setcontext (3p) - Linux Manuals

setcontext: get and set current user context

PROLOG

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME

getcontext, setcontext - get and set current user context

SYNOPSIS

#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.

The following sections are informative.

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.

RATIONALE

None.

FUTURE DIRECTIONS

None.

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .