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.\"     @(#)sigaltstack.2	8.1 (Berkeley) 6/4/93
.\"
.Dd $Mdocdate: October 19 2022 $
.Dt SIGALTSTACK 2
.Os
.Sh NAME
.Nm sigaltstack
.Nd set and/or get signal stack context
.Sh SYNOPSIS
.In signal.h
.Bd -literal
typedef struct sigaltstack {
	void	*ss_sp;
	size_t	 ss_size;
	int	 ss_flags;
} stack_t;
.Ed
.Ft int
.Fn sigaltstack "const stack_t *ss" "stack_t *oss"
.Sh DESCRIPTION
.Fn sigaltstack
allows users to define an alternate stack on which signals
delivered to this thread
are to be processed.
If
.Fa ss
is non-zero and
.Dv SS_DISABLE
is set in
.Fa ss_flags ,
the signal stack will be disabled.
A disabled stack will cause all signals to be
taken on the regular user stack.
Trying to disable an active stack will cause
.Fn sigaltstack
to return \-1 with
.Va errno
set to
.Er EPERM .
.Pp
Otherwise,
.Fa ss_sp
specifies a pointer to a space to be used as the signal stack and
.Fa ss_size
specifies the size of that space.
When a signal's action indicates its handler
should execute on the signal stack (specified with a
.Xr sigaction 2
call), the system checks to see
if the thread is currently executing on that stack.
If the thread 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.
.Pp
If
.Fa oss
is non-zero, the current signal stack state is returned.
The
.Fa ss_flags
field will contain the value
.Dv SS_ONSTACK
if the thread is currently on a signal stack and
.Dv SS_DISABLE
if the signal stack is currently disabled.
.Pp
The value
.Dv SIGSTKSZ
is defined to be the number of bytes/chars that would be used to cover
the usual case when allocating an alternate stack area.
The following code fragment is typically used to allocate an alternate stack.
.Bd -literal -offset indent
if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
	/* error return */
sigstk.ss_size = SIGSTKSZ;
sigstk.ss_flags = 0;
if (sigaltstack(&sigstk, NULL) == -1)
	perror("sigaltstack");
.Ed
.Pp
An alternative approach is provided for programs with signal handlers
that require a specific amount of stack space other than the default size.
The value
.Dv MINSIGSTKSZ
is defined to be the number of bytes/chars that is required by
the operating system to implement the alternate stack feature.
In computing an alternate stack size,
programs should add
.Dv MINSIGSTKSZ
to their stack requirements to allow for the operating system overhead.
.Pp
Signal stacks are automatically adjusted for the direction of stack
growth and alignment requirements.
Signal stacks may or may not be protected by the hardware and
are not
.Dq grown
automatically as is done for the normal stack.
If the stack overflows and this space is not protected,
unpredictable results may occur.
.Pp
On
.Ox
some additional restrictions prevent dangerous address space modifications.
The proposed space at
.Fa ss_sp
is verified to be contiguously mapped for read-write permissions
(no execute)
and incapable of syscall entry
(see
.Xr msyscall 2 ) .
If those conditions are met, a page-aligned inner region will be freshly mapped
(all zero) with
.Dv MAP_STACK
(see
.Xr mmap 2 ) ,
destroying the pre-existing data in the region.
Once the sigaltstack is disabled, the
.Dv MAP_STACK
attribute remains on the memory, so it is best to deallocate the memory
via a method that results in
.Xr munmap 2 .
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
.Fn sigaltstack
will fail and the signal stack context will remain unchanged
if one of the following occurs.
.Bl -tag -width [ENOMEM]
.It Bq Er EFAULT
Either
.Fa ss
or
.Fa oss
points to memory that is not a valid part of the process
address space.
.It Bq Er EINVAL
The
.Fa ss_flags
member pointed to by the
.Fa ss
argument contains flags other than
.Dv SS_DISABLE .
.It Bq Er EINVAL
The memory region is not acceptable for use as a stack;
see above.
.It Bq Er ENOMEM
Size of alternate stack area is less than or equal to
.Dv MINSIGSTKSZ .
.It Bq Er EPERM
An attempt was made to disable an active stack.
.El
.Sh SEE ALSO
.Xr sigaction 2 ,
.Xr setjmp 3
.Sh STANDARDS
The
.Fn sigaltstack
function conforms to
.St -p1003.1-2008 .
.Sh HISTORY
The predecessor to
.Fn sigaltstack ,
the
.Fn sigstack
system call, appeared in
.Bx 4.2 .