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.\"     @(#)sigaltstack.2	8.2 (Berkeley) 5/1/95
.\" $FreeBSD: release/9.1.0/lib/libc/sys/sigaltstack.2 211966 2010-08-29 16:38:08Z kib $
.\"
.Dd May 6, 2010
.Dt SIGALTSTACK 2
.Os
.Sh NAME
.Nm sigaltstack
.Nd set and/or get signal stack context
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In signal.h
.Bd -literal
typedef struct {
        char    *ss_sp;
        size_t  ss_size;
        int     ss_flags;
} stack_t;
.Ed
.Ft int
.Fn sigaltstack "const stack_t * restrict ss" "stack_t * restrict oss"
.Sh DESCRIPTION
The
.Fn sigaltstack
system call
allows defining an alternate stack on which signals
are to be processed for the current thread.
If
.Fa ss
is non-zero,
it specifies a pointer to and the size of a
.Em "signal stack"
on which to deliver signals.
When a signal's action indicates its handler
should execute on the signal stack (specified with a
.Xr sigaction 2
system 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
An active stack cannot be modified.
.Pp
If
.Dv SS_DISABLE
is set in
.Fa ss_flags ,
.Fa ss_sp
and
.Fa ss_size
are ignored and the signal stack will be disabled.
A disabled stack will cause all signals to be
taken on the regular user stack.
If the stack is later re-enabled then all signals that were specified
to be processed on an alternate stack will resume doing so.
.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.
.Sh NOTES
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) < 0)
	perror("sigaltstack");
.Ed
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 ``grown'' automatically as is done for the normal stack.
If the stack overflows and this space is not protected
unpredictable results may occur.
.Sh RETURN VALUES
.Rv -std sigaltstack
.Sh ERRORS
The
.Fn sigaltstack
system call
will fail and the signal stack context will remain unchanged
if one of the following occurs.
.Bl -tag -width Er
.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 EPERM
An attempt was made to modify an active stack.
.It Bq Er EINVAL
The
.Fa ss_flags
field was invalid.
.It Bq Er ENOMEM
Size of alternate stack area is less than or equal to
.Dv MINSIGSTKSZ .
.El
.Sh SEE ALSO
.Xr sigaction 2 ,
.Xr setjmp 3
.Sh HISTORY
The predecessor to
.Fn sigaltstack ,
the
.Fn sigstack
system call, appeared in
.Bx 4.2 .