NAME

getcontext, getcontextx, setcontext —

get and set user thread context

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <ucontext.h>

int
getcontext(ucontext_t *ucp);

ucontext_t *
getcontextx(void);

int
setcontext(const ucontext_t *ucp);

DESCRIPTION

The getcontext() function saves the current thread's execution context in the structure pointed to by ucp. This saved context may then later be restored by calling setcontext().

The getcontextx() function saves the current execution context in the newly allocated structure ucontext_t, which is returned on success. If architecture defines additional CPU states that can be stored in extended blocks referenced from the ucontext_t, the memory for them may be allocated and their context also stored. Memory returned by getcontextx() function shall be freed using free(3).

The setcontext() function makes a previously saved thread context the current thread context, i.e., the current context is lost and setcontext() does not return. Instead, execution continues in the context specified by ucp, which must have been previously initialized by a call to getcontext(), makecontext(3), or by being passed as an argument to a signal handler (see sigaction(2)).

If ucp was initialized by getcontext(), then execution continues as if the original getcontext() call had just returned (again).

If ucp was initialized by makecontext(3), execution continues with the invocation of the function specified to makecontext(3). When that function returns, ucp->uc_link determines what happens next: if ucp->uc_link is NULL, the process exits; otherwise, setcontext(ucp->uc_link) is implicitly invoked.

If ucp was initialized by the invocation of a signal handler, execution continues at the point the thread was interrupted by the signal.

RETURN VALUES

If successful, getcontext() returns zero and setcontext() does not return; otherwise -1 is returned. The getcontextx() returns pointer to the allocated and initialized context on success, and NULL on failure.

ERRORS

No errors are defined for getcontext() or setcontext(). The getcontextx() may return the following errors in errno:

[ENOMEM]: No memory was available to allocate for the context or some extended state.

STANDARDS

The getcontext() and setcontext() functions conform to X/Open System Interfaces and Headers Issue 5 (“XSH5”) and IEEE Std 1003.1-2001 (“POSIX.1”). The errno indications are an extension to the standard.

The IEEE Std 1003.1-2004 (“POSIX.1”) revision marked the functions getcontext() and setcontext() as obsolete, citing portability issues and recommending the use of POSIX threads instead. The IEEE Std 1003.1-2008 (“POSIX.1”) revision removed the functions from the specification.

HISTORY

The getcontext() and setcontext() functions first appeared in AT&T System V Release 4 UNIX.