freebsd-dev/lib/libc/gen/getcontext.3

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.Dd March 13, 2013
.Dt GETCONTEXT 3
.Os
.Sh NAME
.Nm getcontext , getcontextx , setcontext
.Nd get and set user thread context
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In ucontext.h
.Ft int
.Fn getcontext "ucontext_t *ucp"
.Ft ucontext_t *
.Fn getcontextx "void"
.Ft int
.Fn setcontext "const ucontext_t *ucp"
.Sh DESCRIPTION
The
.Fn getcontext
function
saves the current thread's execution context in the structure pointed to by
.Fa ucp .
This saved context may then later be restored by calling
.Fn setcontext .
.Pp
The
.Fn getcontextx
function saves the current execution context in the newly allocated structure
.Vt ucontext_t ,
which is returned on success.
If architecture defines additional CPU states that can be stored in extended
blocks referenced from the
.Vt ucontext_t ,
the memory for them may be allocated and their context also stored.
Memory returned by
.Fn getcontextx
function shall be freed using
.Fn free 3 .
.Pp
The
.Fn setcontext
function
makes a previously saved thread context the current thread context, i.e.,
the current context is lost and
.Fn setcontext
does not return.
Instead, execution continues in the context specified by
.Fa ucp ,
which must have been previously initialized by a call to
.Fn getcontext ,
.Xr makecontext 3 ,
or by being passed as an argument to a signal handler (see
.Xr sigaction 2 ) .
.Pp
If
.Fa ucp
was initialized by
.Fn getcontext ,
then execution continues as if the original
.Fn getcontext
call had just returned (again).
.Pp
If
.Fa ucp
was initialized by
.Xr makecontext 3 ,
execution continues with the invocation of the function specified to
.Xr makecontext 3 .
When that function returns,
.Fa "ucp->uc_link"
determines what happens next:
if
.Fa "ucp->uc_link"
is
.Dv NULL ,
the process exits;
otherwise,
.Fn setcontext "ucp->uc_link"
is implicitly invoked.
.Pp
If
.Fa ucp
was initialized by the invocation of a signal handler, execution continues
at the point the thread was interrupted by the signal.
.Sh RETURN VALUES
If successful,
.Fn getcontext
returns zero and
.Fn setcontext
does not return; otherwise \-1 is returned.
The
.Fn getcontextx
returns pointer to the allocated and initialized context on success, and
.Va NULL
on failure.
.Sh ERRORS
No errors are defined for
.Fn getcontext
or
.Fn setcontext .
The
.Fn getcontextx
may return the following errors in
.Va errno :
.Bl -tag -width Er
.It Bq Er ENOMEM
No memory was available to allocate for the context or some extended state.
.El
.Sh SEE ALSO
.Xr sigaction 2 ,
.Xr sigaltstack 2 ,
.Xr makecontext 3 ,
.Xr ucontext 3