sigaltstack(2) System Calls sigaltstack(2)
sigaltstack - set or get signal alternate stack context
int sigaltstack(const stack_t *restrict ss, stack_t *restrict oss);
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:
If ss is not NULL, it points to a structure specifying the alternate
signal stack that will take effect upon successful return from sigalt-
stack(). 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
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 fol-
SS_ONSTACK The thread is currently executing on the alternate sig-
nal 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.
Upon successful completion, 0 is return. Otherwise, -1 is returned and
errno is set to indicate the error.
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
ENOMEM The size of the alternate stack area is less than MIN-
EPERM An attempt was made to modify an active stack.
See attributes(5) for descriptions of the following attributes:
tab() allbox; cw(2.750000i)| cw(2.750000i) lw(2.750000i)|
lw(2.750000i). ATTRIBUTE TYPEATTRIBUTE VALUE Interface StabilityStan-
getcontext(2), mmap(2), sigaction(2), ucontext.h(3HEAD), attributes(5),
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 oper-
ating 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:
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)
SunOS 5.10 1 Nov 2003 sigaltstack(2)