SIGALTSTACK(2) System Calls SIGALTSTACK(2)


NAME


sigaltstack - set or get signal alternate stack context

SYNOPSIS


#include <signal.h>

int sigaltstack(const stack_t *restrict ss, stack_t *restrict oss);


DESCRIPTION


The sigaltstack() function allows a thread to define and examine the
state of an alternate stack area on which signals are processed. If ss is
non-zero, it specifies a pointer to and the size of a stack area on which
to deliver signals, and informs the system whether the thread is
currently executing on that stack. When a signal's action indicates its
handler should execute on the alternate signal stack (specified with a
sigaction(2) call), the system checks whether the thread chosen to
execute the signal handler is currently executing on that stack. If the
thread is not currently executing on the signal stack, the system
arranges a switch to the alternate signal stack for the duration of the
signal handler's execution.


The stack_t structure includes the following members:

int *ss_sp
long ss_size
int ss_flags


If ss is not NULL, it points to a structure specifying the alternate
signal stack that will take effect upon successful return from
sigaltstack(). The ss_sp and ss_size members specify the new base and
size of the stack, which is automatically adjusted for direction of
growth and alignment. The ss_flags member specifies the new stack state
and may be set to the following:

SS_DISABLE
The stack is to be disabled and ss_sp and ss_size are
ignored. If SS_DISABLE is not set, the stack will be
enabled.


If oss is not NULL, it points to a structure specifying the alternate
signal stack that was in effect prior to the call to sigaltstack(). The
ss_sp and ss_size members specify the base and size of that stack. The
ss_flags member specifies the stack's state, and may contain the
following values:

SS_ONSTACK
The thread is currently executing on the alternate signal
stack. Attempts to modify the alternate signal stack while
the thread is executing on it will fail.


SS_DISABLE
The alternate signal stack is currently disabled.


RETURN VALUES


Upon successful completion, 0 is return. Otherwise, -1 is returned and
errno is set to indicate the error.

ERRORS


The sigaltstack() function will fail if:

EFAULT
The ss or oss argument points to an illegal address.


EINVAL
The ss argument is not a null pointer, and the ss_flags member
pointed to by ss contains flags other than SS_DISABLE.


ENOMEM
The size of the alternate stack area is less than MINSIGSTKSZ.


EPERM
An attempt was made to modify an active stack.


ATTRIBUTES


See attributes(5) for descriptions of the following attributes:


+--------------------+-------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------+
|Interface Stability | Standard |
+--------------------+-------------------+
|MT-Level | Async-Signal-Safe |
+--------------------+-------------------+

SEE ALSO


getcontext(2), mmap(2), sigaction(2), ucontext.h(3HEAD), attributes(5),
standards(5)

NOTES


The value SIGSTKSZ is defined to be the number of bytes that would be
used to cover the usual case when allocating an alternate stack area.
The value MINSIGSTKSZ is defined to be the minimum stack size for a
signal handler. In computing an alternate stack size, a program should
add that amount to its stack requirements to allow for the operating
system overhead.


The following code fragment is typically used to allocate an alternate
stack with an adjacent red zone (an unmapped page) to guard against stack
overflow, as with default stacks:

#include <signal.h>
#include <sys/mman.h>

stack_t sigstk;
sigstk.ss_sp = mmap(NULL, SIGSTKSZ, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANON, -1, 0);
if (sigstk.ss_sp == MAP_FAILED)
/* error return */;
sigstk.ss_size = SIGSTKSZ;
sigstk.ss_flags = 0;
if (sigaltstack(&sigstk, NULL) < 0)
perror("sigaltstack");


November 1, 2003 SIGALTSTACK(2)