the structure pointed to by ucp
to the current user context of the calling process. The ucontext_t
type that ucp points to defines the user context and includes the
contents of the calling process' machine registers, the signal mask,
and the current execution stack.

The ucontext_t structure is a part of the system ABI. However, most
architectures have added additional register states such as the
extended vector and floating point registers that are not part of that.
To facilitate getting that state (such as the x86 xsave area) the
getcontext_extd() function exists. Once called, the context will be
initialized and is suitable for use in other context operations just as
though one had called getcontext().

When calling the getcontext() function the ucontext_t is completely
overwritten without regards for what is currently present. This is
different when using getcontext_extd(). Instead, the ucontext_t
structure is read by the kernel and it assumes that the user has
initialized it. This allows the system to consider members of the
ucontext_T (such as the uc_xsave member on x86) to point to properly
sized memory.

To allow for all extended states to be copied out, ucp must be
allocated with ucontext_alloc(3C). Otherwise whether it is declared on
the stack, as global data, allocated dynamically, or part of a
structure, ucp must be zeroed through a call to bzero(3C) or memset(3C)
prior to calling getcontext_extd(). Improper initialization can lead
to memory safety bugs, making it critical that this is done.

The flags member must be zero and is present to allow for what is
copied out to change in the future. This indicates that the system
should attempt to copy out all extended states, though if the
ucontext_t was not allocated with ucontext_alloc(3C), some extended
states may not be. This happens because ucontext_alloc(3C) takes care
of allocating and setting up the ucontext_t to indicate that memory
beyond the ucontext_t is valid and the corresponding flags in the
structure are set.

The setcontext() function restores the user context pointed to by ucp.
A successful call to setcontext() does not return; program execution
resumes at the point specified by the ucp argument passed to
setcontext(). The ucp argument should be created either by a prior
call to getcontext(), or by being passed as an argument to a signal
handler. If the ucp argument was created with getcontext(), program
execution continues as if the corresponding call of getcontext() had
just returned. If the ucp argument was created with makecontext(3C),
program execution continues with the function passed to
makecontext(3C). When that function returns, the process continues as
if after a call to setcontext() with the ucp argument that was input to
makecontext(3C). If the ucp argument was passed to a signal handler,
program execution continues with the program instruction following the
instruction interrupted by the signal. If the uc_link member of the
ucontext_t structure pointed to by the ucp argument is NULL, then this
context is the main context, and the process will exit when this
context returns. The effects of passing a ucp argument obtained from
any other source are unspecified.

RETURN VALUES

On successful completion, setcontext() does not return and getcontext()
and getcontext_extd() returns 0. Otherwise, -1 is returned.

ERRORS

No errors are defined for getcontext() or setcontext().

The getcontext_extd() function only sets errno in some circumstances
when it fails. The function may fail if:

EINVAL flags had invalid values.

USAGE

When a signal handler is executed, the current user context is saved
and a new context is created. If the thread leaves the signal handler
via longjmp(3C), then it is unspecified whether the context at the time
of the corresponding setjmp(3C) call is restored and thus whether
future calls to getcontext() will provide an accurate representation of
the current context, since the context restored by longjmp(3C) may not
contain all the information that setcontext() requires. Signal
handlers should use siglongjmp(3C) instead.

Portable applications should not modify or access the uc_mcontext
member of ucontext_t. A portable application cannot assume that
context includes any process-wide static data, possibly including
errno. Users manipulating contexts should take care to handle these
explicitly when required.

INTERFACE STABILITY

Committed