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[thirdparty/man-pages.git] / man2 / getcontext.2

.\" Copyright (C) 2001 Andries Brouwer (aeb@cwi.nl)
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.TH GETCONTEXT 2 2001-11-15 "Linux 2.4" "Linux Programmer's Manual"
.SH NAME
getcontext, setcontext \- get or set the user context
.SH SYNOPSIS
.B #include <ucontext.h>
.sp
.BI "int getcontext(ucontext_t *" ucp );
.br
.BI "int setcontext(const ucontext_t *" ucp );
.SH DESCRIPTION
In a System V-like environment, one has the two types
\fImcontext_t\fP and \fIucontext_t\fP defined in
.I <ucontext.h>
and the four functions
\fBgetcontext\fP(), \fBsetcontext\fP(), \fBmakecontext\fP()
and \fBswapcontext\fP()
that allow user-level context switching between multiple
threads of control within a process.
.LP
The \fImcontext_t\fP type is machine-dependent and opaque.
The \fIucontext_t\fP type is a structure that has at least
the following fields:
.RS
.nf

typedef struct ucontext {
    struct ucontext *uc_link;
    sigset_t         uc_sigmask;
    stack_t          uc_stack;
    mcontext_t       uc_mcontext;
    ...
} ucontext_t;

.fi
.RE
with \fIsigset_t\fP and \fIstack_t\fP defined in
.IR <signal.h> .
Here \fIuc_link\fP points to the context that will be resumed
when the current context terminates (in case the current context
was created using \fBmakecontext\fP()), \fIuc_sigmask\fP is the
set of signals blocked in this context (see
.BR sigprocmask (2)),
\fIuc_stack\fP is the stack used by this context (see
.BR sigaltstack (2)),
and \fIuc_mcontext\fP is the
machine-specific representation of the saved context,
that includes the calling thread's machine registers.
.LP
The function \fBgetcontext\fP() initializes the structure
pointed at by \fIucp\fP to the currently active context.
.LP
The function \fBsetcontext\fP() restores the user context
pointed at by \fIucp\fP.
A successful call does not return.
The context should have been obtained by a call of \fBgetcontext\fP(),
or \fBmakecontext\fP(), or passed as third argument to a signal
handler.
.LP
If the context was obtained by a call of \fBgetcontext\fP(),
program execution continues as if this call just returned.
.LP
If the context was obtained by a call of \fBmakecontext\fP(),
program execution continues by a call to the function \fIfunc\fP
specified as the second argument of that call to \fBmakecontext\fP().
When the function \fIfunc\fP returns, we continue with the
\fIuc_link\fP member of the structure \fIucp\fP specified as the
first argument of that call to \fBmakecontext\fP().
When this member is NULL, the thread exits.
.LP
If the context was obtained by a call to a signal handler,
then old standard text says that "program execution continues with the
program instruction following the instruction interrupted
by the signal".
However, this sentence was removed in SUSv2,
and the present verdict is "the result is unspecified".
.SH "RETURN VALUE"
When successful, \fBgetcontext\fP() returns 0 and \fBsetcontext\fP()
does not return.
On error, both return \-1 and set \fIerrno\fP
appropriately.
.SH ERRORS
None defined.
.SH NOTES
The earliest incarnation of this mechanism was the
\fIsetjmp\fP()/\fIlongjmp\fP() mechanism.
Since that does not define
the handling of the signal context, the next stage was the
\fIsigsetjmp\fP()/\fIsiglongjmp\fP() pair.
The present mechanism gives much more control.
On the other hand,
there is no easy way to detect whether a return from \fBgetcontext\fP()
is from the first call, or via a \fBsetcontext\fP() call.
The user has to invent her own bookkeeping device, and a register
variable won't do since registers are restored.
.LP
When a signal occurs, the current user context is saved and
a new context is created by the kernel for the signal handler.
Do not leave the handler using \fIlongjmp\fP(): it is undefined
what would happen with contexts.
Use \fIsiglongjmp\fP() or
\fIsetcontext\fP() instead.
.SH "CONFORMING TO"
SUSv2, POSIX.1-2001.
.SH "SEE ALSO"
.BR sigaction (2),
.BR sigaltstack (2),
.BR sigprocmask (2),
.BR longjmp (3),
.BR makecontext (3),
.BR sigsetjmp (3)