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.\" $FreeBSD: src/lib/libc/gen/makecontext.3,v 1.4 2002/12/19 09:40:21 ru Exp $
.\"
.Dd September 10, 2002
.Dt MAKECONTEXT 3
.Os
.Sh NAME
.Nm makecontext , swapcontext
.Nd modify and exchange user thread contexts
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In ucontext.h
.Ft void
.Fo makecontext
.Fa "ucontext_t *ucp"
.Fa "void \*[lp]*func\*[rp]\*[lp]void\*[rp]"
.Fa "int argc" ...
.Fc
.Ft int
.Fn swapcontext "ucontext_t *oucp" "const ucontext_t *ucp"
.Sh DESCRIPTION
The
.Fn makecontext
function
modifies the user thread context pointed to by
.Fa ucp ,
which must have previously been initialized by a call to
.Xr getcontext 3
and had a stack allocated for it.
The context is modified so that it will continue execution by invoking
.Fn func
with the arguments provided.
The
.Fa argc
argument
must be equal to the number of additional arguments provided to
.Fn makecontext
and also equal to the number of arguments to
.Fn func ,
or else the behavior is undefined.
.Pp
The
.Fa "ucp->uc_link"
argument
must be initialized before calling
.Fn makecontext
and determines the action to take when
.Fn func
returns:
if equal to
.Dv NULL ,
the process exits;
otherwise,
.Fn setcontext "ucp->uc_link"
is implicitly invoked.
.Pp
The
.Fn swapcontext
function
saves the current thread context in
.Fa "*oucp"
and makes
.Fa "*ucp"
the currently active context.
.Sh RETURN VALUES
If successful,
.Fn swapcontext
returns zero;
otherwise \-1 is returned and the global variable
.Va errno
is set appropriately.
.Sh ERRORS
The
.Fn swapcontext
function
will fail if:
.Bl -tag -width Er
.It Bq Er ENOMEM
There is not enough stack space in
.Fa ucp
to complete the operation.
.El
.Sh SEE ALSO
.Xr setcontext 3 ,
.Xr ucontext 3