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.Dd March 23, 2020
.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 of type
.Vt int
provided to
.Fn makecontext
and also equal to the number of arguments of type
.Vt int
to
.Fn func ;
otherwise ,
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
.Sh STANDARDS
The
.Fn makecontext
and
.Fn swapcontext
functions conform to
.St -xsh5
and
.St -p1003.1-2001 .
.Pp
The
.St -p1003.1-2004
revision marked the functions
.Fn makecontext
and
.Fn swapcontext
as obsolete, citing portability issues and recommending the use of
.Tn POSIX
threads instead.
The
.St -p1003.1-2008
revision removed the functions from the specification.
.Pp
.Bf -symbolic
The standard does not clearly define the type of integer arguments
passed to
.Fa func
via
.Fn makecontext ;
portable applications should not rely on the implementation detail that
it may be possible to pass pointer arguments to functions.
.Sh HISTORY
The
.Fn makecontext
and
.Fn swapcontext
functions first appeared in
.At V.4 .