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