mirror of https://github.com/mkerrisk/man-pages
133 lines
4.9 KiB
Groff
133 lines
4.9 KiB
Groff
.\" Copyright (C) 2001 Andries Brouwer (aeb@cwi.nl)
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.TH GETCONTEXT 2 2001-11-15 "Linux 2.4" "Linux Programmer's Manual"
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.SH NAME
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getcontext, setcontext \- get or set the user context
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.SH SYNOPSIS
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.B #include <ucontext.h>
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.sp
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.BI "int getcontext(ucontext_t *" ucp );
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.br
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.BI "int setcontext(const ucontext_t *" ucp );
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.SH DESCRIPTION
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In a System V-like environment, one has the two types
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\fImcontext_t\fP and \fIucontext_t\fP defined in
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.I <ucontext.h>
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and the four functions
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\fBgetcontext\fP(), \fBsetcontext\fP(), \fBmakecontext\fP()
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and \fBswapcontext\fP()
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that allow user-level context switching between multiple
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threads of control within a process.
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.LP
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The \fImcontext_t\fP type is machine-dependent and opaque.
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The \fIucontext_t\fP type is a structure that has at least
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the following fields:
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.RS
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.nf
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typedef struct ucontext {
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struct ucontext *uc_link;
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sigset_t uc_sigmask;
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stack_t uc_stack;
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mcontext_t uc_mcontext;
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...
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} ucontext_t;
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.fi
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.RE
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with \fIsigset_t\fP and \fIstack_t\fP defined in
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.IR <signal.h> .
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Here \fIuc_link\fP points to the context that will be resumed
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when the current context terminates (in case the current context
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was created using \fBmakecontext\fP()), \fIuc_sigmask\fP is the
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set of signals blocked in this context (see
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.BR sigprocmask (2)),
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\fIuc_stack\fP is the stack used by this context (see
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.BR sigaltstack (2)),
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and \fIuc_mcontext\fP is the
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machine-specific representation of the saved context,
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that includes the calling thread's machine registers.
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.LP
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The function \fBgetcontext\fP() initializes the structure
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pointed at by \fIucp\fP to the currently active context.
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.LP
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The function \fBsetcontext\fP() restores the user context
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pointed at by \fIucp\fP.
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A successful call does not return.
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The context should have been obtained by a call of \fBgetcontext\fP(),
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or \fBmakecontext\fP(), or passed as third argument to a signal
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handler.
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.LP
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If the context was obtained by a call of \fBgetcontext\fP(),
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program execution continues as if this call just returned.
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.LP
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If the context was obtained by a call of \fBmakecontext\fP(),
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program execution continues by a call to the function \fIfunc\fP
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specified as the second argument of that call to \fBmakecontext\fP().
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When the function \fIfunc\fP returns, we continue with the
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\fIuc_link\fP member of the structure \fIucp\fP specified as the
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first argument of that call to \fBmakecontext\fP().
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When this member is NULL, the thread exits.
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.LP
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If the context was obtained by a call to a signal handler,
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then old standard text says that "program execution continues with the
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program instruction following the instruction interrupted
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by the signal".
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However, this sentence was removed in SUSv2,
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and the present verdict is "the result is unspecified".
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.SH "RETURN VALUE"
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When successful, \fBgetcontext\fP() returns 0 and \fBsetcontext\fP()
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does not return.
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On error, both return \-1 and set \fIerrno\fP
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appropriately.
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.SH ERRORS
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None defined.
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.SH NOTES
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The earliest incarnation of this mechanism was the
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\fIsetjmp\fP()/\fIlongjmp\fP() mechanism.
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Since that does not define
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the handling of the signal context, the next stage was the
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\fIsigsetjmp\fP()/\fIsiglongjmp\fP() pair.
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The present mechanism gives much more control.
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On the other hand,
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there is no easy way to detect whether a return from \fBgetcontext\fP()
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is from the first call, or via a \fBsetcontext\fP() call.
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The user has to invent her own bookkeeping device, and a register
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variable won't do since registers are restored.
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.LP
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When a signal occurs, the current user context is saved and
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a new context is created by the kernel for the signal handler.
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Do not leave the handler using \fIlongjmp\fP(): it is undefined
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what would happen with contexts.
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Use \fIsiglongjmp\fP() or
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\fIsetcontext\fP() instead.
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.SH "CONFORMING TO"
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SUSv2, POSIX.1-2001.
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.SH "SEE ALSO"
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.BR sigaction (2),
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.BR sigaltstack (2),
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.BR sigprocmask (2),
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.BR longjmp (3),
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.BR makecontext (3),
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.BR sigsetjmp (3)
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