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.\"     @(#)sigstack.2	6.5 (Berkeley) 3/10/91
.\"
.Dd March 10, 1991
.Dt SIGSTACK 2
.Os BSD 4.2
.Sh NAME
.Nm sigstack
.Nd set and/or get signal stack context
.Sh SYNOPSIS
.Fd #include <sys/signal.h>
.Bd -literal
struct sigstack {
	caddr_t ss_sp;
	int     ss_onstack;
};
.Ed
.Ft int
.Fn sigstack "const struct sigstack *ss" "struct sigstack *oss"
.Sh DESCRIPTION
.Fn Sigstack
allows users to define an alternate stack on which signals
are to be processed.  If
.Fa ss
is non-zero,
it specifies a
.Em "signal stack"
on which to deliver signals
and tells the system if the process is currently executing
on that stack.  When a signal's action indicates its handler
should execute on the signal stack (specified with a
.Xr sigvec 2
call), the system checks to see
if the process is currently executing on that stack.  If the
process is not currently executing on the signal stack,
the system arranges a switch to the signal stack for the
duration of the signal handler's execution. 
If
.Fa oss
is non-zero, the current signal stack state is returned.
.Sh NOTES
Signal stacks are not ``grown'' automatically, as is
done for the normal stack.  If the stack overflows
unpredictable results may occur.
.Sh RETURN VALUES
Upon successful completion, a value of 0 is returned.
Otherwise, a value of -1 is returned and 
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn Sigstack
will fail and the signal stack context will remain unchanged
if one of the following occurs.
.Bl -tag -width [EFAULT]
.It Bq Er EFAULT
Either
.Fa ss
or
.Fa oss
points to memory that is not a valid part of the process
address space.
.El
.Sh SEE ALSO
.Xr sigvec 2 ,
.Xr setjmp 3
.Sh HISTORY
The
.Nm
function call appeared in
.Bx 4.2 .