illumos: 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