NAME

getcontext, setcontext - get and set current user context

SYNOPSIS

[OB XSI] #include <ucontext.h>

int getcontext(ucontext_t *ucp);
int setcontext(const ucontext_t *ucp);

DESCRIPTION

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

The setcontext() function shall restore the user context pointed to by ucp. A successful call to setcontext() shall 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 makecontext(), 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(), program execution continues with the function passed to makecontext(). When that function returns, the thread shall continue as if after a call to setcontext() with the ucp argument that was input to makecontext(). If the uc_link member of the ucontext_t structure pointed to by the ucp argument is equal to 0, then this context is the main context, and the thread shall exit when this context returns. The effects of passing a ucp argument obtained from any other source are unspecified.

RETURN VALUE

Upon successful completion, setcontext() shall not return and getcontext() shall return 0; otherwise, a value of -1 shall be returned.

ERRORS

No errors are defined.

EXAMPLES

Refer to makecontext().

APPLICATION 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(), then it is unspecified whether the context at the time of the corresponding setjmp() call is restored and thus whether future calls to getcontext() provide an accurate representation of the current context, since the context restored by longjmp() does not necessarily contain all the information that setcontext() requires. Signal handlers should use siglongjmp() or setcontext() instead.

Conforming applications should not modify or access the uc_mcontext member of ucontext_t. A conforming 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.

Use of contexts to create alternate stacks is not defined by this volume of IEEE Std 1003.1-2001.

The obsolescent functions getcontext(), makecontext(), and swapcontext() can be replaced using POSIX threads functions.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

bsd_signal(), makecontext(), setcontext(), setjmp(), sigaction(), sigaltstack(), siglongjmp(), sigprocmask(), sigsetjmp(), the Base Definitions volume of IEEE Std 1003.1-2001, <ucontext.h>

CHANGE HISTORY

First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

The following sentence was removed from the DESCRIPTION: "If the ucp argument was passed to a signal handler, program execution continues with the program instruction following the instruction interrupted by the signal."

Issue 6

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/45 is applied, updating the SYNOPSIS and APPLICATION USAGE sections to note that the getcontext() and setcontext() functions are obsolescent.