typedef struct sigaltstack {
void *ss_sp;
size_t ss_size;
int ss_flags;
} stack_t;
int
sigaltstack(const stack_t * restrict ss
, stack_t * restrict oss
)
)
allows users to define an alternative stack on which signals
are to be processed.
If
ss
is non-zero,
it specifies a pointer to and the size of a
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
sigaction(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
SS_DISABLE
is set in
ss_flags
,
ss_sp
and
ss_size
are ignored and the signal stack will be disabled.
Trying to disable an active stack will cause
sigaltstack
to return -1 with
errno
set to
EINVAL
.
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 alternative stack will resume doing so.
If
oss
is non-zero, the current signal stack state is returned.
The
ss_flags
field will contain the value
SS_ONSTACK
if the process is currently on a signal stack and
SS_DISABLE
if the signal stack is currently disabled.
SIGSTKSZ
is defined to be the number of bytes/chars that would be used to cover
the usual case when allocating an alternative stack area.
The following code fragment is typically used to allocate an alternative stack.
if ((sigstk.ss_sp = malloc(SIGSTKSZ)) == NULL)
/* error return */
sigstk.ss_size = SIGSTKSZ;
sigstk.ss_flags = 0;
if (sigaltstack(&sigstk,0) < 0)
perror("sigaltstack");
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
MINSIGSTKSZ
is defined to be the number of bytes/chars that is required by
the operating system to implement the alternative stack feature.
In computing an alternative stack size,
programs should add
MINSIGSTKSZ
to their stack requirements to allow for the operating system overhead.
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.
)
will fail and the signal stack context will remain unchanged
if one of the following occurs.
EFAULT
]
ss
or
oss
points to memory that is not a valid part of the process
address space.
EINVAL
]
ENOMEM
]
MINSIGSTKSZ
.
)
function conforms to
X/Open Portability Guide Issue 4, Version 2 (``XPG4.2'') .
)
system call, appeared in
4.2BSD.